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Prebiotic chemistry in eutectic solutions at the water-ice matrixa
César Menor-Salván*b and Margarita R. Marín-Yaseli
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
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A crystalline ice matrix at subzero temperatures can maintain a liquid phase where organic solutes and
salts concentrates to form eutectic solutions. This concentration effect converts the confined reactant
solutions in the ice matrix, sometimes making condensation and polymerisation reactions occur more
favourably. These reactions occur at significant high rates from a prebiotic chemistry standpoint, and the
labile products can be preserved from degradation. The experimental study of the synthesis of nitrogen
heterocycles at the ice-water system showed the efficiency of this scenario and could explain the origin of
nucleobases in the inner Solar System bodies, including meteorites and extra-terrestrial ices, and on the
early Earth. The same conditions can also favour the condensation of monomers to form ribonucleic acid
and peptides. Together with the synthesis of these monomers, the ice world (i.e., the chemical evolution
in the range between freezing point of water and the limit of stability of liquid brines, ≈273 to 210 K) is
an under-explored experimental model in prebiotic chemistry.
Introduction
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Life as we know it depends on interfacial redox and transport
processes between liquid water and a system of lipid membranes
with the associated protein machinery. It seems logical to assume
that life emerged from liquid water solutions where relatively
simple raw materials were synthesised or accumulated. These
solutions could be subjected to water-mineral matrix interfacial
chemistry or concentration and compartmentalisation process,
which ultimately leads to the emergence of life in a complexity
increasing process. Consequently, to determine the possible
compositions of the raw materials for the plausible first steps of
abiotic evolution, pioneering experiments on prebiotic chemistry
have been conducted in water-saturated atmospheres and liquid
solutions1, which are largely supported by a reductive atmosphere
model.
The criticisms regarding an efficient atmospheric-liquid water
origin for the organic components of the first biochemical
processes on Earth arise from the lack of a universally accepted
geochemical model for the Archean atmosphere. Additionally,
the classic prebiotic chemistry approach deals with the problem
of the concentration and stability in liquid water of the plausible
prebiotic reactants. These criticisms and the lack of experimental
evidence supporting a model for the origin of biochemical
pathways have led to two main schools of thought:
The first concept is the possibility of an in situ origin on Earth,
which focus on either water-mineral interfacial processes as a
way for concentration and compartmentalisation of
environmentally synthesised reactants2 or on the origin of
chemoautotrophic pre-biochemical systems3.
The second concept argues that amino acids, nitrogen
heterocycles and simple organic molecules and monomers could
be synthesised by irradiation at very low temperatures in extraThis journal is © The Royal Society of Chemistry [year]
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terrestrial ice layers composed of water and other condensates4.
Ice is the most abundant form of water beyond the asteroid belt5.
The chemistry of ices at low temperatures followed by the
delivery of the organic molecules on Earth by comets, meteorites
and dust particles could have been an important source of
organics on the prebiotic Earth and could have played a key role
in early chemical evolution. The photochemistry and
radiochemistry of outer solar system bodies and interstellar ices
has received substantial attention6.
Despite the research into the photochemical transformations in
ice from an astrochemical point of view, the study of the
chemistry in the range of stability of the ice-water interface has
not received much attention. This may be due to the scarcity of
the defined conditions in the Solar System during the epoch of
active prebiotic chemistry or the difficulties for demonstrating
that these cold conditions existed in Hadean Earth.
The evidence for a liquid water subsurface ocean on Saturn’s
moon Europa7 and the possible presence of water-ammonia
eutectic brines or even a subsurface ocean in other outer giant
planet satellites such as Titan8 or Enceladus9 rekindled the
interest in liquid water prebiotic chemistry. Moreover, the
subsequent proposed steps for the emergence of cellular life have
a limited temperature range, and a hot prebiotic Earth was
regarded to be an unlikely environment for the origin of life by
some authors10. Miller and Orgel stated in 1974 that the
emergence of biological organisation could only occur at
temperatures below the melting point of the polynucleotide
structure. After observing the instability of organic compounds in
the prebiotic stages, these authors concluded that a temperature of
273 K would have been beneficial and that temperatures near the
eutectic point of NaCl solutions (251.3 K) would have been even
better11.
The low temperatures in planetary surface ices could be more
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conductive to the origin and the preservation of molecules that
could be relevant for the emergence of life. In 1994, in one of the
first explorations of the idea of an ice world-based origin of the
life raw materials, Bada et al.12 suggested that ice formations on
early Earth could have preserved organic compounds against
hydrolysis or photochemical degradation. Under plausible
planetary conditions, the presence of liquid water at T<273 K
within an ice matrix creates a potential reactor where the
synthesis or polymerisation of molecules of biological interest
could occur. Herein, we will review our current knowledge of the
chemical models that simulate possible prebiotic synthetic
pathways in liquid water interfacial ice. The experimental
approaches developed in the literature are primarily focused on
the RNA-world hypothesis of an abiotic origin of nucleic acids,
as these studies provide experimental evidence for the abiotic
synthesis and polymerisation of nitrogen heterocycles and
nucleotides. Apart from the molecular evolutionary perspective
for the emergence of life, exploring the chemistry in liquid
inclusions confined in an ice matrix could explain and predict the
composition of objects in the inner Solar System and icy
planetary bodies.
The ice/ liquid water system and its presence on
the early Earth and in the Solar System
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The ice-liquid water system has not received much attention in
the literature, including the chemical physics and
astrochemical/astrobiological literature. In the latter case, the
experimental efforts are focused on low temperature condensates,
where there is no evidence of a liquid interface and the ice is in
its amorphous crystalline state. In the inner Solar System,
including on Earth, ice occurs naturally in the crystalline form
with two primary polymorphs, which are cubic and hexagonal.
The crystallisation of water under current Earth surface
conditions results in hexagonal ice Ih. The ice formed from liquid
or heated from amorphous ice at temperatures between 100 and
130K is crystalline, with a diamond-type cubic structure Ic13.
Cubic ice is metastable at T<70 K and undergoes a
transformation to the amorphous state (the stable form at these
temperatures) via cosmic ray bombardment and ultraviolet
irradiation14. The irradiation diminishes the kinetic barrier
between the metastable cubic ice form and stable amorphous ice
form at lower temperatures15. The crystallisation of ice Ih leads to
the formation of various interfaces, such as ice-ice, iceatmosphere and water-ice, as well as water-ice-mineral, which
results from crystallisation of solutes by ice matrix exclusion or
the presence of suspended mineral grains16. The ice-ice and iceatmosphere interfaces are not a distinct transition. Nuclear
magnetic resonance studies of ice crystals indicate the existence
of a liquid transition between the crystals or between the ice and
the atmosphere. The thickness of this liquid phase becomes
monomolecular at T< 243 K and is thickened by dissolved solutes
excluded from the ice matrix to the interface during
crystallisation16.
The unexpected presence of crystalline ice in the Quaoar
object at the Kuiper Belt, on Enceladus and its suggested
presence in Titan17 imply that the evolution of ices are subject to
occasional heating events. If crystalline ice and if even fluid
water solutions are unambiguously present, the conditions for the
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increase in organic complexity from reactions between precursors
such as cyanide or cyanoacetylene may exist. The young and
active surface of the Jovian moon Europa suggests the possibility
of a subsurface water ocean from the observations of the Voyager
mission and strengthened by the observations with the Galileo
spacecraft18. Recently, it has been stated that Europa possesses
and active dynamic ice-water system with cycles of melting and
refreezing. In addition, a lenticular body of liquid brine in the
Thera Macula region of approximately 20.000-60.000 km3 has
been predicted19. The composition of Europa’s subsurface water,
underlying an ice crust, could be rich in sulphate salts, the source
of surface evaporite deposits20. The details on water composition
and temperature are unknown, but estimations suggest a MgSO4-Na(K) rich water with temperatures in the range 210-270
K21. A model for the formation of liquid ammonia-water pockets
that cause episodic cryomagmatism and a subsurface eutectic
water-ammonia solution has been proposed for the Saturn moon
Titan22. Within this context, both Titan and Europa constitute
important astrobiological targets for direct exploration and
laboratory simulations to predict the chemistry that will be found
and to test our experimental prebiotic chemistry models 23. A
complex prebiotic chemistry has been predicted for Titan that
includes the formation of nucleobases24 and the possibility of a
methane-acetylene based chemical or biochemical evolution25.
From these hypotheses based on atmospheric or surface
chemistry, the prebiotic possibilities of liquid water brines
entrapped under ice has received less attention and is the object
of speculative discussion regarding possible biochemical
evolution and the presence of chemoautotrophic life26.
Some models suggest a Hadean terrestrial atmosphere
composed primarily of high pressure carbon dioxide. If liquid
water were present in oceans over a basaltic crust, a CO2
atmosphere would be unstable and could be depleted as
carbonates in a period of approximately 10 million years due to
hydrothermal circulation and reaction of the CO2 with the crustal
rock. Under these conditions, together with the Hadean faint Sun,
the model developed by Sleep and Zahnle27 agrees with the ideas
suggested by J. Bada in 199412, predicting ice-covered oceans
and an average surface temperature of approximately 220 K, with
freeze-thaw episodes motivated by occasional warming provoked
by high energy impacts. These cold conditions would be
prevented if a methane-rich atmosphere were present during the
Hadean, as methane is a potent greenhouse gas. Evidence thus far
does not support an atmosphere with a high enough concentration
of methane to avoid freezing of the ocean surface. This model
would be amenable for the development of prebiotic chemistry in
an ice matrix based on HCN, cyanoacetylene, acetylene, urea or
cyanate precursors synthesised on Earth or brought in via
extraterrestrial input28.
The freezing of ocean water is a complex process. Modern sea
water begins to freeze at 271.2 K and crystals of pure ice (Ih)
begin to grow, surrounded by liquid brine with sodium chloride
concentrations up to 25%. The liquid solution is concentrated
within the ice structure in channels, which have been observed in
stained samples under the microscope, with diameters ranging
from 10 to 100 µm29. Based on observations of microscopic ice
layers, it is estimated that 1 m3 of sea ice has a network of
channels with a combined surface area of 105 to 106 m2. The
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volume of ice occupied by the brine channels and the brine
conditions within the channels are directly proportional to the
temperature; at 267 K, the brine salinity in sea ice is 100 (on the
practical salinity scale, i.e., dimensionless units that are
equivalent to the ratio between the sample solution and a standard
KCl solution; normal ocean water has a salinity range of 30-35);
at 263 K, the salinity rises to 145, and at 252 K, the salinity
reaches a maximum of 21630. In sea ice, the presence of
interstitial channels filled with liquid water and concentrated
solutes has been observed over a range of temperatures down to
243 K. Sea ice can lead to the formation of solid mineral phases
from the crystallisation of dissolved salts.
During freezing or thawing events, the temperature gradients
and density changes in the ice matrix lead to pressure gradients
and motion of the trapped liquid water that fills the channels and
pores. The freezing process led to the formation of potential
gradients, with pH variations of up to 3 units31.
The boundary between liquid and solid water has a different
refractive index and reveals an interface. Measurements of the
zeta potential (electric potential difference between the fluid brine
and the stationary liquid layer attached to the ice crystals) showed
that the interfacial properties of an ice-water system are
comparable to the interface with hydrophobic and nonionogenic
solids, such as diamond or hydrocarbons32. These properties
could be essential for the solute exclusion from the interstitial
brines in ice and the formulation of a freeze-concentration model
for explaining the prebiotic chemistry observed in the ice matrix.
Observation of the behaviour of stains in ice shows that
organic molecules are excluded from the ice matrix and
concentrated in the interstitial brine, where chromatographic
separation has been noted. Another important property of the
behaviour of organic molecules in ice is that a dilute starting
solution of a given solute always reaches the same molal
concentration in the interstitial solution, which is determined by
the final incubation temperature33. For example, a freezing dilute
urea solution tends to form an interstitial eutectic 8 m solution
with a melting point of 261K. These properties of the ice-water
interface convert the ocean ices, at temperatures within the range
of existence of the interface with liquid brines, into a potential
reactor for the first steps responsible for the emergence of life.
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Synthesis based on hydrogen cyanide
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Prebiotic synthesis of nucleobases and other
nitrogen heterocycles in the ice matrix
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Nucleobases are a small group of one-ring (pyrimidines) and tworing (purines) nitrogen heterocycles that, together with sugars and
phosphate, compose nucleic acids. The pyrimidines include
uracil, thymine and cytosine and purines include adenine and
guanine. Other heterocycles belonging to both groups are
important intermediates in the biochemistry, including xanthine,
hypoxanthine and orotic acid. It is generally assumed that the
earliest living forms on Earth used a genetic code based on
nucleobases34. In addition, nitrogen heterocycles could have been
involved in the first metabolic pathways as cofactors35.
Regardless of the controversy regarding whether life began
with a replicator, as suggested by the RNA-world hypothesis, or
with metabolism, as suggested by later authors36, there is no
evidence to discard the hypothesis of a prebiotic source of
nucleobases or cofactors for the first living system. The first
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logical hypothesis considers that the prebiotic synthesis took
place on Earth, although it is not clear if the environmental
conditions were consistent with efficient in situ synthesis37. The
second logical hypothesis is the delivery of nitrogen heterocycles
to Earth by comets, meteorites and dust particles. This extraterrestrial delivery could compensate for a possible lack of
availability from in situ synthesis. Analysis of carbonaceous
chondrites, a class of meteorites rich in organic carbon and
water38, has demonstrated the presence of N-heterocycles. These
heterocycles include adenine, guanine and triazines (ammeline
and melamine), which were found in the Orgeil meteorite by
Hayatsu in 196439. Subsequent analyses performed from 1965197540 show that the extraction conditions and sample treatments
determine the analytical results. However, the presence of
nucleobases in carbonaceous chondrites is widely accepted. In
2008, Martins et al. demonstrated41 the extra-terrestrial origin of
xanthine and adenine in a Murchinson meteorite sample using
carbon isotope measurements. Recently, Callahan et al.
demonstrated that the suite of purines found in carbonaceous
chondrites is consistent with those obtained using ammonium
cyanide chemistry42. The questions that arise from these results
include how were the nitrogen heterocycles synthesised on Earth
or other bodies in Solar System, and how could the ice-water
interface play a role in this process?
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The synthesis of nucleobases and other nitrogen heterocycles in
the parent body of a meteorite could be a process that is
dependent on the water content and irradiation of precursors. The
seminal work of Juan Oró and co-workers demonstrated that
adenine can be easily synthesised from hydrogen cyanide
(Scheme 1)43. A prebiotic origin for the nucleobases was
thereafter regarded as a realistic possibility44. Additionally, a
Fischer-Tropsch type synthetic mechanism catalysed by mineral
phases at high temperature has been suggested for the origin of
N-heterocycles in meteorites45, but its actual significance is
unclear46 and currently is not a widely accepted route.
Cyanide is the primary precursor involved in our current
models for prebiotic synthesis of nitrogen heterocycles and a
possible precursor to the organic molecules that gave rise to
biochemistry. Cyanide could be generated photochemically or by
spark discharges in methane/nitrogen planetary atmospheres 47. In
addition, free HCN and cyanide polymers have been observed in
comets, dust particles48 and the Titan atmosphere49.
The mechanism of synthesis of adenine from HCN implies that
the first step is polymerisation to the HCN-tetramer
diaminomaleonitrile (DAMN; Scheme 1). This intermediate
could undergo further polymerisation to form dark brown solid
polymers, which upon hydrolysis release nitrogen heterocycles,
including adenine50. This hydrolysis could take place in the icewater interface in the parent body of comets or meteorites during
their journey in the inner Solar System or after these objects
impacted the Earth. Another possible mechanism is the reaction
of DAMN with formamidine51 to afford a 4-amino-5cyanoimidazole (AICN) intermediate. This reaction yields
adenine through the coupling of HCN or formamidine. The
hydrolysis of AICN leads to 4-aminoimidazole-5-carboxamide
(AICA), which could be a xanthine and hypoxanthine precursor52
(Scheme 1). Formamidine has also been identified as an organic
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precursor found in comets53 and prebiotic chemistry laboratory
simulations54. A possible major mechanism for the formation of
adenine from HCN, which was elucidated by Voet and Schwartz
in 1982, is the reaction between the HCN tetramer and its
cyanoimino tautomer or diiminosuccinonitrile (an oxidation
product of HCN tetramer) to yield the carbamimidoyl cyanide
derivative. This molecule cyclises to 4-amino-2-cyanoimidazole5-carbimidoylcyanide. Further addition of the cyanoimino
derivative and ring closure, affords adenine-8-carboxamide
(Scheme 1)55. This product is quantitatively converted to adenine
by hydrolysis. The above mechanism was supported by the
structural
elucidation
of
4-amino-2-cyanoimidazole-5carboxamide and its hydrolysis product, 4-aminoimidazole-2,5dicarboxamide. However, the adenine-8-carboxamide has not yet
been identified in HCN oligomerisation experiments.
The last proposed mechanism is the UV-induced photoisomerisation of the HCN tetramer to 4-amino-5-cyanoimidazole.
The reaction of this imidazole with HCN or with its hydrolysis
product ammonium formate in a melt directly yields adenine56.
Because it is the key reaction in the pathway, the formation of the
HCN tetramer requires a high HCN concentration to avoid the
volatilisation or hydrolysis to ammonium formate, which
competes with the formation of diaminomaleonitrile in dilute
solutions. Therefore, it would have been impossible to reach
sufficiently high HCN concentrations in the open oceans or by
water evaporation57.
One solution to this problem could be to consider alternatives
to aqueous HCN chemistry. The formation of nucleobases from
formamide in the presence of inorganic catalysts at high
temperature creates a robust pathway for adenine, hypoxanthine,
uracil and cytosine among other N-heterocycles 58. One solution
to this problem could be concentrating HCN using the liquid-ice
interfacial properties. During the first attempt to test this
possibility, Sanchez et al. (1966) showed that HCN concentrates
in a frozen eutectic solution. The eutectic solution, which has a
mole fraction of 70 to 80% in HCN, is formed at 249 K and
deposits a dark HCN polymer59. Considering the activation
energy of the HCN polymerisation and the rate constants, the
formation of the HCN tetramer in eutectic fluids should be
complete in a few years. At 173 K, the reaction occurs over the
order of hundreds of millions of years 60. The advantageously
stable conditions in a water-ice interface could surpass the
handicap of prebiotic synthesis at low temperatures and the
problem of concentration and stability at high temperatures.
Additionally, the freezing of dilute glycolonitrile solutions,
produced by addition of HCN and formaldehyde, produces
adenine in low yield (0.004%)61. In a long duration experiment,
Miyakawa et al. maintained a frozen solution of ammonium
cyanide at 195 K over 27 years and at the end of this time period,
identified adenine as well as other purine and pyrimidine
products62. Although the HCN pathway has been extensively
studied for the synthesis of purines, it has been demonstrated that
the polymerisation of cyanide could provide a pathway for the
formation of the pyrimidines including uracil, 5-hydroxyuracil
and orotic acid63. The freezing of cyanide solutions could also
provide a source of amino acids. In 1972, another long-term
experiment involved a solution of NH4CN prepared from HCN
and NH3. These reagents were frozen and subjected to variable
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temperatures of 253K and 195K for 25 years. The analysis
indicated the formation of glycine and small amounts of alanine
and aspartic acid64. The mechanism for the cold synthesis of
amino acids from HCN has not been elucidated, but may include
the hydrolysis of HCN polymers65 and the hydrolysis of 2aminoacetonitrile, which is formed during HCN tetramer
evolution, to glycine (Scheme 1).
Prebiotic laboratory synthesis from frozen cyanide solutions
could be a model for the prebiotic synthesis of nucleobases. This
synthesis could also explain the chemistry observed in icecovered objects within the inner solar system, such as asteroids
and comets during their closest passage to the sun, and in objects
with complex chemistry, including Titan or Enceladus. To
efficiently serve both goals, more experimental work should be
performed to elucidate the mechanisms involved in frozen HCN
solution, to test if the classic pathway through cyanoimidazole
derivatives is reproducible in the ice matrix scenario and to
determine if alternative pathways should also be examined.
Synthesis based on cyanoacetylene/acetylene and the role of
urea
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Cyanoacetylene is the other primary precursor considered for the
synthesis of nucleobases. Cyanoacetylene can be obtained in the
laboratory from methane/nitrogen mixtures by spark discharges 66
by irradiation with short-wave ultraviolet radiation at 185 and
254 nm67; the spectrum of this molecule has been observed in the
interstellar medium68 and by the Voyager mission in Titan’s
atmosphere69, where crystalline condensates of cyanoacetylene
with acetylene may exist70.
The potential prebiotic relevance of cyanoacetylene in origin
of life studies were pointed out by Ferris, Sanchez and Orgel in
1968. They observed that the reaction of cyanoacetylene with
aqueous 1 M sodium cyanate or 1 M urea gave cytosine in up to
5% yield (Scheme 2)71. The prebiotic availability of cyanate
could be explained by the hydrolysis of cyanogen and urea,
which may also be present in cometary and interstellar ices72.
The mechanism of this reaction could be explained by
cyanoacetaldehyde, generated by hydrolysis of cyanoacetylene.
The Miller research demonstrated the eutectic concentration and
reaction of cyanoacetaldehyde with urea in an ice matrix at 253K
to give cytosine and uracil in 0.005% and 0.02% yields,
respectively73. In the same report, cyanoacetaldehyde reacted
with guanidine at 253K to give cytosine in 0.05% yield and uracil
in 10.8% yield, as well as lesser amounts of isocytosine and 2,4diaminopyrimidine after 2 months74. This reaction may proceed
through the cyanoacetaldehyde dimer, 4-(hydroxymethylene)
pentenedinitrile, easily formed by concentrating the
cyanoacetaldehyde solutions (Scheme 2)72.
The basis of these experiments is the freezing of a urea or
guanidine solution. This process provides a concentration
mechanism because the crystalline ice excludes the solute and a
eutectic is formed. At 262K, urea forms an 8 m eutectic solution
in water. This effect could be significant from a prebiotic point of
view, despite the slower reaction rates, as has been shown in
recent experiments.
An unresolved issue with the cyanoacetylene pathway in the
synthesis of nucleobases is its reactivity to nucleophiles75, which
suggests a high number of competitive reactions that lead to the
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formation of amino- or hydroxyacrylonitriles and subsequent
polymers or hydrolysis products; On the other hand, the prebiotic
origin of cytosine was questioned, at least in the liquid water
medium, because its spontaneous and rapid deamination to
uracil76. In part, the reactions in the water-ice interface could
overcome the problem of dilution and degradation associated
with solutions in liquid water pools.
Although much time has elapsed since the first proposal in
1966 of a low temperature prebiotic environment for the origin of
nucleobases, it was not until 2000 that the product of the classic
approach of spark discharges in methane/nitrogen based
atmosphere was subjected to eutectic freezing77 at 253K for 5
years. The frozen spark discharge product showed a more
extensive mixture of amino acids and the presence of adenine,
which was absent in the control experiment at room temperature.
The first experimental simulation of prebiotic synthesis in iceliquid water directly from nitrogen/methane atmosphere by spark
discharges was performed in 200978. The sparking on a freezing
dilute urea solution under a nitrogen/methane atmosphere leads to
the formation of cytosine, uracil and 2,4,6-trihydroxypyrimidine
(barbituric acid) as the main identified pyrimidines, in addition to
adenine. The experiments showed that using the freeze-thaw
conditions, the observed sequence of pyrimidine yield obtained
was cytosine > uracil > 2,4-diaminopyrimidine > 2,4,6trihydroxypyrimidine. The formation or pyrimidines by oxidative
alteration of cytosine (UV irradiation, hydroxyl radical addition
or other free radical mechanism and further oxidation to
barbituric acid) could explain the results observed79. The
formation of cytosine as the main pyrimidine suggests that the
low temperature conditions could reduce the rate of deamination
to uracil and favour subsequent chemical evolution steps, as
suggested by Bada12.
The triazine series (cyanuric acid, ammelide, ammeline and
melamine) are also obtained in high yields (Scheme 3). The
formation of triazines appears to be dependent on the freezing of
urea solution. The triazines are not biological compounds, but
they could mimic nucleobases behaviour in nucleic acids and
their potential prebiotic role has been discussed80. Their presence
in meteorites remains contentious81.
The key factor appears to be the freezing process itself and not
the temperature of the final ice obtained, as the temperature was
selected to be right below the freezing point of 0.1 M urea. In a
liquid urea solution at room temperature, there is no evidence of
nucleobases. Instead, the formation of hydantoins, nitriles and
tholins
(reddish-brown,
insoluble,
heteropolymeric
or
macromolecular materials formed by sparking or irradiation of
simple carbon sources, as methane) is prevalent.
The behaviour of urea in the ice-water interface is the key
factor because urea tends to form dimers or oligomers in a
concentration-dependent manner82. Urea molecules in aqueous
fluids tend to form hydrogen bonds with neighbouring water
molecules at both the amino and the carbonyl groups83.
Spectroscopic studies show that at urea concentrations higher
than 1 M, the urea-urea molecular interactions are significant.
The urea-urea molecular interaction with subsequent formation of
dimers or clusters of urea molecules becomes dominant at
eutectic concentration84. During freezing, the urea is segregated
from pure ice to accumulate in supercooled microfluid inclusions
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of a supersaturated solution. This system is governed by
dehydration and association of solute molecules85. Thus, the
extent of urea dimerisation (18% in 0.1 M urea solution at
standard temperature86) is expected to increase and to become
quantitatively a few degrees below the onset of freezing.
Consequently, we expect an apparently paradoxical similarity
between the process observed in molten urea84 and urea clusters
entrapped in an ice matrix when the latter are subjected to direct
sparking or irradiation.
This behaviour could explain the sequence of products
obtained (cyanuric acid > ammelide > ammeline > melamine),
which is the same sequence observed when urea is heated above
its melting point. The spark discharges into the ice, then, could
thermally decompose urea clusters in ammonium cyanate. Further
decomposition of ammonium cyanate leads cyanic acid. The
cyanic acid reacts with urea to form the biuret and with the
formed biuret to form cyanuric acid (a cyanic acid trimer), which
is the main triazine observed84. Several routes to ammelide are
possible: reaction of cyanuric acid and ammonia or cyanic acid
and urea or biuret. The process, in which the decomposition
products accelerate the formation of triazines, could explain the
high concentration of cyanuric acid obtained in these conditions.
Other parallel pathway is the formation of melamine by
cyanamide polymerization. The melamine hydrolysis yield
cyanuric acid (Scheme 3). These pathways and the same reaction
sequence, with the same relative abundance of triazines, have
been studied in molten urea87 at temperatures between 406 and
460K. In this case, an alternative route for forming purines could
result from the condensation of amino acids and biuret, a reaction
that occurs at high temperature88; however, this alternative still
has not been studied in ice-water systems and could be an
unlikely possibility because of the high activation energy of such
condensations. We also cannot discard other alternative pathways
parallel to the polymerisation of concentrated urea solutions. For
example, the production of cyanic acid during atmospheric
discharges or thermal alteration of tholins89 and the subsequent
reaction in freezing urea solutions could be an alternative source
of cyanuric acid. Additional laboratory studies are necessary for
clarifying the mechanisms involved in the cold synthesis of
triazines and purines in the ice matrix.
The effect of concentration of solutes in the ice matrix,
together with the low availability of water vapour could explain
the preferential synthesis of polycyclic aromatic hydrocarbons
(PAHs) by sparking methane/nitrogen atmosphere over an ice
matrix90. The model of PAH synthesis is interesting because it
could confirm the theoretical synthesis of aromatics by acetylene
insertion mechanisms proposed for the Titan’s atmosphere91. In
laboratory experiments at sub-zero temperatures65, the acetylene
addition mechanism could explain the preferential formation of
aromatics and poly(triacetylene) polymers (Scheme 4) by two
possible mechanisms. First, a single aromatic ring could be
generated from acetylene and vinyl radical and PAH growth by H
abstraction and acetylene addition (Berthelot synthesis, similar to
PAHs formation in flames). The second mechanism involves
polyyne growth. The presence of water ice induces oxidations
leading to the formation of aromatic polar species as
benzaldehyde or acetophenone. The reaction in ice, in contrast to
the dry high temperature synthesis of PAHs, leads to hydroxyl-
Journal Name, [year], [vol], 00–00 | 5
5
10
15
20
25
30
35
40
45
50
rich poly(triacetylene) based polymers. Overall, these ice-water
laboratory experiments reveal the expected chemical species in
surface or subsurface ices on solar system objects or extrasolar
planetary bodies.
The activation of methane/nitrogen atmospheres by spark
discharges could lead to various chemistries involving reactive
intermediates, including HCN, cyanoacetylene and acetylene.
The preference for the hydantoins in liquid urea solutions at room
temperature versus pyrimidines in frozen solution experiments
could be due to the acetylene formation and subsequent alteration
by means of ozone and hydroxyl radicals at higher temperatures
to form alpha-dicarbonyl compounds as glyoxal 92. The reaction
of glyoxal with urea in mild acidic conditions yields hydantoin93,
whose further oxidation yields 5-hydroxyhydantoin and parabanic
acid (Scheme 3). These three hydantoins are always found
together in all the experiments reported in the literature. Its
formation could be explained also as alteration products of uracil
by hydroxyl and other free radicals generated in water solutions
by photolysis or irradiation94,79. At lower temperatures, the
degradation of pyrimidines to hydantoins and the oxidation of
acetylene could be diminished, due to the lower availability of
reactive oxygen species generated from the excitation of water. In
consequence, hydantoins could be the final products of alteration
of pyrimidines in prebiotic conditions subjected to UV-irradiation
or other energetic processes. Regarding acetylene, the
polymerisation could be the preferred reaction pathway, as shown
by the formation of poly-triacetylene and aromatic hydrocarbons
at the ice-water matrix previously described. In this environment,
the HCN or cyanoacetylene pathways could dominate, as long
with other alternative mechanisms, as the synthesis of uracil by
reaction of urea with acetylene dicarboxylic acid 95. This acid is
the aqueous hydrolysis product of dicyanoacetylene, which is an
exotic product of methane/nitrogen atmospheres observed in the
Titan atmosphere96. The role of acetylene derivatives has not
been studied in the ice-water scenario, and further experiments
are necessary to explore the possible alternative pathways related
to acetylene in prebiotic synthesis in an ice matrix and to put it in
context with the classic mechanisms involving cyanide and
cyanoacetylene. The products identified in the simulations of
methane/nitrogen atmospheres over the ice-water interface
include dicarboxylic and hydroxycarboxylic acids, amino acids
and pyrazines, suggesting an additional mechanism to those
suggested above.
In summary, the advantages of an ice-water interface in
prebiotic synthesis include the reduction in the formation of
polymers and tholin with a preference for ring systems (nitrogen
heterocycles or aromatic rings) by the effect of concentration of
diluted reactants such as HCN, urea or cyanate. Combined with
other rocks or minerals, the freezing of liquid water solutions
could favour mineral surface-organic solute interactions97
60
65
70
75
80
85
90
Scheme 5 (uncaptioned)
95
100
105
The ice-water system in the origin of nucleic acids
55
The ice matrix is an appropriate environment for the synthesis
of nitrogen heterocycles, as demonstrated by the synthesis of
triazines and nucleobases in freezing urea solutions. Could the
ice-water interface be a favourable environment for the assembly
of the first biologically relevant informational polymers?
The success in the synthesis of nucleobases from a feedstock
6 | Journal Name, [year], [vol], 00–00
of active nitrogen species available prebiotically led to the
establishment of a similar retrosynthetic analysis for RNA and to
the search for prebiotically plausible syntheses of a primordial
informational, self-replicating polymer. If the discovery of an
abiotic pathway to the origin of the first nucleotides and the
constitutional self-assembly of RNA is achieved, the RNA-world
hypothesis (a term coined by Walter Gilbert in 1986)98, which
proposes a molecular evolutionary step involving autocatalytic
RNA molecules prior to the origin of protein synthesis and
metabolic machinery, will be strengthened. The current state of
prebiotic chemistry does not provide a complete model for an
abiotic origin of RNA, and the first formulations of an RNA
world have been re-evaluated99. However, some argue that it
may be premature to conclude that the prebiotic RNA world is
unlikely to be a step in the emergence of life100.
In this context, the ice-water interface has been evaluated
thoroughly as a matrix for the polymerisation of highly activated
nucleotides. The first demonstration of this possibility was
performed by Gryaznov and Letsinger in 1993101. In their
experiment, the coupling of an alpha-bromoacyl-activated
oligonucleotide (bromoacetylamino-3’-desoxythimidine in the 3’terminus) with another oligonucleotide with a phosphorothioate
group in the 5’-terminus proceeded without a template in a frozen
saline solution at 255 K in 5 days. The reaction was explained as
a result of the high local concentration of reactants in the fluid
cavities in the ice matrix.
The enhancing effect of the ice matrix in the formation of
RNA oligomers was demonstrated by Kanavarioti et al. in a very
remarkable experiment in which oligouridylates up to 22 bases
long were synthesised by incubating a uridine 5´monophosphorimidazolide solution at 255 K at a pH range
between 6 and 8 in the presence of magnesium and lead
cations102:
110
The study of the ribonuclease A digestion products showed that
the oligomers obtained are mainly linear and that 30% carry at
least one 3´-5´ linkage. The fluorescence microscopy observation
of an ice layer in the experimental conditions with acridine
orange staining indicated that the organic solutes were
concentrated in the eutectic lattice structure included in the ice
matrix. The authors concluded that the formation of eutectic
solutions of reactants in the ice matrix facilitated the
oligomerisation. The polymerisation most likely occurs in the
liquid concentrated solutions between the ice crystals, and not by
the adsorption of reactants onto the ice surfaces, as previously
suggested by Stribling and Miller103, who studied the template
directed synthesis of poly(U) in diluted solutions concentrated by
freezing close to the NaCl eutectic. The ice also has an effect on
the metal catalysis. The reaction in the ice-water medium requires
Pb2+ as a catalyst and not Mg2+. This phenomenon is different
from reactions in solution, which require both magnesium and
lead cations. A possible interpretation of this observation is that
the molecular associations in an ice matrix tend to be more stable
than the corresponding ones in solution. An open question that
arises is the role of certain metal cations (for example lead) as
prebiotic catalysts. The lead catalysis in the polymerisation of
This journal is © The Royal Society of Chemistry [year]
5
10
15
20
activated nucleotides could be related to the mechanism of
leadzymes104 and suggests that metal ion catalysis is central in a
hypothetical RNA world.
If pyrimidine and purine-activated nucleotides are used in the
water-ice interface at 255 K during 38 days in the presence of
Mg2+ and Pb2+, a mixed-sequence polynucleotide with
approximately the same proportion of purine and pyrimidines
residues is obtained105. Monnard and Szostak106 studied the
template-directed RNA polymerisation in water-ice at 256.4 K, a
temperature that permits the maintenance of a stable water-ice
interface for long periods of time. They found that lead and
magnesium ions catalyse the elongation of a RNA hairpin with a
5’-overhang as a template.
Similarly, the non-enzymatic synthesis of polyadenosine in a
sea-ice matrix, directed by poly(U) was performed, using
adenosine-5´-monophosphate
(2-methyl)
imidazolide
as
monomer. Temperature fluctuations established the freeze-partial
thaw cycles during one year. The results show high molecular
weight poly(A) formation, with chain lengths of as many as 420
residues107:
60
65
70
75
Scheme 6 (uncaptioned)
25
30
35
40
45
50
55
The freezing-concentration model could also govern the
conformational rearrangement pathway of the formed polymers.
Freezing a 21-nt RNA hairpin solution at 203 K followed by
incubation at 263 K results in the conversion to the duplex dimer
form108. The formation of frozen microenvironments during
prebiotic evolution could be a key factor in the possible prebiotic
evolution of informational polymers.
80
85
The formation of peptides in the ice matrix
The linking of monomer units to form simple polymers likely
defined an important step in the origins of life, and many
conditions have been proposed, including dehydration agents109,
sulphide minerals110, melting111 or hydrothermal systems112.
Further studies suggest important roles for catalytic surfaces,
such as clays, or interfaces created by wet–dry cycling of
monomers on mineral surfaces113.
Based on this idea, Schwendinger and Rode found a
particularly simple process of salt-induced peptide formation,
using 40-50 mM amino acid solutions where NaCl at
concentrations above 3 M can act as a dehydrating or
condensation agent, using dissolved Cu(II) as a catalyst 114.
Experiments carried out by Fox demonstrated that the melting of
amino acids at temperatures in the range of 400 to 433 K, to
allow melting without decomposition, produces a type of polymer
called ‘proteinoids’. This phenomenon will occur provided that
acidic or basic amino acids are present in excess115. However, the
so-called ‘proteinoids’ are mainly heteropolymers containing
only very small quantities of peptide bonds116. The melting of a
mixture of urea and alanine yields the dipeptide Ala-Ala81.
The largest number of proposals and related experiments
performed in order to model the prebiotic peptide formation in
solution involves the postulated existence of coadjutant
condensation reagents in a homogenous catalytic process. These
reagents include cyanamide and cyanoguanidine, which may act
as prebiotically plausible condensing agents117.
This journal is © The Royal Society of Chemistry [year]
90
95
100
105
110
A problem associated with high temperature processes is the
decomposition of amino acids and the hydrolysis of peptides,
which constitutes a limitation for the organisation of larger
polymers118. The synthesis in freezing solutions could prevent
undesirable side reactions, hydrolysis of the formed peptide bond,
and the decomposition of amino acids as well as reduce the rate
of amino acid racemisation119. This idea is connected to a
different approach to the problem of amino acid condensation
that was introduced years ago: the salt-induced peptide formation
reaction. Salty brines could have played a role in the
polymerisation of amino acids. However, the formation of a
peptide bond is not straightforward at low temperatures without
condensing agents, and the experiments performed were carried
out at high temperatures in drying conditions.
Could freezing of the primitive oceans have produced the
concentrated salty brines with the associated condensing agents
needed to promote the salt-induced polymerisation process?
Orgel et al. studied the oligomerisation of beta-amino acids in
aqueous solutions under eutectic conditions using activation by
the
water-soluble
reagents
EDAC
(1-ethyl-3-(3dimethylaminopropyl)-carbodiimide)
and
carbonyldiimidazole120. The oligomerisation of beta-amino acids (LAspartic acid, beta-amino adipic acid, beta-glutamic acid) using
these condensing agents proceeds efficiently at 253 K (under
eutectic freezing), even from dilute solutions of the substrates.
This reaction produces peptides in the range 15 to 20 units
(maximum: 45) in length with a yield of over 50%. The
efficiency of polymerisation and the length distribution of the
oligomers was almost unaffected by the solute concentration over
a broad range of 0.1 to 100 mM at 253K. According to these
results, the EDAC reagent constitutes the model of a group of
activating agents whose function is the direct reaction with the
carboxyl group of amino acids. Cyanogen, cyanamide and
cyanoguanidine are prebiotically plausible members of this
group. The elucidation of the pathway shows that the first step is
the direct attack of the carboxyl group on the carbodiimide to
form an O-acylisourea. The free amino group of another amino
acid attacks this activated species to form a peptide bond. In the
case of alpha amino acids, the carboxyl group of the dipeptide
can be activated and then cyclise efficiently to give a
diketopiperazine, thus inhibiting oligomerisation121. Cyclisation
of an activated dimer of beta-amino acids is not straightforward
because an eight-membered ring does not form readily.
In 1996, Vajda et al. synthesised four protected dipeptides and
a protected tripeptide in frozen dioxane and other organic
solvents122. The data demonstrated that the coupling rates in
frozen dioxane at 254 K exceed by approximately one order of
magnitude the rates in liquid solution at 313 K. Vajda suggested
that enhanced reaction rates and/or yields, diminution of
racemisation, and the suppression of side reactions can be
expected in frozen systems, and these possibilities substantially
increase the importance of peptide formation in eutectic frozen
solutions123. However, no further investigation on these
possibilities has been performed.
Concluding remarks
Prebiotic chemistry in the range of stability of a liquid water-ice
interface (277 to 243K in common laboratory conditions) has
Journal Name, [year], [vol], 00–00 | 7
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20
25
been proposed since the pioneering experiment in the field. These
ideas were proposed to overcome the concentration and stability
problems associated with liquid water prebiotic chemistry. The
experiments performed demonstrated that the synthesis of
aromatic hydrocarbons, purines and pyrimidines and other
nitrogen heterocycles of potential prebiotic interest (such as
triazines) are favoured in the ice matrix by classic cyanide and
cyanoacetylene pathways following a freezing-concentration
model. Despite these results, the experimental prebiotic chemistry
in the solute-concentrated solutions that fill the space confined by
ice matrix has received relatively little attention in the elaboration
of the models for the origin of organics in Solar System bodies
and prebiotic evolution. Consequently, it is necessary to clarify
the mechanisms involved and the role of reactants as well as to
perform more experiments under plausible prebiotic conditions,
especially if geochemical models support stable icy environments
on the prebiotic Earth.
The concentration of reactant solutions by freezing also enhances
the polymerisation of activated nucleotides and the formation of
small peptides in the presence of an activating agent. The
prebiotic relevance of these polymerisation reactions and the gap
between the nucleobase synthesis and the organisation of the first
biopolymers is a matter for discussion. Nevertheless, the ice
world constitutes an interesting prebiotic chemistry scenario that
awaits further investigation.
Acknowledgements
We acknowledge the Centro de Astrobiologia (CSIC-INTA) and
the grants of the project AYA2009-13920-C02-01 from the
Ministerio de Ciencia e Innovación (MICINN, Spain).
30
Notes and references
a
Part of the prebiotic chemistry themed issue
Centro de Astrobiología (INTA-CSIC), INTA, E-28850 Torrejón de
Ardoz, Spain. Tel: 32 91520 6458; E-mail: menorsc@cab.inta-csic.es
b
35
1
2
S. Miller, Science, 1953, 117, 528.
J.P. Ferris, R.A. Sanchez, L.E. Orgel, Journal of Molecular Biology,
1968, 33, 693; D. Clarke and J. Ferris, Icarus, 1997, 127, 158.
3 W. Martin and M.J. Russell, Philosophical Transactions of the Royal
Society B, 2006, 362, 1887.
4 G.M. Muñoz Caro, U. Meierhenrich, W.A. Schutte, W. H.P.
Thiemann and J.M. Greenberg, Astronomy and Astrophysics, 2004,
413, 209.
5 J.I. Lunine, Meteorites and the early solar system vol II, 309.
6 P. Klan and I. Holoubek, Chemosphere, 2002, 46, 1201 and
references therein.
7 C.F. Chyba and C. B. Phillips, Origins of Life and Evolution of the
Biosphere, 2002, 32, 47.
8 G. Tobie, O. Grasset, J.I. Lunine, A. Mocquet and C. Sotin, Icarus,
2005, 175, 496.
9 F. Postberg, S. Kempf, J. Schmidt, N. Brilliantov, A. Beinsen, B.
Abel, U. Buck and R. Srama, Nature, 2009, 459, 1098.
10 V. Moulton, P.P Gardner, R.F. Pointon, L.K. Creamer, G.B. Jameson
and D. Penny, Journal of Molecular Evolution, 2000, 51, 416.
11 S.L. Miller and L. Orgel, The Origins of Life on the Earth, Prentice
Hall, New Jersey, 1974.
12 J.L. Bada, Earth and Planetary Science Letters, 1994, 226, 1.
8 | Journal Name, [year], [vol], 00–00
13 M. Blackman and N.D. Lisgarten, Proceedings of the Royal Society
A, 1957, 239, 93.
14 A. Kouchi and T. Kuroda, Nature, 1990, 344, 134.
15 H. E. Stanley, MRS Bulletin, 1999, 24, 22.
16 D.M. Anderson and A. Banin, Origins of Life and Evolution of
Biospheres, 1975, 6, 23; D.M. Anderson, Life in the Universe, J.
Billigham ed. MIT press, Cambridge, Massachusetts, 1981.
17 W. Zheng, D. Jewitt and R.I. Kaiser, Journal of Physical Chemistry
A, 2009, 113, 11174.
18 M. G. Kivelson, K. K. Khurana, C.T. Russell, M. Volwerk, R. J.
Walker and C. Zimmer, Science, 2000, 289, 1340.
19 B.E. Schmidt, D.D. Blankenship, G.W. Patterson and P.M. Schenk,
Nature, 2011, 479, 502.
20 T. B. McCord, G.B. Hansen, F.P. Fanale, R.W. Carlson, D.L.
Matson, T.V. Johnson, W.D. Smythe, J.K. Crowley, P.D. Martin, A.
Ocampo, C.A. Hibbitts, J.C. Granahan and the NIMS team, Science,
1998, 280, 1242.
21 G.M. Marion, C.H. Fritsen, H. Eicken and M.C. Payne, Astrobiology,
2003, 3, 785.
22 G. Mitri, A.P. Showman, J.I. Lunine and R.M.C. Lopes, Icarus,
2008, 196, 216.
23 R. Shapiro, D. Schulze-Makuch, Astrobiology, 2009, 9, 1.
24 S. Pilling, D.P. Andrade, A.C. Neto, R. Rittner, Journal of Physical
Chemistry, 2009, 113, 11161.
25 C.P. McKay and H. Smith, Icarus, 2005, 178, 214; R.S. Oremland
and M.A. Voytek, Astrobiology, 2008, 8, 45.
26 D. Schulze-Makuch and L.N. Irwin, Eos Transactions, American
Geophysical union, 2001, 82, 150; C.F. Chyba and C. B. Phillips,
Proceedings of the National Academy of Science USA, 2001, 98,
801.
27 K. Zahnle, L. Schaeffer and B. Fegley, Cold Spring Harbor
Perspectives in Biology, 2010, 2, a004895; N.H. Sleep, K.J. Zahnle
and P.S. Neuhoff, Proceedings of the National Academy of Science
USA, 2001, 98, 3666; E.G. Nisbet and N.H. Sleep, Nature, 409, 1083.
28 C.F. Chyba, P. J. Thomas, L. Brookshaw and C. Sagan, Science,
1990, 249, 366.
29 H. Trinks, W. Shröder, C.K. Biebricher, Origins of life and Evolution
of Biospheres, 2005, 35, 429.
30 H. Eicken, J. Kolatschek, J. Freitag, F. Lindemann, H. Kassens and I.
Dmitrenko, Geophysical Research Letters, 2002, 27, 1919.
31 V.K. Bronshteyn and A.A. Chernov, Journal of Crystal Growth,
1991, 112, 129.
32 J. Drzymala, Z. Sadowski, L. Holysz and E. Chibowski, Journal of
Colloid and Interface Science, 1999, 220, 229.
33 P.A. Monnard and H. Ziock, Chemistry and Biodiversity, 2008, 5,
1521.
34 J.P. Dworkin, A. Lazcano and S. Miller, Journal of Theoretical
Biology, 2003, 222, 127.
35 B.E.H. Maden, Trends in Biochemical Sciences, 1995, 20, 337; A.
Eschenmoser, Angewandte Chemie, 1988, 27, 5.
36 S.A. Benner, A. D. Ellington and A. Tauer, Proceedings of the
National Academy of Science USA, 1989, 86, 7054.
37 J.F. Kasting and L. Brown, Origins of Life and Evolution of
Biospheres, 1996, 26, 219; R. Stribling and S. Miller, Origins of Life
and Evolution of Biospheres, 1986, 17, 261.
38 O.R. Norton, The Cambridge Encyclopedia of Meteorites,
Cambridge: Cambridge University Press, 2002.
39 R. Hayatsu, Science, 1964, 146, 1291.
40 R. Hayatsu, M.H. Studier, L.P. Moore and E. Anders, Geochimica et
Cosmochimica Acta, 1975, 39, 471.
41 Z. Martins, O. Botta, M.L. Fogel, M.A. Sephton, D.P. Glavin, J.S.
Watson, J.P. Dworkin, A. W. Schwartz and P. Ehrenfreund, Earth
and Planetary Science Letters, 2008, 270, 130.
42 M.P. Callahan, K.E. Smith, H.J. Cleaves, J. Ruzicka, J.C. Stern, D.P.
Glavin, C.H. House and J.P. Dworkin, Proceedings of the National
Academy of Sciences USA, 2011,
43 J. Oró and A.P. Kimball, Archives of Biochemistry and Biophysics,
1961, 94, 217.
This journal is © The Royal Society of Chemistry [year]
44 L. E. Orgel, Origins of Life and Evolution of Biospheres, 2001, 34,
361.
45 R. Hayatsu and E. Anders, Topics in Current Chemistry, 1981, 99, 137.
46 A.W. Schwartz and G.J.F. Chittenden, Biosystems, 1977, 9, 87-92.
47 R.A.Sanchez, J. Ferris and L.E. Orgel, Journal of Molecular Biology
1967, 30, 223.
48 C.N. Matthews and R.D. Minard, Faraday Discussions, 2006, 133,
393.
49 D.E. Shemansky, A.I.F. Stewart, R. West, L. W. Esposito, J.T.
Harlett and X. Liu, Science, 2005, 308, 978.
50 S. Miyakawa, H.J. Cleaves and S. Miller, Origins of Life and
Evolution of Biospheres, 2002, 32, 209; M Ruiz-Bermejo, J.L. de la
Fuente, C. Rogero, C. Menor-Salvan, S. Osuna-Esteban and J.A.
Martin-Gago, Chemistry and Biodiversity, 2012, 9, 25.
51 R.A. Sanchez, J.P. Ferris and L.E. Orgel, Journal of Molecular
Biology, 1967, 30, 223.
52 R. Saladino, C. Crestini, F. Ciciriello, G. Constanzo and E. Di
Mauro, Chemistry and Biodiversity, 4, 694; S. Yuasa, D. Flory, B.
Basile and J. Oró, Journal of Molecular Evolution, 1984, 21, 76; J.
Oró and P. Kimball, Archives of Biochemistry and Biophysics, 96,
293.
53 G.F. Joyce, Nature, 1989, 338, 217.
54 A.W. Schwartz, A.B. Voet and M. Veen, Origins of Life and
Evolution of Biospheres, 1984, 14, 91.
55 A.B. Voet and A. W. Schwartz, Bioorganic Chemistry, 1983, 12, 8.
56 G. Zubay and T. Muy, Origins of Life and Evolution of Biospheres,
2001, 31, 87; R.A. Sanchez, J.P. Ferris and L.E. Orgel, Journal of
Molecular Biology, 1968, 38, 121.
57 L.E. Orgel, Critical Reviews in Biochemistry and Molecular Biology,
2004, 39, 99.
58 R. Saladino, C. Crestini, V. Neri, F. Ciciriello, G. Constanzo and E.
Di Mauro, ChemBioChem, 2006, 7, 1707; H.L. Barks, R. Buckley,
G. A. Grieves, E. Di Mauro, N.V. Hud and T.M. Orlando,
ChemBioChem, 2010, 11, 1240.
59 R.A. Sanchez, J. Ferris and L.E. Orgel, Science, 1966, 153, 72.
60 F. Raulin, P. Brunston, P. Paillous and R. Sternberg, Advanced Space
Research, 1995, 15, 321.
61 A.W. Schwartz, H. Joosten and A.B. Voet, BioSystems, 1982, 15,191.
62 S. Miyakawa, H.J. Cleaves and S.L. Miller, Origins of Life and
Evolution of Biospheres, 2002, 32, 209.
63 A.B. Voet and A. W. Schwartz, Origins of Life and Evolution of
Biospheres, 1981, 12, 45.
64 M. Levy, S. L. Miller, K. Brinton and J.L. Bada. Icarus, 2000, 145,
609.
65 J.P. Ferris, P.C. Joshi, E.H. Edelson and J.G. Lawless, Journal of
Molecular Evolution, 1978, 11, 293; J. Oró and Kamat, Nature, 1961,
190, 442.
66 R.A. Sanchez, J.P. Ferris and L.E. Orgel, Science, 1966, 154, 784.
67 B.N. Tran, J.C. Joseph, M. Force, R. C. Briggs, V. Vuitton and J.P.
Ferris, Icarus, 2005, 177, 106.
68 B.E. Turner, The Astrophysical Journal, 1971, 163, L35; W.J.
Lafferty and F. J. Lovas, Journal of Physical and Chemical
Reference Data, 1978, 7, 441; M. Morris, B.E. Turner, P. Palmer and
B. Zuckerman, The Astrophysical Journal, 1976,0 205, 82; Y.
Osamura, K. Fukuzawa, R. Terzieva and E. Herbst, The
Astrophysical Journal, 1999, 519, 697.
69 P.S. Monks, P.N. Romani, F.L. Nesbitt, M. Scanlon and L.J. Stief,
Journal of Geophysical Research, 1993, 171, 15; D.W. Clarke and
J.P. Ferris, Icarus, 1995, 115, 119.
70 R.K. Khanna, Icarus, 2005, 178, 165.
71 J.P. Ferris, R.A. Sanchez and L.E. Orgel, Journal of Molecular
Biology, 1968, 33, 693.
72 M. Nuevo, J. H. Bredehöft, U. J. Meierhenrich, L. d'Hendecourt and
W. H.-P. Thiemann. Astrobiology, 2010, 10, 245.
73 K. E. Nelson, M.P. Robertson, M. Levy and S.L. Miller, Origins of
Life and Evolution of Biospheres, 2001, 31, 221.
This journal is © The Royal Society of Chemistry [year]
74 H.J. Cleaves, K.E. Nelson and S.L. Miller, Naturwissenschaften,
2006, 93, 228.
75 A. Benidar, J.C. Guillemin, O. Mo and M. Yañez, Journal of
Physical Chemistry A, 2995, 109, 4705; H. Möllendal, B. Khater and
J.C. Guillemin, Journal of Physical Chemistry A, 2007, 111, 1259.
76 R. Shapiro, Proceedings of National Academy of Science USA, 1999,
96, 4396.
77 M. Levy, S.L. Miller, K. Brinton and J. L. Bada, Icarus, 145, 609.
78 C. Menor-Salvan, M. Ruiz-Bermejo, M.I. Guzman, S. Osuna-Esteban
and S. Veintemillas, Chemistry European Journal, 2009, 15, 4411.
79 J. Hong, D.G. Kim, C. Cheong and K.J. Paeng, Microchemical
Journal, 2001, 68, 173.
80 G. K. Mittapalli, K. R. Reddy, H. Xiong, O. Muñoz, B. Han, F. De
Riccardis, R. Krishnamurthy and A. Eschenmoser, Angewandte
Chemie, 2007, 119, 2522; M. Hysell, J.S. Siegel and Y. Tor, Organic
and Biomolecular Chemistry, 2005, 3, 2946.
81 Z. Martins, O. Botta, M. L. Fogel, M. A. Sephton, D. P. Glavin, J. S.
Watson, J. P. Dworkin, A. W. Schwartz and P. Ehrenfreund, Earth
and Planetary Science Letters, 2008, 270, 130.
82 Y. Hayashi, Y. Katsumoto, S. Omori, N. Kishii and A. Yasuda,
Journal of Physical Chemistry A, 2007, 111, 1076.
83 A.K. Soper, E.W. Castner and A. Luzar, Biophysical Chemistry,
2003, 105, 649.
84 Y. Kameda, M. Sasaki, S. Hino, Y. Amo and T. Usuki, 2006, Bulletin
of the Chemical Society of Japan, 2006, 79, 1367; G. Grdadolnik and
Y. Marechal, Journal of Molecular Structure, 2002, 615, 177.
85 M. I. Guzmán, L. Hildebrandt, A.J. Colussi and M.R. Hoffmann,
Journal of American Chemical Society, 2006, 110, 931.
86 R.H. Stokes, Journal of Physical Chemistry, 1965, 69, 4012.
87 P.M. Schaber, J. Colson, S. Higgins, D. Thielen, B. Anspach and J.
Brauer, Thermochimica Acta, 2004, 424, 131.
88 I.M. Lagoja and P. Herdewijn, Chemistry and Biodiversity, 2007, 4,
818.
89 J.L. de la Fuente, M. Ruiz-Bermejo, C. Menor-Salvan, and S. OsunaEsteban, Journal of Thermal Analysis and Calorimetry, 2012,
DOI: 10.1007/s10973-011-2141-1
90 C. Menor-Salvan, M. Ruiz-Bermejo, S. Osuna-Esteban, G. MuñozCaro and S. Veintemillas, Chemistry and Biodiversity, 2008, 5, 2729.
91 E.H. Wilson , S.K. Atreya and A. Coustenis, Journal of Geophysical
Research, 2003, 108, 1.
92 D. Cremer, R. Crehuet and J. Anglada, Journal of the American
Chemical Society, 2001, 123, 6127.
93 E. Ware, Chemical Reviews, 1950, 46, 403.
94 G.A. Infante, P. Jirathana, E.J. Fendler and J. H. Fendler, Journal of
the Chemical Society, Faraday Transactions 1, 1974, 70, 1162.
95 A. S. Subbaraman, Z. A. Kazi, A.S.U. Choughuley and M.S. Chadha,
Origins of Life and Evolution of Biospheres, 1980, 10, 343.
96 Z. Guennoun, N. Pietri, I. Couturier-Tamburelli and J.P. Aycard,
Chemical Physics, 2004, 300, 23.
97 M.D. Brasier, R. Matthewman, S. McMahon and D. Wacey,
Astrobiology, 2010, 11, 725; N. Lahav and S. Chang, Journal of
Molecular Evolution, 19, 36.
98 W. Gilbert, Nature, 1986, 319; G.F. Joyce, Nature, 2002, 418, 214.
99 S.A. Benner, A.D. Ellington and A. Tauer, Proceedings of the
National Academy of Science USA, 1989, 86, 7054.
100 C. Anastasi, F.F. Buchet, M. Crowe, A.L. Parkes, M.W. Powner, J.
M. Smith and J. D. Sutherland, Chemistry and Biodiversity, 2007, 4,
721.
101 S.M. Gryaznov and R.L. Letsinger, Journal of American Chemical
Society, 1993, 115, 3808.
102 A. Kanavarioti, P.A. Monnard and D. W. Deamer, Astrobiology,
2001, 1, 271.
103 R. Stribling and S.L. Miller, Journal of Molecular Evolution, 1991,
32, 282.
104 W.G. Scott, Current Opinion in Chemical Biology, 1999, 3, 705.
105 P.A. Monnard, A. Kanavarioti and D.W. Deamer, Journal of the
American Chemical Society, 2003, 125, 13734.
Journal Name, [year], [vol], 00–00 | 9
106 P.A. Monnard and J.C. Szostak, Journal of Inorganic Biochemistry,
2008, 102, 1104.
107 H. Trinks, W. Shröder and C.K. Biebricher, Origins of Life and
Evolution of Biospheres, 2005, 35, 429.
108 X. Sun, J.M. Li and R.M. Wartell, RNA, 2007, 13, 2277.
109 J. Hulshof and C. Ponnamperuma, Origins of Life and Evolution of
Biospheres, 1976, 7, 197.
110 C. Huber and G. Watchtershauser, Science, 1998, 281, 670.
111 Mita et al International Journal of Astrobiology, 2005.
112 E. Imai, H. Honda, K. Hatori, A. Brack and K. Matsuno, Science,
1999, 283, 831; Y. Ogata, E. Imai, H. Honda, K. Hatori and K.
Matsuno, Origins of Life and Evolution of the Biosphere ,2000, 30,
527.
113 J. Bujdák and B. M. Rode, Origins of Life and Evolution of
Biospheres, 1999, 29, 451.
114 M.G. Schwendinger and B.M. Rode, Analitycal Sciences, 1989, 5,
411.
115 S.W. Fox and H.J. Harada, Journal of the American Chemical
Society, 1960, 82, 3745.
116 B.M. Rode, Peptides, 1999, 20, 773.
117 T. Vajda, Cellular and Molecular Life Sciences, 1999, 56, 398.
118 C. P.Ivanov and Slavcheva, Origins of Life and Evolution of the
Biosphere, 1977, 8, 13.
119 J.L. Bada, G.D. McDonald, Icarus, 1995, 114, 139.
120 R. Liu and L. E. Orgel, Origins of Life and Evolution of the
Biosphere, 1998¸28, 47.
121 P. Greenstein and M. Winitz, Chemistry of the amino acids Vol.1,
John Wiley & Sons, New York, 1961.
122 T. Vajda, Cryo-Letters, 1996, 17, 295–302; T. Vajda, G. Szókán.
and M. Hollo´ si, 1998, Journal of Peptide Science, 4, 300.
123 T. Vajda, Cell and Molecular Life Sciences, 1999, 56, 398.
SCHEME CAPTIONS
Scheme 1 Synthesis of purines by polymerisation of cyanide to HCN
tetramer and formation of cyanoimidazole derivatives. The related
formation of glycine, formamidine and glycolonitrile, were observed
in ice-water experiments.
Scheme 2 Cyanoacetylene as a precursor for pyrimidines. The reaction of
cyanoacetylene with urea or ammonium cyanate yields cytosine,
whose deamination leads to uracil. The reaction with guanidine
directly forms 2,4-diaminopyrimidine. Is possible that reaction goes
through a pentanedinitrile intermediate.
Scheme 3 Urea as precursor of nitrogen heterocycles. Possible pathways
to pyrimidines, hydantoins and triazines in frozen urea solution under
a methane/nitrogen atmosphere.
Scheme 4 Polycyclic aromatic hydrocarbons and acetylene polymers
detected from sparking a methane/nitrogen atmosphere on water-ice
matrix.
10 | Journal Name, [year], [vol], 00–00
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