1.Indimlnrr. Sci., Jan.-Fcb. 2001, 81, 3142.
0 lndian Instihlte of science
.
,
i
Supramolecular assmhtkm
design. Case studies with hydr
S~ANUBHATTACHARYA
apsmnuU of organic cbcmisay. Indian Institutc ofSEicncc. Bangdore 5 . 9 012, snd Chemical Biology Unit,
Jawsbarlnl N+ Centre for Advanad Soirotific m a r c h , Bpngdore 560 012 India.
tmail:sb@ngcbem.5iac.met.in;Phone: 9 1 - 8 0 - 3 ~ 6 f 4Pax:91-80-3310653
Ldminc is able to
1. ~ ~ u c i i o n
Amphiphilic molecules comprise a polar headgroup region that likes solvents like water and a
tail-lie region that does not. W h e n suspended in water these molecules self-organize into
thermodynamically stable aggregates that range from micelles, bilayers or a number of more
complex self-assembled entities.' Another class of materials formed as a sequel of intra- or
inter-molecular self-assembly is known as gels.'.
Gels are found EI both natural and manmade sources. For instance, 'Jell-0, a popular food, is a good example of gel. htoplasm and
animal flesh are other examples of gel. It i s rather strange that despiti gels W i g so commonplace, investigations relating the.physical inf&atiou on theii mole&~I&S d c w are rather
h i t & . Several reasons could h& attributed to this. Por instance, gelatoh of water aie usually
large molecules (i.e. protehs and polymers) whose complicated intermolrpdakassociations are
:often
difticult . to
~ . decipher. In addition,such interactions are generally not sfatic'but cin change
.. . . .
(often inevwibly) with time, t h e m stimuli or mechanical s k s . Moreover, miterials in the
be stu&+y
atomic reiolution X-ray diffraction analysis,'the main'sonrceof
gel-state &t
our most precise
structursl
infomition.
.
.
. .
It has been recently realized that certain low molecular mass, nonpolymeric molecules also
posw.s..+e capaoity to gelate specific solvents.c13 A few of this class of g@-forming moleN$!are $so ampbiphilic. Since gels derived from such low molecular mass system are non@yt@c and.cou1d o+n be synthesized byapplyinggrinciples of self-assembly, they represent atfnrctive systems for materials design. ConsequeNy, de.nbvo niol&ulai. design
of such
.
systems is attracting a lot of current attention.
'Text of lcchue delivcnd,?t Ihc Annual Pwulty Meeting of the Jawaharlal N e b Cent+ for Advanced Scientific Research at Bangdm duriog Novambcr ZMX).
SANTANU BHATTACHARYA
38
Due to our interest in the design of novel self-assembling molecular entities, we embarked
upon a program of synthesizing supramolecular aggregates, which also possess the capacity to
retain large volume of solvents. My presentation here is rather succinct as most of the work is
already published. Interested readex can get debails fmm the reference scction pertaining to Our
publications in this area for experimental and other information.
2. Background
We have been intrigued by protein lipidation in cellular surfaces, a phenomenon, which is reNonlipidated
sponsible for sustaining the activity of several membrane-associated
s @e o@n of
proteins are cytosolic, and hence biologically inactive. To w
at molecular level, we have been systematically examiniq tbe self-a&embly
acid amides of several natural amino acids.
hothex phenomenon, which is commonly observed on cellular surfaces, involves carbohydrate-based modifications (glycosylation) at the N-temiinns of proteins. These modified
residues along with glycolipid assemblies have been implicated in diverse intercellular
recognition events.15 However, it is often difficult to isolate these lipids in pure form from
natural sources. Therefore, it has been considered important to synthesize and examine c h d cally pure sugar-based amphiphiles that self-assemble in suitable solvents to produce membrkous organizations. The self-organizimg properties are strongly dependent on the nfolecnlar
'
structures of such lipids. This is particularly relevant because naturally oc
sylated lipids cannot form stable bilayer membranes by themselves.
supporting lipid is usually added to the glycolipid in order t 0 ~ p r o d ~ : s t a bMnyer
le
vesic l e ~ . ' ~S~e~v' d sugar-derived amphiphiles are also
attention rr&ently due ui their
significant immunorndulatory and antincuplastic
.,,
.
.~
3. Ourwork
During the studies undeaaken in my laboratory, it was found that a hydmphobically modified
amino acid derivative, N-undecynoyl-Gserine, (1) formed fibrous mic~struclufes and gel
when dissolved in chloioform or other related chlorinated
corresponding histidine amphiphile, (2) did not, however,
chlorinated hy&ocarbons. It was also found that
fatty acyl derivatives of Lserine such as undecenoyl
similar gelation of the chlorinated hydrocarbons i
amino acid at the headgroup in such 'mphiphil
we synthesized the corresponding dimeric anal
...
1
.
.
.,
2
,. .
HYDRCUELS ANDORGANffiEL3
39
N-undccynoyl-Lserine derivative. This derivative did not produce gels in the above solvents
although the same system when Buspehdedin aqumus media produced tubular aggregates.
range of lipopbilic derivatives of Lphenylalanine. Of
Subsequently, we synthesi
these, only the n-hexadecylamide of N-benyloxycarbon 1 Lphenyl alanine possessed the capacity to effectively @late a numtoer d organic so1vd.J;~ b judicious variation of the protecting p u p around the Lphenyl alanine moiety of this derivative it w s revealed that the
presence of chirality, w t h m and amide linkages was essential €or e f k t i v e gelation.=
Amphipbilk sugar derivatives have been interesting due to their significance in areas of
The sugar-linked amphiphiles
self-assembly and molecular recognition in biological ~ystems.2~
are also often produced on a commercial scale and its starting materials are obtained from r e
newable raw materials.” The aldopyranose rings at the head group level of these molecules
have multiple hydroxyl.groups with defmed orientation unliie their acyclic analogues. Consequently, the formation of strong cooperative hydrogen bonding networks between the amphiphiles is feasible. Spontaneous association of these amphiphiles is thus anticipated in water as
a result of cooperating noncovalent forces, i.e. hydrogen bonds among sugar moieties and hydrophobic association between the long hydrocarbon chains. The pronounced chiral character
of the headgroup should also help generate novel nonspheroidal mirrOst~~ctures.~
To develop novel water-gelating molecules, four single- and two double-chained disaccbaride amphiphiles (4-9) were synthesized% and their hydrogel-forming behaviour was extenaivfly investigated. These amphlphiles were based on maltose and lactose. Since the gels
fodned from some of these systems showed the ability to ‘trap’ water molecules upon gelation,
they were described as ‘hydrogels’. When these gels were heated to -70°C, the samples turned
into Clear, isotropic fluids and upon gradual cooling, the hydrogels could be reproduced. Thus
these systems were also ‘themo-reversible’.The low molecular mass &iW 565) of the gelators
compand to that of a typical polymeric gellator-forming substance implies pronounced aggm
SANTANU BHATTACHARYA
40
gation of the disaccharide amphiphiles intn larger micr~struchlresduring gelationdn cmier to
discern the aggregate textum and mwphdogjrs, the specimen bydrogel sanplss wcao examined by high-resolution scanning electton microscopy (SEM). SEM studies confirm
ence of fibrous structures in such hydmgels.
The possible reason8 for the exceptionally high water-gelating capacities (> 6ooo molecules of water per gelator molecule) exhibited by these. N-alkyl disaccharide amphiphiles being
the presence of large interlamellar spaces into which presumably the water molecules get enChain
trapped due to surface tension. In contrast to their single-chain counterparts, the
lactosyl and maltosylamme amphiphiles upon solubilization in EtOH-H20 don3
ogels,
with reduced mechanical strengths. Interestingly, the corresponding microstructures were
found to be quite different from the corresponding hydrogels of their single-chain counteparts.
Rheological studies provided further insights into the viscoelastic behavior of these hjrdrogels.
Varying the chain length of the alcohol cosolvent could modulate the gelation capacities, melting temperatures and the mechanical properties of these hydrogels.
@*F<*ecH
, 3$
8
(*cI@31)
9
Interestingly, variation in the conditions of the preparations of aqueous suspensions of the
above compounds also afforded vesicular and related giant microstructure^?^ These aggregates
maintained inner aqueous compartments as revealed from the dye entrapment studies. Some of
these suspensions also manifested the ability to effectively complex transition metal ions.
Most recently, in o w laboratory, a small biocompatible organic molecule hss been shown
for the first time lo cause gelation of oil and organic solvents in preference to water.% The material, a simple amino acid derivative, N-lanroyl-Lalanine, could, with fuaher modificatiou,
ultimately be used in the control of oil and other organic spills.
As discussed earlier, countless low molecular mass-gelahg agents h v e been developed
for transforming liquid organic solvents into a gelatinous state. but these work preferentially
It has until now not been
only with a single compound, given a mixture of oil and
possible to gelate one component selmvely over another. This is even more pertinent when
one of those components is water.
The problem with gelathg any organic solvent mixed with water is that often hydrogen
bonding is required in order to form a gel network and water molecules simply compete for
domi~~anw
when such noncovalent interactions are imprtant, w e discovered, howe4er, that
the fatty acid-derived amino acid, N-lauroyl-Lalanine, could simply and very effectively gelate nonpolar organic solvents such as aromatic and aliphatic hydmmbons like hexane, benzene and petrol.
41
Scanning electron m i m p p h images of @li?
the existence of fibrous,entangled nkmww&m
effective it might be. w d e x p e r i m e m + . a - * s
possible to selectively gelate q&c
bath h
eme and toluene ,revealed
n flgurcd 'thatsince. the agent was 90
of solvents
be.
,
-
..
.
,
,
.,
fi-
but aimon BS the
ewlj belated leav'b8SerVation
~twe&wnrar fuundthat
we arewing uheating*&,oil. But we do emphasize that these results point t b w a y
to B novel means of wntrolling oil ,andother toxic spillage once they can twa& the -ya~rial
to
work passively or with an additive.
'ws
4. Conclusion
This is a brief account of our work on the design of gel-forming systems and on the systehtic
evaluation of their
chains in the ensemble.
..
AeLaowledgmmt.
I am thankful to my coworkers (the co-authors of references 20-22 and 26-28) for their hard
work which formed the basis'of these studiss. panial funding of this work-cemc from the
Swamajayanti Fellowship grant of the Department of Science a n d T c c h n o l o ~ ~ ~ ~ ~ $ iofi m e n
India.
*
,
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42
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