Research Signpost
37/661 (2), Fort P.O.
Trivandrum-695 023
Kerala, India
Topical Series in Health Science 1 (TSHS-1), 2013: 27-37 ISBN: 978-81-308-0521-4
Editors: Francis Agyemang-Yeboah, Henry Asare-Anane and Sylvester Yaw Oppong
3. Caffeine: The wonder compound,
chemistry and properties
1
Francis Agyemang-Yeboah1 and Sylvester Yaw Oppong2
Department of Molecular Medicine, School of Medical Science, College of Health
Sciences, Kwame Nkrumah University of Science and Technology, Kumasi
Ghana; 2Head, Department of Chemical Pathology, University of
Ghana Medical School, P.O. Box 4236, Accra, Ghana
Abstract. Caffeine (C8H10N4O2) is a natural product from several
plants. It is chemically called trimethylxanthine. It is also known as
theine, mateine, guaranine, or methyltheobromine. Its natural
sources include coffee beans, guarana, cocoa, tea etc. It occurs as a
white powder with a bitter taste but odourless. It has a density of
1.2g/cm3 with a volatility of 0.5% and a vapour pressure of 101kPa
at 178oC. It has an almost a neutral pH of 6.9 and a water solubility
of 2.17%. Caffeine has a boiling point of 178oC and a melting
point of 238oC. It sublimes at its boiling point. Several studies have
shown that caffeine exhibits profound metabolic, physiologic and
neurologic effects. Various review studies on caffeine have also
highlighted that caffeine exhibits diverse analytical, mechanical
and chemical properties either in-vivo or in-vitro. For example, it
has been shown that by the formation of co-crystal of caffeine and
other chemicals like methyl gallate, the mechanical properties of
caffeine can be improved. Again, caffeine can effectively scavenge
hydroxyl radicals via the “Fenton reaction” suggesting that a
caffeine-derived oxygen-centred radical is formed in the reaction
of caffeine with •OH radical which perhaps provides a biochemical
Correspondence/Reprint request: Dr. Francis Agyemang-Yeboah, Head of Department of Molecular Medicine
School of Medical Science, College of Health Sciences, Kwame Nkrumah University of Science and Technology
Kumasi, Ghana. E-mail: fayeboah@yahoo.co.uk
28
Francis Agyemang-Yeboah & Sylvester Yaw Oppong
basis for the understanding of the reported anticarcinogenic properties of caffeine. A
parallel stacking of caffeine (CAF) a poorly water soluble solute, with riboflavin
(RBF) in aqueous solutions showed that such a molecular stacking improves the
hydrotropic solubilization of CAF, possibly, by the phenomenon of molecular
hydrotropy.
List of abbreviation
CAF- Caffeine; IUPAC- International Units of Pure and Applied
Chemistry; LLE - liquid–liquid extraction; MDMA - 3,4-methylenedioxymethamphetamine; NMR - Nuclear Magnetic Resonance; NNHC - N,Nheterocyclic carbine; RBF- riboflavin; ROS- reactive oxidative specie;
SERS - Surface Enhanced Raman Spectroscopic.
Background introduction and physical properties of caffeine
Caffeine is a chemical compound which is naturally found in plant
sources including coffee, cocoa, tea, cola nuts, guarana etc. Chemically, it
can be classified as belonging to the heterocyclic group of compounds called
the purines. It has a chemical formula; C8H10N4O2. Its IUPAC(International
Units of Pure and Applied Chemistry) name is 3,7-dihydro-1,3,7-trimethyl-1Hpurine-2,6-dione and a common name of Trimethylxanthine. Caffeine can also
be classified as an alkaloid since it occurs as a metabolite of nitrogen
metabolism. It has a molecular weight of approximately 194.2 g/mol. It is
soluble both in water and organic solvents such as alcohol and chloroform.
Interestingly, caffeine can be obtained from its natural sources by extraction,
however, it can also be synthesized from uric acid as a precursor. In its purified
state, it occurs as a white powder with a bitter taste but odourless. Caffeine has
a boiling point of 178oC and a melting point of 238oC. It sublimes at its boiling
point. It has a density of 1.2g/cm3 with a volatility of 0.5% and a vapour
pressure of 101kPa at 178oC. It has an almost a neutral pH of 6.9 and a water
solubility of 2.17%. It was first isolated and purified by the German Chemist
Friedrich Ferdinand Runge in 1819 (Weinberg & Bealer 2001). Its molecular
structure and space-filling model is shown in Figure 1 and 2.
Analytical properties of caffeine
By employing a proton and carbon nuclear magnetic resonance (NMR)
spectroscopy and a spiking method to extract and quantify the chemical
components of coffee from roasted coffee bean extract (RCBE), caffeine
was confirmed as a coffee component and isolated as a caffeine-chlorogenate
Caffeine: The wonder compound, chemistry and properties
29
Figure 1. Caffeine Molecular Structure. Source: A LoopingIcon, Wikipedia Commons.
Figure 2. Space-filling model of caffeine. Source: A LoopingIcon, Wikipedia Commons.
complex (Wei et. al; 2011). Caffeine has also been found to be contained in
green tea seeds of Camellia sinensis (Hasegawa et. al., 2011). Using X-Ray
crystallographic analysis on equimolecular suspension of (-)-gallocatechin-3O-gallate (GCg) and caffeine, it was also shown that the resulting complex
was capable of showing π-π interactions between the A, B' rings of GCg and
the two six-membered rings of caffeine. This threw more light on the
stereochemical structure and intermolecular interaction of caffeine (Tsutsumi
et. al; 2011). Studies of the N-heterocyclic carbene Pt(II) complexes from
caffeine has further shown that the complex has an opposite stereochemistry
and a shorter Pt-C(carbene) bond compared to that of an analogous
benzimidazole-derived N,N-heterocyclic carbene (NNHC) Pt complex,
suggesting a lower trans influence of pyridyl N, compared to cyclometallated
carbon and an increased Pt-NHC π-backbonding in caffeine (Hu et. al; 2011).
A study done in Glasgow, employing whole coffee fruit extracts and
30
Francis Agyemang-Yeboah & Sylvester Yaw Oppong
subsequent characterization of its caffeine components showed the presence
of caffeoylquinic acids, feruloylquinic acids, dicaffeoylquinic acids,
caffeoylferuloylquinic acids etc. along with a methyl ester of 5-caffeoylquinic
acid. These extracts also demonstrated some antioxidant activity (Mullen
et. al; 2011). A pH dependent Raman spectroscopic study of caffeine showed
that the pH value can dramatically affect the surface enhanced Raman
spectroscopic (SERS) properties of caffeine, but barely affect the normal
Raman spectrum of caffeine aqueous solution. The study concluded that such
characteristics can essentially affect the reorientation of caffeine molecule to
a Ag colloid surface, but cannot impact the vibration of functional groups and
chemical bonds in caffeine molecule (Kang et. al; 2011). A parallel stacking
of caffeine (CAF) a poorly water soluble solute, with riboflavin (RBF) in
aqueous solutions showed that such a molecular stacking improves the
hydrotropic solubilization of CAF, possibly, by the phenomenon of molecular
hydrotropy whereby the presence of a large amount of a solute perhaps
improves the solubility of a component (Cui, 2010). The study demonstrates
that CAF and RBF undergo molecular parallel stacking in the aqueous
solution and that such a stacking are found to be both structural and dynamic
under a given condition suggesting that perhaps the self-stacking of CAF is
the primary effect, and incorporation of RBF is the secondary effect.
Mechanical, chemical, solubility and other properties of
caffeine
Studies have clearly demonstrated that by the formation of co-crystal of
caffeine and other chemicals like methyl gallate, the mechanical properties of
caffeine can be improved dramatically (Sun and Hou, 2008), (Figure 3). In a
study in which bulk co-crystal was prepared by suspending caffeine and
methyl gallate in ethanol, it was observed that within an appropriate compact
pressure range, the tablet tensile strength of the co-crystal improved to about
twice the original. Figure 4 shows an excellent agreement between
experimental spectra patterns of (a) caffeine and (b) 1:1 cocrystal. The study
proposed that the poor tablet tensile strength before co-crystallization was
associated with high elastic recovery and low plasticity and that the good
plasticity and tabletability of the co-crystal validated the selection criterion,
i.e., the presence of slip planes in the crystal structure (Sun and Hou, 2008),
(Figure 3 and 5). This offers the potential of engineering crystals of fine
chemicals for superior physico-mechanical properties (Sun and Grant, 2001),
(Reddy et. al; 2005). This may be useful in pharmaceutical and drug discovery
31
Caffeine: The wonder compound, chemistry and properties
Figure 3. Crystal structure of caffeine-methyl gallate co-crystal. Broken lines indicate
hydrogen bonds. The flat molecule layers correspond to crystallographic planes.
Source: Calvin Sun et al (2008). Improving Mechanical Properties of Caffeine and
Methyl Gallate Crystals by Cocrystallization. Copyright permission: "Reprinted with
permission from American Chemical Society”.
Table 1. Some caffeine sources and content.
Source/Product
Caffeine
Content
Source/Product
Caffeine
Content
Cocoa(Powder)
0.1-0.5 %
Black Tea
85-370mg/l
Chocolate(Bitter)
875mg/Kg
Green Tea
100-210mg/Kg
17mg/Kg
Chocolate(Milk)
Cocoa
Source/Product
Caffeine
Content
Coffee(Brewed)
130-680mg/l
85mg/l
Coffee(Decaf)
13-30mg/l
White Tea
68mg/l
Coffee(Instant)
130-400mg/l
Decaf Tea
17mg/l
Coffee(Espresso)
3400mg/l
where the phenomenon of co-crystallization can be employed to modify
physical properties of drugs while maintaining their pharmacological
activities.
In a study to compare the solubilities of caffeine in supercritical carbon
dioxide at different temperatures and over a pressure range with xanthine
compounds of similar chemical structures (e.g. theophylline and
theobromine), it was observed that although, the chemical structures of the
xanthines are very similar, their solubilities in supercritical carbon dioxide vary
substantially. The solubilities of caffeine in CO2 are one order of magnitude
32
Francis Agyemang-Yeboah & Sylvester Yaw Oppong
Figure 4. Excellent agreement between experimental the calculated PXRD patterns of
(a) caffeine (form II) and (b) 1:1 cocrystal. Source: Calvin Sun et al (2008).
Improving Mechanical Properties of Caffeine and Methyl Gallate Crystals by
Cocrystallization. Copyright permission: "Reprinted with permission from American
Chemical Society”.
Caffeine: The wonder compound, chemistry and properties
33
Figure 5. DSC thermograms of caffeine, methyl gallate, and their 1:1 cocrystal.
Source: Calvin Sun et al (2008). Improving Mechanical Properties of Caffeine and
Methyl Gallate Crystals by Cocrystallization. Copyright permission: "Reprinted with
permission from American Chemical Society”.
higher than those of theophylline and two orders of magnitude higher than
those of theobromine (Johannsen and Brunner, 1994).
In another study to elucidate the luminescence emission properties of
caffeine, theophylline was determined in a mixture with caffeine by roomtemperature luminescence. It was observed that, the excitation energy
absorbed by the theophylline was transferred to europium (III), which then
emitted its characteristic luminescence. The study concluded that the
enhanced luminescence emission intensity was quantitatively related to
theophylline concentration without interference from caffeine or emission
from the sample matrix (Perry and Winefordner, 1990).
Studies have also been carried out to investigate some of the chemical
reactions of caffeine to ascertain whether or not such reactions pose any toxic
risks. A reaction of caffeine (1,3,7-trimethylxanthine) with the reactive
oxidative specie (ROS), the hydroxyl radical (·OH), was investigated by
electron spin resonance (ESR) spin trapping (Shi et al; 1990). It was
observed that, the ·OH was generated by the Fenton reaction (Fe2+ + H2O2) as
well as by the reaction of chromium(V) with H2O2 and that caffeine
effectively scavenges ·OH suggesting a caffeine-derived oxygen-centered
34
Francis Agyemang-Yeboah & Sylvester Yaw Oppong
radical is formed in the reaction of caffeine with ·OH. This perhaps provides
a biochemical basis for the understanding of the reported anti-carcinogenic
properties of caffeine.
Trace determination and effects of caffeine in fluids, tissues,
and surface water samples
A novel autoanalyzer for sequential determination of traces of caffeine in
fluids like soft drinks have been developed (Lucena et. al; 2004). The
application of the multi-parametric autoanalyzer is based on the on-line
coupling of a continuous solid-phase extraction unit with two detectors in
series: UV–vis and evaporative light scattering (ELSD) detectors. The study
observed that, that caffeine is selectively retained on the sorbent column,
allowing the elution of the other components in the effluent. Caffeine was
subsequently eluted with an acetonitrile stream. The study proposed that the
auto-analyzer is a useful tool for the determination of macro-components like
caffeine on account of its high productivity-related analytical properties.
In a related development, a High Performance Liquid Chromatography
(HPLC) method which enables an easy and cost-effective means of
preparing gram quantities of epigallocatechin gallate from green tea
(Camellia sinensis) has also been employed (Copeland et. al; 1998). In this
technique, a decaffeinated aqueous brew of commercial green tea was treated
with caffeine. The resultant precipitate was re-dissolved after decaffeination
with chloroform and further purified by solvent partition with ethyl
hexanoate and propyl acetate. A yield in the region of 400 mg and 80% purity
of epigallocatechin gallate was obtained.
A method based on liquid–liquid extraction (LLE) coupled to reverse
phase liquid chromatography and atmospheric pressure, chemical ionization
mass spectrometry has been applied to determine trace amounts of caffeine in
surface water samples (Gardinali and Zhao, 2002). The rationale behind the
development of such a method was to evaluate the use of unmetabolized
caffeine as a potential dissolved phase tracer of human waste contamination.
The method allowed for the determination of caffeine at levels as low as
4.0 ng/l (ppt) in both salt and freshwater.
The tissue enhancement effect of ultrasound on permeation of caffeine
(CAF) through hairless mouse skin in vitro has been studied and compared
with other chemical enhancers (Monti et. al; 2001). The study revealed that
compared with that of other enhancers, high-frequency ultrasound
enhancement of caffeine on transdermal flux was not statistically significant,
while low frequency produced a small but significant increase of the
Caffeine: The wonder compound, chemistry and properties
35
enhancement factor. The effect of ultrasound on caffeine permeation, however,
was lower than that produced by chemical enhancers like oleyl alcohol.
Summary
Caffeine is a chemical compound which is naturally found in plant
sources including coffee, cocoa, tea, cola nuts, guarana etc.
Chemically, caffeine can be classified as belonging to the heterocyclic
group of compounds called the purines.
Caffeine has a chemical formula; C8H10N4O2.
pH- dependent Raman spectroscopic of caffeine can affect its surface
enhanced Raman spectroscopic (SERS) properties
Parallel stacking of caffeine (CAF) with riboflavin (RBF) in aqueous
solutions improves its hydrotropic solubilization by the phenomenon of
molecular hydrotropy
Caffeine can effectively scavenge hydroxyl radicals via the “Fenton
reaction”
A parallel stacking of caffeine a poorly water soluble solute, with
riboflavin in aqueous solutions improves the hydrotropic solubilization of
caffeine possibly by the phenomenon of molecular hydrotropy.
Multi-parametric autoanalyzers has been used for the determination of
macro-components like caffeine on account of its high productivityrelated analytical properties
References
1.
2.
3.
4.
5.
Copeland, E.L., Clifford, M.N. and Williams, C.M. (1998). Preparation of (–)epigallocatechin gallate from commercial green tea by caffeine precipitation and
solvent partition. Food Chemistry:61:81-87.
Cui, Y. (2010) Parallel stacking of caffeine with riboflavin in aqueous solutions:
the potential mechanism for hydrotropic solubilization of riboflavin. Int J
Pharm:397:36-43.
Gardinali, P.R. and Zhao, X. (2002). Trace determination of caffeine in surface
water samples byliquid chromatography–atmospheric pressure chemical
ionization–mass spectrometry (LC–APCI–MS). Environmental International:28:
521-528.
Hasegawa, T., and Akutsu, K., Kishi, Y., Nakamura K. (2011).Constituents of the
green tea seeds of Camellia sinensis. Nat Prod Commun:6:371-4.
Hu, J.J., and Bai, S.Q., Yeh, H.H., Young, D.J., Chi, Y. and Hors, T.S. (2011). Nheterocyclic carbene Pt(II) complexes from caffeine: synthesis, structures and
photoluminescent properties. Dalton Trans:40:4402-6.
36
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
Francis Agyemang-Yeboah & Sylvester Yaw Oppong
Johannsen, M. and Brunner, G (1994). Solubilities of the xanthines caffeine,
theophylline and theobromine in supercritical carbon dioxide. Fluid Phase
Equilibria:95: 215-226.
Kang, J., and Gu, H., Zhong, L., Hu, Y., and Liu, F. (2011).The pH dependent
Raman spectroscopic study of caffeine. Spectrochim Acta Mol Biomol Spectro
Sc.;78:757-62.
Lucena, R., Cárdenas, S., Gallego, M. and Valcárcel, M. (2004). Continuous flow
autoanalyzer for the sequential determination of total sugars, colorant and
caffeine contents in soft drinks. Analytica Chimica:530:283-289.
Ming, Z., and Lautt, W.W. (2010). Caffeine-induced natriuresis and diuresis via
blockade ofhepatic adenosine-mediated sensory nerves and a hepatorenal reflex.
Can J Physiol Pharmcol: 88(11):1115-21.
Monti, D., Giannelli, R., P. Chetoni, P. and Burgalassi, S. (2001). Comparison of
the effect of ultrasound and of chemical enhancers on transdermal permeation of
caffeine and morphine through hairless mouse skin in vitro. International Journal
of Pharmaceutics:229:131-137.
Mullen, W., and Nemzer, B., Ou, B., Stalmach, A., Hunter, J., Clifford, M.N.,
and Combet, E. (2011). The antioxidant and chlorogenic acid profiles of whole
coffee fruits are influenced by the extraction procedures. J Agric Food
Chem:59:3754-62.
Perry, L.M. and Winefordner, J.D. (1990). Selective determination of
theophylline in the presence of caffeine by sensitized luminescence of
europium(III). Talanta:37:965-969
Reddy, C., Basavoju, M. S., and Desiraju, G. R. (2005). Sorting of polymorphs
based on mechanical properties. Trimorphs of 6-chloro-2,4-dinitroaniline Chem.
Commun.:19: 2439- 2441.
Shakeel, F., and Faisal, M.S. (2010). Caffeine: a potential complexing agent for
solubility and dissolution enhancement of celecoxib. Pharm Biol:48:113-5.
Shi, X., Dalal, N.S. and Jain, A.C. (1990). Antioxidant behaviour of caffeine:
Efficient scavenging of hydroxyl radicals. Food & Chemical Toxicology:29:1-6
Seifert, J.G., and Nelson, A., Devonish, J., Burke, E.R., and Stohs, S.J. (2011).
Effect of acuteadministration of an herbal preparation on blood pressure and
heart rate in humans. Int J Med Sci :8:192-7
Snel, J., and Lorist, M.M. (2011). Effects of caffeine on sleep and cognition. Proq
Brain Res:2011;190:105-17.
Sun, C. C., and Grant, D. J. W. (2001). Influence of crystal structure on the
tableting properties of sulfamerazine polymorphs. Pharm. Res.:1:274-280.
Sun, C.C. and Hou, H. (2008). Improving Mechanical Properties of Caffeine and
Methyl Gallate Crystals by Cocrystallization. Cryst. Growth Des.: 8:1575–1579.
Tsutsumi, H., and Sato, T., Ishizu, T.(2011).Stereochemical structure and
intermolecularinteraction of complexes of (-)-Gallocatechin-3-O-gallate and
caffeine. Chem Pharm Bull (Tokyo):59:100-5.
Caffeine: The wonder compound, chemistry and properties
37
21. Wei, F., and Furihata, K., Hu, F., Miyakawa, T., Tanokura, M.(2011). TwoDimensional (1)H-(13)C Nuclear Magnetic Resonance (NMR)-Based
Comprehensive Analysis of Roasted Coffee Bean Extract. J Agric Food Chem.
17:9065-73.
22. Weinberg, B.A., and Bealer, B.K. (2001). The world of caffeine. New York &
London: Routledge. ISBN 0-415-92722-6.