Absolute Asymmetric Photoreactions of Aliphatic Amino Acids
A R T I C L E S
which is defined as the normalized difference in molecular
absorption coefficients between optical isomers toward l- and
r-CPL at a given wavelength:
In this comprehensive study on the photoreactions of aliphatic
amino acids such as glycine (Gly), alanine (Ala), valine (Val),
leucine (Leu), and isoleucine (Ile), we have identified all major
photoproducts, determined their yields, and clarified the pho-
todecomposition mechanisms at acidic and neutral pHs. On this
basis, we elucidate the relationship between the process resulting
in enantiomeric enrichment by CPL irradiation (AAP) and the
photodecomposition mechanism. Consequently, this report aims
to contribute to the understanding of the photochemistry of
amino acids and also to give insight into how the origin of
homochirality in biomolecules may have occurred by CPL
irradiation.
g ) |ꢀl - ꢀr|/ꢀ ) ∆ꢀ/ꢀ
where ꢀ ) (ꢀl + ꢀr)/2 and 0 e g < 2.9,12,14,21-23 It has been
reported that enantiomeric enrichment of some amino acids can
be experimentally obtained by CPL irradiation.24-26 However,
a detailed understanding of the reaction mechanism operating
in the AAP of amino acids has yet to be achieved. In our recent
paper,24 we reported the pH dependency of the obtained optical
purity (op) of racemic leucine (rac-Leu) upon irradiation by l-
and r-CPL. The CPL is generated by a polarizing undulator
installed in an electron storage ring, NIJI II; this is the most
suitable CPL source for the UV and VUV regions.9,27 The AAP
of aliphatic amino acids having a γ-hydrogen, such as Leu,
proceeds via γ-hydrogen abstraction by the ester carbonyl group
(Norrish type II mechanism) in acidic solution, while at neutral
or alkaline pHs, no op is observed.24 At that time, the
photodecomposition products required to explain the details of
photodecomposition of aliphatic amino acids at pHs 7 and 11
had not be identified. We speculated that some and/or all parts
of decomposition of Leu at pHs 7 and 11 proceeded via achiral
thermal process(es) and/or via indirect photochemical process-
(es). However, irrespective of the solution pH, if a g factor exists
and the reaction proceeds via the excited state (governed by
the g factor), AAP with CPL should occur. Consequently, we
have undertaken to identify the photodecomposition products
with the aim to clarify the detailed nature of the mechanism
occurring at pH 7.
Since the early part of the past century, several photodecom-
position mechanisms have been proposed, but the correct
pathway(s) have yet to be unambiguously determined.28,29 It has
been thought that photodeamination is the main mechanism in
the case of various amino acids.28-32 It has been reported that
NH3 and glycolic acid (1h) are photoproducts of glycine
(Gly), and NH3 and lactic acid (2h) are photoproducts of
alanine.28,29,31-35 However, methane, carbon dioxide, and other
small molecules have also been reported as photodecomposition
products.28 In the cases of Val, Leu, and Ile, NH3 has been
detected, but other, as-yet unidentified and unexplained products
have also been detected. Hence, it would be premature to say
that the photodecomposition mechanism of amino acids has been
clarified in detail.24,28,29
Experimental Section
Materials. Gly (1, Wako), rac-Ala (2, Wako), rac-Val (3, Wako),
rac-Leu (4, Wako), rac-Ile (5, Wako), D-Ala (Wako), L-Ala (Wako),
D-Leu (Wako), L-Leu (Wako), rac-aspartic acid (Wako), rac-threonine
(Wako), rac-serine (Wako), rac-glutamic acid (Wako), rac-lysine
(Aldrich), rac-histidine (Aldrich), rac-arginine (Wako), ammonia
(Wako), methylamine (1a, Wako), ethylamine (2a, Wako), isobutyl-
amine (3a, Aldrich), isoamylamine (4a, Aldrich), 2-methylbutylamine
(5a, Aldrich), formic acid (Wako), acetic acid (1c, Wako), propionic
acid (2c, Wako), isovaleric acid (3c, Aldrich), 4-methylvaleric acid (4c,
Aldrich), 3-methylvaleric acid (5c, Aldrich), isobutyric acid (Wako),
2-methylbutyric acid (Wako), 2-chloropropionic acid (Aldrich), acrylic
acid (1e, Aldrich), 3,3-dimethylacrylic acid (3e, Aldrich), 4-methyl-2-
pentenoic acid (4e, TCI), glycolic acid (1h, Aldrich), lactic acid (2h,
Aldrich), 2-hydroxy-3-methylbutyric acid (3h, Aldrich), 2-hydroxy-
isocaproic acid (4h, Aldrich), pyruvic acid (2k, Aldrich), 2-oxo-4-
methylvaleric acid (4k, Pfaltz & Bauer), (S)-sec-butylamine (Aldrich),
(S)-2-methylbutyric acid (Aldrich), (R)-lactic acid solution (R-2h,
Aldrich), (S)-lactic acid solution (S-2h, Aldrich), (S)-2-hydroxyisoca-
proic acid (S-4h, Aldrich), isoamyl alcohol (Wako), isobutanol (Wako),
isobutyraldehyde (Aldrich), 2-methylbutyraldehyde (TCI), 2-methyl-
propene (Aldrich), and 2,2,3,3-tetramethylbutane (TCI) were used as
received. In this paper, amino acids are numbered sequentially as R-CH-
(NH2)COOH, 1 (R ) H); 2 (R ) CH3); 3 (R ) i-Pr); 4 (R ) i-Bu); 5
(R ) sec-Bu), while the relevant carboxylic acids, hydroxycarboxylic
acids, and ketocarboxylic acids derived therefrom are identified by
suffixes a, h, and k, respectively.
In most cases, the acidic solutions of amino acids were prepared
with 0.1 or 0.01 M standard HCl solution, the alkaline solutions were
prepared with 0.1 M standard NaOH solution, and the neutral solutions
(pH 7) was prepared with distillated water. The solutions of 4a and
S-4h were prepared with 1 M standard HCl solution, 0.1 M standard
HCl solution, distillated water, and 1 M standard NaOH solution. For
CPL and linearly polarized light (LPL) irradiation, the concentrations
of amino acids were adjusted to be 5.01-5.78 mM, except for Ala
solutions for CPL irradiation (11.6 mM) and Gly (11.7-12.7 mM).
The pH of the solutions was measured using Horiba pH meter F-12 at
24.2-24.5 °C. The measured pHs deviated only slightly from the formal
values indicated in the text: Leu, pH 1 ) 1.00, pH 2 ) 2.00, pH 3 )
3.02, pH 7 ) 6.74, pH 11 ) 10.92; Ala, pH 1 ) 1.00, pH 2 ) 2.01,
pH 3 ) 3.01, pH 7 ) 6.76, pH 11 ) 10.91; Gly, pH 1 ) 1.01, pH 7
) 6.89; Val, pH 1 ) 1.03, pH 7 ) 6.91; Ile, pH 1 ) 1.03, pH 7 )
6.90.
(21) Rau, H. Chem. ReV. 1983, 83, 535.
(22) Kuhn, W. Trans. Faraday Soc. 1930, 26, 293.
(23) Kuhn, W.; Knopf, E. Z. Phys. Chem., Abt. B 1930, 7, 291.
(24) Nishino, H.; Kosaka, A.; Hembury, G. A.; Shitomi, H.; Onuki, H.; Inoue,
Y. Org. Lett. 2001, 3, 921.
(25) Flores, J. J.; Bonner, W. A.; Massey, G. A. J. Am. Chem. Soc. 1977, 99,
3622.
(26) Norden, B. Nature 1977, 266, 567.
(27) Yuri, M.; Yagi, K.; Yamada, T.; Onuki, H. J. Electron Spectrosc. Methods
1996, 80, 425.
(28) Schaich, K. M. CRC Crit. ReV. Food Sci. Nutr. 1980, 13, 189.
(29) Mclaren, A. D.; Shugar, D. Photochemistry of Proteins and Nucleic Acids;
Pergamon Press Inc: Oxford, 1964.
Photolysis. Each solution (2 mL) of amino acid was melt-sealed in
a rectangular quartz cell (1 cm × 1 cm × 4 cm) under an argon
atmosphere (for volatile product analysis, the rectangular quartz cell
was capped with a Teflon/silicon rubber stopper) and was irradiated
with the CPL and LPL generated with the Onuki-type polarizing
undulator installed in the electron storage ring NIJI II at the Metrology
Institute (former name, Electrotechnical Laboratory).27
(30) Johns, R. B.; Looney, F. D.; Whelan, D. J. Biochim. Biophys. Acta 1967,
147, 369.
(31) Weizmann, C.; Bergmann, E.; Hirsheberg, Y. J. Am. Chem. Soc. 1936, 58,
1675.
(32) Weizmann, C.; Hirsheberg, Y.; Bergmann, E. J. Am. Chem. Soc. 1938, 60,
1799.
(33) Mandel, I.; McLaren, A. D. J. Am. Chem. Soc. 1951, 73, 1826.
(34) Kolomiichenko, M. A. Ukr. Biokhim. Zh. 1968, 40, 57.
(35) Lapinskaya, E. M.; Khenokh, M. A. Zh. EVol. Biokhim. Fiziol. 1971, 7,
14.
Analysis. The circular dichroism (CD) spectra were measured using
JASCO 720 WI and 725 CD spectrometers. The UV spectra were
9
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