Racemization of amino acids

Racemization is the process by which one enantiomer converts to the other. In normal circumstances, organic compounds which may exhibit optical activity (such as simple sugars) exist in solution as equal numbers of d- and l- forms. 

There is now considerable interest in the relationship between the preservation of collagen and the rate of racemization of its constituent amino acids, and further data may be expected to clarify these problems. 

Because at ambient temperatures the racemization rates of all amino acids are slow, it is usually found that aspartic acid is most useful archaeologically, but over much longer geological timescales aspartic acid may become racemic 

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In 1983, Yamada et al. developed an efficient method for the racemization of amino acids using a catalytic amount of an aliphatic or an aromatic aldehyde [50]. This method has been used in the D KR of amino acids. Figure 4.25 shows the mechanism of the racemization of a carboxylic acid derivative catalyzed by pyridoxal. Racemization takes place through the formation of Schiff-base intermediates. 

Figure 4.25 Racemization of amino acids through formation of Schiff-base intermediate.

Ohtani B, Kawaguchi J, Kozawa M, Nishimoto S, Inui T, Izawa K (1995) Photocatalytic racemization of amino acids in aqueous polycrystaUine cadmium(ll) sulfide dispersions. J Chem Soc Faraday Trans 91 1103-1109... 

There is, however, a drawback to this method of dating. The half-life (see Textbox 14) of the racemization process is greatly affected by temperature it is shorter at higher temperatures and slows down as the temperature decreases. An uncertainty of 2°C in the temperature history of dead remains can lead to an error of about 50% in the age determined. For the racemization of amino acids to be applicable for dating, therefore, it is crucial that the temperature history of the environment where the amino acids (that is, skin or bone) have been deposited should be well known. 

O Connell T. C., R. E. M. Hedges, and G. J. van Klinken (1997), An improved method for measuring racemization of amino acids from archaeological bone collagen, Ancient Biomolec. 1, 215-220. 

P.E. Hare, P.H. Abelson, Racemization of Amino Acids in Fossil Shells, Carnegie Institution of Washington Year Book, 66, 526 528 (1967). 

J.L. Bada, Racemization of Amino Acids in Nature, Interdisciplinary Science Reviews, 7(1), 30 46 (1982). 

There have also been reports [36, 37] that racemization of amino acids occurs more rapidly using MW heating than conventional heating at the same temperature. Chen et al. [36] observed that racemization of amino acids in acetic acid the presence of benzaldehyde was accelerated by MW heating. Lubec et al. [37] reported that some D-proline and ris-4-hydroxy-D-proline were found in samples of infant milk formula when they were heated in a MW oven. On the other hand, conventionally heated samples did not contain these unnatural D-amino acids. This report caused concern, and received media attention because D-proline is neurotoxic and suggested that MW heating of some foods could have deleterious effects on their nutritional value and the health of the consumer. 

D/L enantiomers increases with time. This racemization of amino acids is used for amino acid racemization dating (see Pollard and Heron 1996, 271 301). 

J Kovac, GL Mayers, RH Johnson, RE Cover, UR Ghatak. Racemization of amino acid derivatives. Rate of racemization and peptide bond formation of cysteine active esters. J Org Chem 35, 1810, 1970. 

M Dzieduszycka, M Smulkowski, E Taschner. Racemization of amino acid residue penultimate to the C-terminal one during activation of N-protected peptides, in H Hanson, HD Jakubke, eds. Peptides 1972. North-Holland, Amsterdam, 1973, pp 103-107. 

The effect of temperature on the rate of racemization of amino acids in fossils was investigated and the implications of the findings on fossil dating were analyzed313. The high rate of conversion of L-aspartic acid into its D-isomer, observed in uncontaminated bone samples taken from catacombs in Rome (IV century BC) was attributed to collagen decomposition due to the humidity of the catacombs314. 

Another possible mechanism for the racemization of amino acid esters involves the in situ, transient, formation of Schiff s bases by reaction of the amine group of an amino acid ester with an aldehyde. Using this approach, DKR of the methyl esters of proline 5 and pipecolic acid 6 was achieved using lipase A from C. ant-arclica as the enantioselective hydrolytic enzyme and acetaldehyde as the racemiz-ing agent (Scheme 2.4). Interestingly, the acetaldehyde was released in situ from vinyl butanoate, which acted as the acyl donor, in the presence of triethylamine. The use of other reaction additives was also investigated. Yields of up to 97% and up to 97% e.e. were obtained [6]. 

Polymers containing chiral groups are useful for resolving racemic mixtures into the individual enantiomers [Kiniwa et al., 1987 Mathur et al., 1980 Wulff et al., 1980]. For example, the copper(II) complex of XXXXIII (either the R- or S-enantiomer) resolves racemates of amino acids [Sugden et al., 1980], The separation is based on the formation of a pair of diastereomeric complexes from the reaction of the polymer reagent with the two enantiomers. One of the enantiomers is complexed more strongly than the other and this achieves separation of the enantiomers. 

Excessive heat due to roasting can cause racemization of amino acid residues (25). Most amino acids are only available in the L form. Consequently, complete racemization could be equivalent to a 50% decrease in availability for the residues affected. 

Generally, the enhanced risk of racemization of amino acid chlorides, due to their known susceptibility to cyclization into oxazolones, as well as the instability of urethane-protected derivatives easily convertible into the corresponding /V-carboxy anhydrides, 1081 has resulted in very limited application of this technique in depsipeptide chemistry in spite of the successful examples discussed. 

The electron-withdrawing effect of the metal ion causes considerable enhancement of the acidity of the a C—H bond of coordinated amino acids. The most obvious illustration of this effect is that most (but not all, vide infra) metal ions increase the rates of racemization of amino acids. With Co111 complexes it has been clearly demonstrated that the rate of racemization is similar to the rate of H-D exchange at the a C atom (conveniently followed by1H NMR), results which are interpreted in terms of a symmetrical carbanion intermediate (equation 14). 

It may also be surprising how easily this racemization may occur. Friedman and Liardon (126) studied the racemization kinetics for various amino acid residues in alkali-treated soybean proteins. They report that the racemization of serine, when exposed to 0.1M NaOH at 75°C, is nearly complete after just 60 minutes. However, caution must be used when examining apparent racemization rates for protein-bound amino acids. Liardon et al. (127) have also reported that the classic acid hydrolysis, employed to liberate constituent amino acids, causes amino acids to racemize to various degrees. This will necessarily result in D-isomer determinations that are biased high. Widely applicable correction factors are not possible since the racemization behavior of free amino acids is different from that of amino acid residues in proteins (which can be further affected by sequence). Of course, this is not a problem for free amino acid isomer determinations since the acid hydrolysis is unnecessary. Liardon et al. also describe an isotopic labeling/mass spectrometric method for determining true racemization rates unbiased by the acid hydrolysis. For an extensive and excellent review of the nutritional implications of the racemization of amino acids in foods, the reader is directed to a review article written by Man and Bada (128). 

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