Isoleucine racemization

Table V. Isoleucine Racemization Dates on Total Hydrolysates of Fossil Chione from Interglacial Marine Terraces in San Diego County (20)...

The alleu/iso ratios for these latter two terrace exposures are close to the nonlinear portion of the kinetics plot (30). Therefore, it may not be possible to date absolutely older terraces in the San Diego area by the extent of isoleucine racemization in total hydrolysates of Chione. Because of the slower kinetics of isoleucine racemization in the protein... 

In the desulfurization of 3-substituted thiophenes several stereoisomers may be formed in certain cases. Both meso and racemic compounds have been obtained from the desulfurization of 3,4-diaryl-substituted thiophenes. It is claimed, however, that only meso, -diphenyladipic acid is obtained upon desulfurization of 3,4-di-phenyl-2,5-thiophenedicarboxylic acid and only di-isoleucin from 3-thienylglycine. The formation of small amounts of dimeric products in the desulfurization has been discussed with reference to the mechanism of this reaction. ... 

However, the use of a HPLC separation step enabled a remarkable acceleration of the deconvolution process. Instead of preparing all of the sublibraries, the c(Arg-Lys-O-Pro-O-P-Ala) library was fractionated on a semipreparative HPLC column and three fractions as shown in Fig. 3-2 were collected and subjected to amino acid analysis. According to the analysis, the least hydrophobic fraction, which eluted first, did not contain peptides that included valine, methionine, isoleucine, leucine, tyrosine, and phenylalanine residues and also did not exhibit any separation ability for the tested racemic amino acid derivatives (Table 3-1). 

Bada, J. L. (1972), Dating of bones using the racemization of isoleucine, Earth Planet. Sci. Lett. (July). 

J.L. Bada, The Dating of Fossil Bones using the Racemization of Isoleucine, Earth and Planetary Science Letters, 15, 223 231 (1972). 

Locquin has since obtained d-isoleucine from this racemic compound which was identical with Ehrlich s natural product, and this therefore has the above constitution. 

The numerous preparations of mono-, di-, tri-, and hexafluoro derivatives of valine, norvaline, leucine, norleucine, and isoleucine, using classical methods of amino acid chemistry (e.g., amination of an a-bromoacid, " azalactone, Strecker reaction, amidocarbonylation of a trifluoromethyl aldehyde, alkylation of a glycinate anion are not considered here. Pure enantiomers are generally obtained by enzymatic resolution of the racemate, chemical resolution, or asymmetric Strecker reaction. ... 

A novel technique for dating archaeological samples called amino acid racemiza-tion (AAR) is based on the stereochemistry of amino acids. Over time, the configuration at the a-carbon atom of a protein s amino acids is lost in a reaction that follows first-order kinetics. When the a carbon is the only chirality center, this process corresponds to racemization. For an amino acid with two chirality centers, changing the configuration of the a carbon from L to D gives a diastereomer. In the case of isoleucine, for example, the diastereomer is an amino acid not normally present in proteins, called alloisoleucine. 

While enantiomerically pure a-substituted isocyanoacetates have been used in Passerini condensation without significant racemization [4-6], the same class of compounds is believed to be configurationally unstable under the conditions of U-4CRs [7]. However, one notable exception is the reaction shown in Scheme 1.1, where L-isoleucine-derived isocyanide 2 has been condensed without such problems with pyrroline 1 [8]. The bulkiness of this isocyanide or the use of a preformed cyclic imine, thus avoiding the presence of free amine in solution, may be the reasons for the absence of racemization. 

Nutritional and Physiological Effects of Alkali-Treated Proteins. The first effect of the alkaline treatment of food proteins is a reduction in the nutritive value of the protein due to the decrease in (a) the availability of the essential amino acids chemically modified (cystine, lysine, isoleucine) and in (b) the digestibility of the protein because of the presence of cross-links (lysinoalanine, lanthionine, and ornithinoalanine) and of unnatural amino acids (ornithine, alloisoleucine, / -aminoalanine, and D-amino acids). The racemization reaction occurring during alkaline treatments has an effect on the nitrogen digestibility and the use of the amino acids involved. 

The catabolism of certain amino acids (e.g., valine, isoleucine, methionine) and odd-chain fatty acids (17 0) produces propionyl-CoA. Propionyl-CoA enters the TCA (citric acid) cycle following conversion to succinyl-CoA, as shown in Fig. 28-7. Propionyl-CoA is first carboxylated to produce D-methylmalonyl-CoA, which in turn is then racemized to L-methylmalonyl-CoA. In an intramolecular rearrangement reaction catalyzed by L-methylmalonyl-CoA mutase, a vitamin B12-... 

---