Alloisoleucine racemization

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. 

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. 

Fig. 5.38 Epimerization (E) and racemization (R) of isoleucine (major pathway is epimerization of L-isoleucine and D-alloisoleucine large arrow indicates biogenic input).

At equilibrium, the concentration of alloisoleucine does not equal that of isoleucine (58). The rate expression for Equation 13 is similar to that given in Equation 9 except k (D-amino acid) is replaced by k (D-allo-isoleucine). Since the solutions initially contained only L-isoleucine, when the amount of racemization is small, the term k (D-alloisoleu-cine) can be neglected in the rate expression and in this case the integrated rate equation is... 

In order to calculate amino acid residence times from Equation 21, the ratio of the d to l enantiomers of the various amino acids are required as a function of depth in the oceanic water column. Unfortunately, there have been no investigations of the amino acid enantiomers dissolved in any natural waters. The analyses are difficult because most d- and l-amino acids are not separable by the usual amino acid analytical techniques. One exception is isoleucine, which forms alloisoleucine when it racemizes (Equation 13). Isoleucine and alloisoleucine are separable on the buffered columns of the automatic amino acid analyzer (88). However, as can be seen from Table V, only very small amounts of alloisoleucine would be produced from the racemization of isoleucine, unless... 

Recently, I have been doing some preliminary investigations of the alloisoleucine/isoleucine ratio in a sample taken from a depth of 2500 meters in the Atlantic Ocean. After isolating the amino acids by a procedure similar to that used by Chau and Riley (2), the sample was analyzed for the presence of alloisoleucine on the Beckman-Spinco automatic amino acid analyzer. The results indicate that a small amount of alloisoleucine appears to be present in the sample. It is impossible to make any conclusions from this one experiment, however, since the analysis of a blank which had been carried through the same isolation steps as the sea water sample contained a significant amount of isoleucine. Also, several dipeptides appear at about the same location as does alloisoleucine on the chromatogram from the automatic amino acid analyzer. Many samples from the worlds oceans will have to be analyzed before it can be determined whether the racemization of amino acids can be used to calculate amino acid residence times in the sea. 

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