Isoleucine racemization results

Amino acids are racemized by concentrated acid (50, 51, 52) and base (52, 53, 54) at elevated temperatures, and some preliminary experiments have shown that at 116° aspartic acid is racemized slowly at neutral pH values (38, 55). Also, the amino acids in fossil shells are partially racemized, with the amounts of racemization increasing with the age of the shell (56, 57, 58) racemization is essentially complete in shells of Miocene age. Since the kinetics of racemization of amino acids have not been investigated in detail at any pH, I have recently carried out a detailed study of the kinetics of racemization of aspartic acid between pH 0 and 13 and also the kinetics of racemization of phenylalanine, alanine, and isoleucine at pH 7.6. The results of these investigations are reported herein. 

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. 

These results suggest that the slow racemization of amino acids in the sedimentary column can be used to estimate the sedimentation rate, and in turn the age, of both marine and fresh water sediments. In sediments from areas in the deep ocean where sedimentation rates are very slow (i.e., a few mm per 1000 years), the amount of racemization of isoleucine would be the easiest to determine since the investigations can... 

The aliphatic side chains in alanine and leucine have no major influence but branching at the ) -carbon atom in valine and isoleucine can enhance racemization because the combination of electron release and steric hindrance results in reduced coupling rates. The ensuing increase in the life-time of the reactive intermediate provides an extended opportunity for proton abstraction by base. It is obvious from these examples that the effect of individual side chains, the influence of various methods of coupling and the conditions of the peptide bond forming reaction (solvents, concentration, temperature, additives) must be studied in well designed experiments. Several model systems have been proposed for this purpose. 

The results are of significance in connection with the chemistry of bacitracin A, providing a better understanding of its racemization and loss of activity in acidic media. Furthermore, in a projected total synthesis of this material, racemization of the iV-terminal isoleucine residue cannot presumably be avoided during the thiazoline-ring closure, and pure bacitracin A must be isolated by a final fractionation of the diastereo-isomers. ... 

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