Racemization of free and protein-bound amino acids in strong mineral acid.

A new GC/MS technique was applied to measure the racemization of amino acids in strong mineral acid. The inversion rate constants of 15 free amino acids were determined under standard hydrolysis conditions (110 degrees, 6 N HCl in evacuated vacuum-sealed tubes). The variations in the inversion rate constants of all tested amino acids with the exception of serine and threonine were related to two factors: side-chain electron-withdrawing capacity ( sigma ) and steric hindrance in the vicinity of the alpha-hydrogen atom. The rate constants for serine and threonine were much lower than expected. Hydrolysis-induced racemization of protein-bound amino acids was investigated with alpha-lactalbumin and beta-lactoglobulin. Significant differences were observed as compared with the racemization rates of free amino acids. Such discrepancies were also observed between the two proteins. In the early stage of alpha-lactalbumin hydrolysis, 10% inversion of methionine was measured as compared with less than 0.5% in the case of beta-lactoglobulin. We attributed the particular behaviour of methionine in alpha-lactalbumin to the neighborhood of a cysteine residue on the carboxyl side.