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

Abstract

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°, 6N 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 (s̀*) and steric hindrance in the vicinity of the α-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 α-lactalbumin and β-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 α-lactalbumin hydrolysis, 10% inversion of methionine was measured as compared with less than 0.5% in the case of β-lactoglobulin. We attributed the particular behaviour of methionine in α-lactalbumin to the neighbourhood of a cysteine residue on the carboxyl side.