Determination of the rate of acidic catalyzed racemization of protein amino acids

Determination of the enantiomeric composition of the amino acids of protein or peptide hydrolyzates by gas chromatography involves a hydrolytic step, in the course of which racemization may occur, leading to erroneously high values of D‐enantiomer. Racemization of both protein‐bound and free amino acids contribute to this formation of D‐enantiomers. In order to estimate the latter contribution, the time‐dependance of racemization for all protein amino acids during exposure to strong acid has been determined. The expected pseudo‐first‐order kinetic was confirmed in all cases. The rate constants are compared with those obtained for insulin and albumin and found to be of similar magnitude, except for the initial period of hydrolysis, when cleavage of peptide bonds is still incomplete. Most of the racemate generated during hydrolysis can be accounted for by a study of the time‐dependance of racemization and extrapolation to zero‐time. Racemization of the protein amino acids is decreasing in the following order: Asp = Cys, Pro, Glu, Met, His, Leu, Lys, Phe, Ala, Tyr, Arg, Trp, Val, Ile, Ser, Thr.