Kinetic and thermodynamic study of the reaction of pyridoxal 5′-phosphate with L-tryptophan

The apparent rate constants for formation (k1) and hydrolysis (k2) of the Schiff bases formed by reaction of pyridoxal 5′-phosphate with L-tryptophan were determined at various pH values, at different temperatures and at constant ionic strength (0.1 M). Also obtained were the elementary rate constants for formation and hydrolysis of the Schiff bases corresponding to the different chemical species present in the media, and the pK values of the Schiff's bases. The activation and thermodynamic parameters for the formation and hydrolysis of the Schiff's bases also were determined. Some of the ΔH0 and ΔS0 values for the individual processes were found to be positive. In basic media the enthalpic factor is unfavorable but the entropic contribution leads to a negative ΔG0. Copyright © 2004 John Wiley & Sons, Ltd.