Pyridoxamine and 2-Oxalopropionic Acid in Aqueous Systems: Conditional Imine Formation Constants and Rate Constants of Vitamin B6 Catalyzed Decarboxylation

Article abstract of DOI:10.1021/ja00341a026

The conditional Schiff base formation constants (K_c) of pyridoxamine (PM) and 2-oxalopropionic acid (OPA) were determined spectrophotometrically in basis aqueous solutions.These equilibrium constants exhibit pH dependence with maximum ketimine formation occuring near pH 9.0 (K_c = 9.6).The kinetics of vitamin B₆ catalyzed decarboxylation of OPA have been studied spectrophometrically, and the observed rate constant of decarboxylation (k_obsd) exhibited pH dependence with the maximum occuring at approximatly the pH corresponding to maximum ketimine formation (k_obsd = 15.6 * 10^-4 s^-1 at 1.01 * 10^-4 M OPA and 1.00 * 10^-5 M PM).Microscopic rate constants (k_SB) for the decarboxylation of the Schiff base species present in solution also showed pH dependence with the greatest catalytic activity at pH 7.0 (k_SB = 38.4 s^-1) and the least at pH 12.47 (k_SB = 0.2 s^-1).The relative magnitudes of the k_SB's determined are discussed in terms of the various protonated forms of the OPA:PM ketimine,and the corresponding pK_a's have been estimated from the rate constant-pH profiles.