REACTION-KINETICS OF METAL-ION CATALYZED BETA-PHENYLSERINE-PYRIDOXAL MODEL SYSTEM

Both β-elimination and dealdolation reactions were found to occur in the vitamin B-6 catalyzed reactions of β-phenylserine. The products of the reactions were found to consist of benzaldehyde, pyridoxylideneglycine (or glycine and pyridox-al), and phenylpyruvic acid. The reaction rate constants for the parallel elimination and dealdolation reactions in D2O were determined by proton NMR over pD ranges of 4-10 for metal-free systems and 3-10 for metal-catalyzed systems. The observed (overall) rate constants in the metal-free systems for the disappearance of phenylserine are [formula omitted] at neutral pD. For the dealdolation and elimination reactions the rate constants varied from [formula omitted] respectively, at neutral pD to [formula omitted] at higher pD. Further rate enhancement in the presence of metal ions at low pD was 60-fold for Al(III) and 50-fold for Zn(II), while at high pD it was only 2.3-fold for Al(III) and 1.4-fold for Zn(II). The ratios of dealdolation to β elimination are about 2:1 in metal-free systems and 4:1 and 5:1 in Al(III) and Zn(II) systems, respectively. The significance of these results relative to proposed reaction mechanisms is discussed. © 1978, American Chemical Society. All rights reserved.