Pyridoxal and pyridoxal 5′-phosphate catalyzed β-deuteration of α-aminobutyric acid and homoserine. I. Metal-free systems

As the initial step in the investigation of metal ion- and vitamin B6-catalyzed, β, γ-elimination in α-amino acids, the pyridoxal-and pyridoxal 5′-phosphate-catalyzed β-deuteration of α-aminobutyric acid and homoserine in D2O in metal-free systems has been studied by nuclear magnetic resonance. The reaction kinetics of α-H, β-H, and 4′-CH exchange rates were measured by following the rates of deuteration of these positions. The kinetic data obtained for these reactions lead to a proposed mechanism for β-deuteration. The relative rates of the deuteration reactions observed, and the order in which deuteration products were obtained are: α-H exchange > transamination (4′-CH exchange) > β-H exchange. The observed rates of β-proton exchange exhibit specific base catalysis and are dependent on the activation of the α-hydrogen of the amino acid moiety in the aldimine, and on the activation of the β-hydrogens of the amino acids in the ketimine.