A Kinetic Study of a Zn2+-catalyzed Transamination Reaction between Pyridoxamine Analogs with a Pyridinophane Structure and α-Keto Acids

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Abstract
The kinetics of the nonenzymatic transamination reaction from pyridoxamine analogs with a pyridinophane structure to α-keto acids catalyzed by Zn2+ were investigated by monitoring the changes in the absorption spectra in methanol. It was found that these reactions obeyed first-order kinetics for the formation of the Zn2+ chelate of an aldimine. No appreciable change in the reaction rate was observed when the concentration of the α-keto acid was increased, indicating that the isomerization of the ketimine chelate to the aldimine chelate is the ratedetermining step. There was a considerable enhancement of the reaction rate when the molar ratio of the zinc ion to the pyridoxamine analogs was reduced from 1/1 to 0.5/1. The reaction rates corresponding to the various α-keto acids employed in the presence of an amount of Zn2+ equimolar to the pyridoxamine analog decreased in this order; phenylpyruvic acid>pyruvic acid≈4-methyl-2-oxopentanoic acid>3-methyl-2-oxobutanoic acid, whereas those in the presence of half-molar equivalents of Zn2+ to the pyridoxamine analog decreased in this order; pyruvic acid>phenylpyruvic acid ≈4-methyl-2-oxopentanoic acid>3-methyl-2-oxobutanoic acid. Furthermore, the use of the pyridoxamine analog with a linear “ansa” bridge resulted in a larger reaction rate than the use of the one with a branched “ansa” bridge. The solvent isotope effect is also described.

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