A PROTON RESONANCE INVESTIGATION OF EQUILIBRIA, SOLUTE STRUCTURES, AND TRANSAMINATION IN AQUEOUS SYSTEMS PYRIDOXAMINE-PYRUVATE-ZINC(2) AND -ALUMINUM(3)

As part of a proton resonance investigation of the mechanism of nonenzymatic transamination, the pmr spectra of systems initially containing 0.1 M pyridoxamine and 0.1 M pyruvate in the presence and absence of 0.05 M Zn(II) and Al(III) in D2O solution at pD 1-13 have been examined. Formation of the ketimine Schiff base, pyruvidene-N-pyridoxamine, in the metal-free system and complexes containing one and two complexed ketimine species are observed. Both free and complexed ketimine undergo H-D exchange at the pyruvidene methyl carbon. The presence of metal ions effects greater stabilization of ketimine over a wider pD range, due to complexation, compared to the metal-free system. Exchange between coordinated and free ketimine in the Zn(II) system is fast on the pmr time scale and slow in the Al(III) system. Certain of the acid-base equilibria of free ketimine and the Zn(II) and Al(III) complexes have been investigated using the pD dependencies of chemical shifts. The Zn(II) and Al(III) ketimine complexes convert at rates which qualitatively increase with increasing pD to new species which have been definitely identified by their characteristic pD dependencies of chemical shifts to be the corresponding aldimine complexes produced by transamination. The results of this study together with those of a previous investigation of pyridoxal-alanine-Zn(II) and -Al(III) systems serve to provide further definition of the course of nonenzymatic transamination and, therewith, offer strong confirmatory evidence for the proposed mechanism of this reaction. © 1969, American Chemical Society. All rights reserved.