An NMR investigation of the metal ion and proton catalyzed reactions of some vitamin B-6 Schiff base systems

Abstract

The alpha and beta deuteration of the alpha amino acids valine and alpha aminobutyric acid is easily accomplished by heating D₂O solutions which are 1 M in the amino acid, 0.2 M in pyridoxal, and 0.1 M in aluminum(III) ion. The effects of the deuterium which is introduced make possible the direct inference of the sites being deuterated not only by changing the spin-spin coupling but also by introducing a slight shielding of protons vicinal to the substituted site. The extension of this technique to other amino acids and to peptides is discussed, and a mechanism which involves transamination of the aluminum(III) complexes of the aldimine Schiff base, to the ketimine complex, exchange of the beta protons of the ketimine, and transamination back to the aldimine form is presented. At pD 9, the alpha proton exchange is much more rapid than the beta exchange. This is attributed to the predominance of the pyridoxal, aluminum(III) catalyzed racemization reaction over transamination in this range. ...