Proton resonance detection and identification of intermediates in the pyridoxal catalyzed dehydration of serine and serine-indole synthesis of tryptophan
Reiney, Carolyn Gerhardt
Gansow, Otto A.
Master of Arts
In an investigation of a proposed mechanism for pyridoxal catalyzed serine dehydration and serine-indole synthesis of tryptophan, pmr spectra of solutions containing 0.05M pyridoxal, serine, tryptophari, and/or indole in the presence and absence of 0.025M Zn(II) or Al(III) throughout the pD range 1-13 have been studied. Formation of the aldimine Schiff bases, N-pyridoxylidene-serine and N-pyridoxylidenetryptophan and their metal complexes are readily observed. Formation of Schiff bases of metal-free systems were also observed by ultraviolet-visible spectroscopy. Compared to the metal-free systems, the metal ion has, due to complex formation, the effect of increasing the extent of aldimine formation at any pD below 13. The pD dependencies of chemical shifts in the metal-free and metal-containing systems have been interpreted in terms of formation of free aldimine and its mono and bis complexes, and of several acid-base equilibria of these species. Exchange between free and coordinated aldimine in the Zn(II) system is fast on the pmr time scale and slow in the Al(III) system. Solute structures for the free aldimine and the complexes are proposed. The abnormally high field chemical shift of the azomethine proton in the N-pyridoxylidenetryptophan Schiff base, in the presence and absence of metal ion, has been explained in terms of the shielding effect of the indole ring current. Based on the pmr results, a highly constrained hydrogen bonded structure for the aldimine metal complex has been proposed. The bis(N-pyridoxylideneserinato) and bis(N7pyridoxylidenetryptophan) aluminum(III) complexes are demonstrated to exist as three diastereomers which are distinguishable from the pmr chemical shifts of pyridoxylidene 2-CH3 resonances. Exchange of the alpha-proton of the amino acid resulting in racemization occurs with N-pyridoxylidenetryptophan in the metal-free and aluminum-containing systems.