Reactions between pyridoxal and L-amino acids in the absence and in the presence of zinc(II): The role of the metal in the prevention of Pictet-Spengler cyclization

The reaction of pyridoxal with l-histidine or l-tryptophan resulted in Pictet-Spengler cyclization, with formation of potassium 4-[3-hidroxy-5-(methylpyridin-4-yl]-4,5,6,7-tetrahydropyrido[3,4-c]imidazole-6-carboxylate (1) and 1-[2-methyl-3-hydroxy-5-(hydroxymethyl)pyridine-4-yl]-1,2,3,4-tetrahydro-β-carboline-3-carboxylic acid (2). Similar reactions in the presence of zinc(II) resulted in the formation of [Zn2(Pdx-l-His)2(H2O)2]·4H2O (C1) and [Zn(Pdx-l-Trp)(H2O)]·1.5H2O (C2) with pyridoxal-l-histidine (Pdx-l-His) and pyridoxal-l-tryptophan (Pdx-l-Trp) Schiff bases.