ZINC-COMPLEXES OF CYSTEINE, HISTIDINE, AND DERIVATIVES THEREOF - POTENTIOMETRIC DETERMINATION OF THEIR COMPOSITIONS AND STABILITIES

The stabilities of Zn complexes of cysteine and histidine have been determined together with those of three derivatives of each in which one of their three donor functions (carboxyl, amino, and mercapto and imidazole respectively) has been blocked. Using potentiometric titrations of aqueous solutions, the 1:1 and 1:2 complexes of all four cysteine- and all four histidine-derived ligands are observed among the various species present (ligands, 1:1 and 1:2 complexes, and protonated derivatives thereof). All cysteine-derived complexes are more stable than the corresponding histidine-derived complexes by 1-2 orders of magnitude for the 1:1 composition and by 1-6 orders of magnitude for the 1:2 composition. For the cysteine series, the sequence of stabilities is cysteine > cysteine ethyl ester much greater than N(alpha)-acetylcysteine much greater than S-methylcysteine. For the histidine series, the corresponding sequence is histidine > histidine methyl ester > NN(imidazole)-dimethylhistidine > N(alpha)-acetylhistidine. The order of stabilities can be explained by the relative strengths of the Zn-S vs. Zn-N coordination, by charge effects, and by chelate nn sizes.