Zine 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 with those of three derivatives of each n which one of their three donor functions (carboxyl, amino, and mercapto and imidazole, respectively) has been blocked. Using potentionmetric titrations of aqueous solutions, the 1:1 and 1:2 complexes of all for cysteine‐ and all four histidine‐derived ligands are observed among te various species present (ligands, 1:1 and 1:2 complexes, and protonatd 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 order of magnitude for the 1:2 composition. For the cysteine series, the sequence of stabilities is cysteine > cysteine ethyl ester ≫ N (α)‐acetylcysteine ≫ S Methylcysteine. For the histidine series, the corresponding sequence is histidine > histidine methyl ester > N , N (imidazole)‐dimethylhistidine > N (α)‐acetylhistidine. The order of stabilities can be explained by the relative strengths of the Zn–S vs . Zn–N coordination, y charge effects, and by chelate ring sizes.