Zine Complexes of Cysteine, Histidine, and Derivatives Thereof: Potentiometric determination of their compositions and stabilities
✍ Scribed by Peter Gockel; Heinrich Vahrenkamp; Andreas D. Zuberbühler
- Publisher
- John Wiley and Sons
- Year
- 1993
- Tongue
- German
- Weight
- 561 KB
- Volume
- 76
- Category
- Article
- ISSN
- 0018-019X
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✦ Synopsis
Abstract
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.