CRITICAL EVALUTION OF THE STABILITY-CONSTANTS OF METAL-COMPLEXES OF AMINO-ACIDS WITH POLAR SIDE-CHAINS

Available experimental data on proton and metal complex equilibria of the seven essential amino acids with polar side chains (i.e. cysteine, cystine, methionine, serine, threonine, asparagine and glutamine) have been critically evaluated. First, the introduction outlines the main structural features of these amino acids as ligands and calls attention to their important roles in biology. A general discussion is also reported on the determination of formation constants and their reliability. In particular, the critical role played by protonation constants as reference parameters for the determination of complex formation constants by glass electrode potentiometry has been emphasized. Coordination enthalpies and entropies are also introduced, and the selection criteria employed in the course of the review are described. For each of the seven amino acids, a specific chapter then reports (i) protonation constants and their associated enthalpic and entropic changes, (ii) formation constants for binary metal complex species and ternary mixed-ligand complexes with the associated thermodynamic quantities whenever available. For every entry, all data available in the literature are first reported. Then, a selection of recommended or tentative values is proposed for as many sets of specific experimental conditions as possible, in accordance with the criteria given in the introduction and based on considerations developed in corresponding discussions.