A study of the effect of amino acid structure on the stabilities of the complexes formed with metals of group II of the periodic classification

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Abstract
The stability constants of metal complexes of a wide range of amino acids were measured by a potentiometric method. The sequence of relative stability for any particular amino acid complexes was confirmed as mercury > beryllium> zinc> cadmium. The introduction of a beta-hydroxyl group into the amino acid structure slightly reduces the stability of the complexes. An aromatic substituent decreases the stability constants. The addition of a para-hydroxyl group further reduces the constant, but this effect is less marked when an ortho-hydroxyl group of 2 ortho-iodine atoms are also present in the aromatic ring. Terminal amino groups in a simple amino acid reduce, but the carbamido group increases, complex stabilities. No evidence was found for complex formation through groups other than the alpha-amino group except for histidine and cysteine. Terminal carboxyl groups increase complex stability constants. Amide groups reverse this effect. Histidine has high stability constants and forms complexes through both the glyoxaline and alpha-amino groups. Cysteine produces complexes with the largest stability constants recorded in the alpha-amino acid field.