Stability of fused rings in metal chelates. VI. Structures and stability constants of the copper(II) chelates of dipeptides containing glycine and/or β-alanine

Abstract

The reactions of the dipeptides containing glycine and/or β-alanine with an equimolar amount of copper(II) nitrate in acid solution were investigated potentiometrically, and the stability constants for the copper(II) chelates formed in acid solution were determined at 25 °C by the method of Datta and Rabin. The chelates with the composition [Cu.peptide(OH2)] were isolated in the pure state. The dipeptides exhibited analogous reactivities, and the stability constants, log K1, for the charged chelates, CuHL+, were found to be between 5.5–5.7. Comparison of the values for glycylglycine and β-alanyl-glycine with those for glycine and β-alanine suggests that these dipeptides act as terdentate ligands forming fused-ring chelates, where the coordinating sites are probably the amino nitrogen, amide nitrogen, and carboxyl oxygen, although the complexing ability of the amide NH group may be weak. The constants, Kc, for the dissociation of the amide hydrogen from the charged chelates to form the deprotonated chelates, CuL, with the rigid 5-5-, 5-6-, or 6-6-membered ring system decrease in the order glycylglycine > β-alanylglycine > glycyl-β-alanine > β-alanyl-β-alanine. The log K1Kc values indicate that the tendency of forming the deprotonated chelates also decreases in this order.