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, K_c, 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 K₁K_c values indicate that the tendency of forming the deprotonated chelates also decreases in this order.