Structure-volume relationships: volume effects produced by the interaction of Cu(II) with dipeptides

The volume changes, ΔV, produced by the formation of two types of 1:1 Cu(II):dipeptide complexes were determined dilatometrically. A volume increase of about 11 ml/mole results from the formation of one category of complex in which the Cu(II) is chelated to the terminal amino and carbonyl oxygen of the peptide. A ΔV of about 20 ml/mole results when a tridentated chelate is formed by the coordination of Cu(II) to the terminal amine, the peptide nitrogen and the carboxylate oxygen of the dipeptide; during this process a proton is expelled from the peptide backbone. These volume parameters exhibit a small dependence on the type and location of nonpolar radicals incorporated in the dipeptide. The values for the protonation of the amine radical or of dipeptides exhibited a small dependence on structure, −3.2