Log β, ΔH and ΔS values of mixed complexes of Cu(II) with histamine and some aliphatic aminoacids

Abstract

The stability constants of the parent and mixed complexes of Cu(II) with histamine, and glycine, alanine, α-aminobutyric acid or norvaline have been determined potentiometrically. The ΔH and ΔS values have been obtained by calorimetry. From the thermodynamic data it has been found that the stability of the Cu(II)-histamine parent complex is partly determined by back-coordination. The stabilities of the Cu(II)-histamine-aminoacid mixed complexes are much higher than the statistically expected values. From the formational enthalpy values of the mixed complexes it may be concluded that the bond strengths are not equal to the average of the bond strengths in the Cu A 2 and Cu B 2 type parent complexes. The electrostatic characters of the Cu(II)-aminoacid bonds in the mixed complexes are substantially increased compared with those in the parent complexes. The entropy change of the mixed complex formation is higher by about ten e.u. than the statistically expected value. The enthalpy changes ΔH M of the reactions Cu A 2 + Cu B 2 ⇌ 2Cu AB ( A = aminoacid; B = histamine) are positive and depend on the number of carbon atoms in the aminoacid.