STUDY OF H2O2 INTERACTION WITH COPPER(II) COMPLEXES WITH DIAMINO-DIAMIDE TYPE LIGANDS, DIASTEREOISOMERIC DIPEPTIDES, AND TRIPEPTIDES

Copper(II) complexes with LL or LD dipeptides, tripeptides, L-aminoacy lamides, or N,N'-bis(aminoacyl)alcanediamine were studied in their rea ction with hydrogen peroxide by ESR spectroscopy. Shifts in the magnet ic parameters g(parallel to) and A(parallel to) or differences in the quenching percentages of the ESR signal intensity, due to the formatio n of copper(I) species, suggested that the decomposition mechanism of H2O2 proceeds through the formation of a five-coordinated adduct and a subsequent electron transfer. This last process gave rise to a decomp osition process which involved not only H2O2, but also the ligand coor dinated to copper. It was surprising to find that, at the longest inte raction time, this decomposition reaction always produced a similar co pper(II) complex with (g) over bar(parallel to) = 2.330 +/- 0.005 and (A) over bar(parallel to) = 164 +/- 4 x 10(-4) cm(-1) in spite of the different ligands. Voltammetric measurements confirmed what had been s een by ESR spectroscopy, and suggested that the decomposition mechanis m did not involve the formation of copper(III) species. Furthermore, t he only copper(If) complex with diastereoisomeric dipeptides, which wa s able to promote the copper oxidation, was that formed by LL- or LD- Ala-Trp, thus suggesting that d-pi interaction plays a favorable role in the oxidation process. The complexes which showed catalytic activit y in the hydrogen peroxide decomposition were those obtained from LL- or LD- Ala-Ala or Ala-Leu, i.e., copper(II) complexes with dipeptides having aliphatic side chains. This fact strongly supports the hypothes is of the formation of a ligand radical species due to the breakage of the weak copper(I)-peptide nitrogen bond, radical starting the degrad ation of the ligand itself.