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-aminoacylamides, or N,N'-bis(aminoacyl)alcanediamine were studied in their reaction with hydrogen peroxide by ESR spectroscopy. Shifts in the magnetic parameters g parallel and A parallel or differences in the quenching percentages of the ESR signal intensity, due to the formation 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 decomposition process which involved not only H2O2, but also the ligand coordinated to copper. It was surprising to find that, at the longest interaction time, this decomposition reaction always produced a similar copper(II) complex with g = 2.330 +/- 0.005 and A = 164 +/- 4 x 10(-4) cm-1 in spite of the different ligands. Voltammetric measurements confirmed what had been seen by ESR spectroscopy, and suggested that the decomposition mechanism did not involve the formation of copper(III) species. Furthermore, the only copper(II) complex with diastereoisomeric dipeptides, which was 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 activity 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 hypothesis of the formation of a ligand radical species due to the breakage of the weak copper(I)-peptide nitrogen bond, radical starting the degradation of the ligand itself.