Electrochemical and ESR studies on Cu(II) complexes of bleomycin and its related compounds

The 1:1 Cu(II) complexes of bleomycin (BLM) A2, BLM B2, epi-BLM B2, iso-BLM B2, depyruvamide-BLM A2, deglyco-BLM B2 and the structurally related peptides (P-5m, P-3A and P-3) have been comprehensively investigated by ESR and electrochemical methods. ESR spectra for Cu(II) complexes of BLM A2, epi-BLM B2, depyruvamide-BLM A2 and P-3 revealed the axially symmetric g-anisotropies. In contrast, ESR features of the iso-BLM B2, deglyco-BLM B2, P-5m and P-3A complexes, which lack the sixth ligation by the 3-O-carbamoyl group of mannose, exhibited rhombic g-anisotropies with decrease of the A parallel values. The cyclic voltamograms showed that all of the Cu(II) complexes underwent the well defined quasi-reversible one-electron Cu(II)/Cu(I) coupled redox reaction. The inverse of the redox potential, which measures the effective strength of the ligand field splitting, gave a linear relation with the observed g parallel value of each Cu(II) complex except for depyruvamide-BLM and P-3. The present results confirmed that the 3-O-carbamoyl group of the mannose moiety of the BLM molecule contributes to the stability of the metal site in BLM-Cu(II) complexes by ligation at the sixth coordination site.