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The 1 : 1 bleomycin-A2-Cu(II) complex shows an absorption maximum at 595 nm (e 120), circular dichroism extrema at 555 nm (Ac +1.21) and 665 nm (-0.61), and electron spin resonance (ESR) signal with g,,=2.211, g1 =2.055, and A" =178 x 10-4 cm-1. The formation constant (log K=12.630) and deprotonation constant (pKc=3.585) of the 1: 1 bleomycin-Cu(II) complex were determined by computer analysis of potentiometric data. The results of potentiometric titration also indicate that the stability of bleomycin-metal complexes is in the order Fe(II) < Co(II) < Ni(II) < Cu(II) > Zn(II) and that these divalent metal complexes have a similar coordination environment.The bleomycin-Cu(II) complex has substantially a square-pyramidal configuration in which the secondary amine nitrogen, pyrimidine(N-1) ring nitrogen, deprotonated peptide nitrogen of histidine residue, and histidine imidazole(N-1) nitrogen coordinate to Cu(II) as planar ligand donors, and the a-amino nitrogen as axial donor. The specific Cu(II)-binding site of bleomycin has been compared with that of human serum albumin.

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