Abstract
The 1:1 bleomycin-A2-Cu(II) complex shows an absorption maximum at 595 nm (ε 120), circular dichroism extrema at 555 nm (Δε+1.21) and 665 nm (-0.61), and electron spin resonance (ESR) signal with g11=2.211, g⊥=2.055, and A11=178×10-4 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)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 α-amino nitrogen as axial donor. The specific Cu(II)-binding site of bleomycin has been compared with that of human serum albumin.
