Cobalt-bleomycin DNA system. Evidence of DNA bound superoxo and .mu.-peroxo cobalt-bleomycin complexes

Co(II) interacts with bleomycin in aqueous solution, in the presence of air, to give a short-lived mononuclear superoxo Co(III) complex (I). Then, two molecules of complex I react together, with the loss of oxygen, to yield the dinuclear mu-peroxo Co(III) complex (II); the dimerization follows a second-order rate law with k2 = 200 +/- 50 M-1 s-1 at 25 degrees C. The rate of dimerization is lowered by a factor of 2000 when DNA is present at a molar ratio of [nucleotide]/[Co] higher than 16. These results and studies of circular dichroism and electron paramagnetic resonance spectra of complexes strongly suggest the binding of the superoxo complex to DNA (I') as well as that of the mu-peroxo complex (II'); the binding of 1 molecule of complex II for every 2.9 base pairs in DNA has been determined with an apparent equilibrium constant of 8.4 x 10(4) M-1.