Reaction of Co(II)bleomycin with dioxygen

The reaction of Co(II)bleomycin with dioxygen has been investigated. Dioxygen binds to the Co(II) complex within the time of mixing according to electron spin resonance and uv-visible spectroscopy and dioxygen analysis. Then, two dioxygenated cobalt centers react, releasing 1 mol of O2 and forming an intermediate characterized by a few highly shifted 1H NMR resonances and loss of the ESR spectrum. This is thought to be a dioxygen-bridged dimer of cobalt bleomycin molecules. Time-dependent absorbance and dioxygen measurements yield the same second order rate constant for this step of the reaction. According to uv-visible and NMR spectral analysis, the intermediate decays into diamagnetic products in a first order rate process. High performance liquid chromatography and 1H NMR studies demonstrate that the product contains two bleomycin species of equal concentration. One component is Co(III)bleomycin, designated Form II. The other is the peroxide adduct of Co(III)bleomycin, Form I, as determined by direct determination of hydrogen peroxide, which is slowly released from the product at low pH. In contrast, hydrogen peroxide is readily detected during the reaction of Co(II)Blm with O2. In isolation, Form I is unstable at pH 7 and is converted within 24 h into a mixture of Form I and Form II.