Orientation of dioxygen bound to cobalt(II) bleomycin-DNA fibers.

The molecular oxygen adduct of Co(II)-bleomycin is stable for long periods when bound to salmon sperm DNA at large ratios of polymer to drug. According to ESR studies of orientation of the paramagnetic complex associated with DNA fibers, the oxygen-oxygen bond is restricted to a plane perpendicular to the fiber axis. Thus, one can define three g values for the adduct 2.104, 2.016, and 2.000, one parallel to the fiber axis and two orthogonal to it. There is no change in orientation over the range of 77 K to ambient temperature. Furthermore, there is no difference in results at a series of relative humidities ranging from less than 76% where bulk DNA alone assumes an A conformation to 95% where it is primarily B DNA. A structural model is presented for the geometry of the metal binding domain of O2-Co-bleomycin in relationship to the fiber axis of DNA.