Structure of cupric bleomycin. Nitrogen and proton couplings from EPR and electron nuclear double resonance spectroscopy.

Low frequency (S-band) EPR and electron nuclear double resonance (ENDOR) data have been used to provide a detailed description of the magnetic environment of the cupric complex of the antitumor antibiotic, bleomycin. S-band EPR spectra show well resolved nitrogen hyperfine structure in both gl and g regions. Computer simulation of these spectra are consistent with binding to four donor nitrogen atoms, three with AN = 10 gauss and one with AN = 15 G. ENDOR data confirm the presence of at least two inequivalent nitrogens with AN approximately 11 and 15 G. In addition, at least six proton couplings were evident in the ENDOR spectrum although these could not be unambiguously assigned. Diminution of the matrix ENDOR signal in D2O relative to that in H2O was found; this demonstrates the accessibility of H2O to the metal site.