EPR studies of copper(II) and cobalt(II) complexes of adriamycin

Cu²⁺ and Co²⁺ complexes of adriamycin (ADM) in aqueous solutions have been examined using EPR spectroscopy. An appreciable amount of Cu²⁺ and Co²⁺ complexes formed in the solutions were found to be in the EPR silent associated form, where the metal ions are antiferromagnetically coupled. The associated form of the Cu²⁺ complex may be neither a simple dimer nor coordination polymer but aggregates of a stacked type. Formation of a complex having Cu²⁺-ADM stoichiometry of 1:2 was observed for the solutions containing excess of ADM as an EPR observable species. The complex having Cu²⁺-ADM stoichiometry of 1:1 was not observed directly by EPR, but the presence of the complex is undeniable, especially at low pH range so far as large excessive ADM is not present. The Co²⁺ complex of ADM observed by EPR is in the high-spin (S = 3 2) state and may have a coordination structure of tetragonal symmetry. The EPR spectra of these complexes apparently show that the Cu²⁺ and Co²⁺ ions are bound at the carbonyl and phenolate oxygen in the 1,4-dihydroxyanthraquinone moiety and the amino nitrogen in the sugar part does not seem to participate in the coordination to the metal ions.