Spectroscopic studies on iron complexes of different anthracyclines in aprotic solvent systems

Iron complexes of daunorubicin, idarubicin, pirarubicin, and doxorubicin in anhydrous DMF were studied by UV/vis, CD, fluorescence, Mossbauer, and EPR spectroscopy. Titration studies of the metal-free anthracyclines showed one (UV-detectable) deprotonation step requiring 2 equiv of base, compared to 1 equiv for quinizarine. Metal complexation was studied at three different metal/ligand ratios, and with increasing amounts of base. The results obtained from optical spectroscopy show the existence of two different complex species and give clear indications for the requirements of metal complexation. Complex species I, formed at a low iron-to-ligand ratio, is less dependent on base addition than complex species II formed with equimolar ferric ion. EPR and Mossbauer experiments provide further insight into the structures of both complex species. Lack of spin density of the Mossbauer samples in EPR indicates spin coupling between the metal centers. Mossbauer spectra consist of single quadrupole doublets with values typical for high-spin ferric ion in an octahedral arrangement. The Mossbauer spectroscopic features at 7 T exclude the presence of S = 0 dimers. Complex I represents a monomeric ferric iron complex whereas complex H is consistent with a more or less aggregrated oligomeric Fe-anthracycline system.