Abstract
EPR and Mössbauer studies demonstrate that iron chelation by adriamycin is complex, with several different chelation structures. At physiological pH in aqueous solution, three different EPR spectra are observed: a spectrum at g = 4.2 of Fe^3+^ in a rhombic crystal field (type 1); a spectrum at g = 2.01 with symmetric Gaussian lineshape linewidth 225 G (1 G = 10^−4^ T), suggestive of Fe^3+^ bound in an octahedral crystal field (type 2); and a broad spectrum centered at g = 2.0 suggestive of ferromagnetically coupled Fe^3+^ (type 3). The type 1 and 2 spectra are observed at adriamycin/Fe^3+^ ratios >4, the type 3 spectrum is observed at ratios <4 and at ratios <2 an increasing amount of Fe^3+^ gives rise to EPR silent iron(III) hydroxide polymers. At 4 K the type 1 and 2 complexes exhibit a broad doublet Mössbauer signal with an isomer shift δ = 0.56 (1) mm s^−1^ and quadrupole splitting δ__E__~Q~ = 0.74 (1) mm s^−1^. The type 3 complex gives rise to a sextet signal with isomer shift Δ__E__~q~ = 0.47 (1) mm s^−1^ and hyperfine splitting HF = 476 (1) kG with exhibits superparamagnetic relaxation behavior with a blocking temperature of 23 K, consistent with a microcrystal size of 25 Å. Cu^2+^ binds to adriamycin at adriamycin/Cu^2+^ ratios >4:1 giving rise to an EPR spectrum with axial symmetry g~∥~ = 2.26, g~⟂~ = 2.066, A~∥~ = 188 G, while 2:1 complexes exhibit a single Gaussian line at g = 2.09 indicative of exchange‐coupled Cu^2+^. The exchange‐coupled Cu^2+^ and ferromagnetically coupled Fe^3+^ complexes can be explained by the formation of stacked 2:1 adriamycin‐metal polymers. On titration of adriamycin with Fe^3+^‐nitrilotriacetate a different spectrum is observed at g = 4.3 and its intensity plateaus at an adriamycin/iron ratio of 2. The iron adriamycin complexes cycle to reduce molecular oxygen and this cycle has been hypothesized to be a mechanism mediating the therapeutic and toxic effects of the drug. Both EPR and Mössbauer experiments demonstrate that the type 1 and 2 chelates reduce their Fe^3+^ to Fe^2+^ while the type 3 chelate does not. Therefore, the stoichiometry and method of complex preparation can profoundly effect the properties of these complexes.