EPR studies of chromium(V) intermediates generated via reduction of chromium(VI) by DOPA and related catecholamines: potential role for oxidized amino acids in chromium-induced cancers

The reductions of K2Cr2O7 by catecholamines, DOPA, DOPA-beta,beta-d2, N-acetyl-DOPA, alpha-methyl-DOPA, dopamine, adrenaline, noradrenaline, catechol, 1,2-dihydroxybenzoic acid (DHBA), and 4-tert-butylcatechol (TBC), produce a number of Cr(V) electron paramagnetic resonance (EPR) signals. These species are of interest in relation to the potential role of oxidized proteins and amino acids in Cr-induced cancers. With excess organic ligand, all of the substrates yield Cr species with signals at g(iso) approximately 1.972 (Aiso(53Cr) > 23.9 x 10(-4) cm(-1)). These are similar to signals reported previously but have been reassigned as octahedral Cr(V) species with mixed catechol-derived ligands, CrV(semiquinone)2(catecholate)]+. Experiments with excess K2Cr2O7 show complex behavior with the catecholamines and TBC. Several weak Cr(V) signals are detected after mixing, and the spectra evolve over time to yield relatively stable substrate-dependent signals at g(iso) approximately 1.980. These signals have been attributed to Cr(O)L2](L = diolato) species, in which the Cr is coordinated to two cyclized catecholamine ligands and an oxo ligand. Isotopic labeling studies with DOPA (ring or side chain deuteration or enrichment with 15N), and simulation of the signals, show that the superhyperfine couplings originate from the side chain protons, confirming that the catecholamine ligands are cyclized. At pH 3.5, a major short-lived EPR signal is observed for many of the substrates at g(iso) approximately 1.969, but the species responsible for this signal was not identified. Several other minor Cr signals are detected, which are attributed (by comparison with isoelectronic V(IV) species) to Cr(V) complexes coordinated by a single catecholamine ligand (and auxiliary ligands e.g. H2O), or to Cr(O)L2]- (L = diolato) species with a sixth ligand (e.g. H2O). Addition of catalase or deoxygenation of the solutions did not affect the main EPR signals. When the substrates were in excess (pH > 4.5), primary and secondary (cyclized) semiquinones were also detected. Semiquinone stabilization by Zn(II) complexation yielded stronger EPR signals (g(iso) approximately 2.004).