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-d(2), N-a cetyl-DOPA, alpha-methyl-DOPA, dopamine, adrenaline, noradrenaline, catecho l, 1,2-dihydroxybenzoic acid (DHBA), and 4-tert-butylcatechol (TBC), produc e a number of Cr(V) electron paramagnetic resonance (EPR) signals. These sp ecies are of interest in relation to the potential role of oxidized protein s and amino acids in Cr-induced cancers. With excess organic ligand, all of the substrates yield Cr species with signals at g(iso) similar to 1.972 (A (iso)(Cr-53) > 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, [Cr-V(semiquinone)(2)(catecholate)](+). Experime nts with excess K2Cr2O7 show complex behavior with the catecholamines and T BC. Several weak Cr(V) signals are detected after mixing, and the spectra e volve over time to yield relatively stable substrate-dependent signals at g (iso) similar to 1.980. These signals have been attributed to [Cr(O)L-2](-) (L = diolato) species, in which the Cr is coordinated to two cyclized catec holamine ligands and an oxo ligand. Isotopic labeling studies with DOPA (ri ng or side chain deuteration or enrichment with N-15), and simulation of th e signals, show that the superhyperfine couplings originate from the side c hain 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 Siso similar to 1.969, but the species responsible for this signal was not identified. Several other minor Cr signals are detected, which are attr ibuted (by comparison with isoelectronic V(IV) species) to Cr(V) complexes coordinated by a single catecholamine ligand (and auxiliary ligands e.g. H2 O), or to [Cr(O)L-2](-) (L = diolato) species with a sixth ligand (e.g. H2O ). Addition of catalase or deoxygenation of the solutions did not affect th e main EPR signals. When the substrates were in excess (pH > 4.5), primary and secondary (cyclized) semiquinones were also detected. Semiquinone stabi lization by Zn(II) complexation yielded stronger EPR signals (g(iso) simila r to 2.004).