Abstract
| - For the speciation of Cr(V) intermediates formed during the intracellular reduction of Cr(VI) to beunderstood, the intramolecular competition between 1,2-diol and 2-hydroxy acid coordination to Cr(V) as afunction of pH has been studied in quinic acid complexes. The Cr(V)-2-hydroxy acid complex, K[Cr(O)(qaH3)2]·H2O (qaH5 = 1R,3R,4R,5R-1,3,4,5-tetrahydroxycyclohexanecarboxylic acid, I), has been isolated andcharacterized. In aqueous solutions at pH values <4.0, K[Cr(O)(qaH3)2]·H2O gives two EPR signals (giso =1.9787, Aiso = 17.2 × 10-4 cm-1; giso = 1.9791, Aiso = 16.4 × 10-4 cm-1). The relative intensities of thesignals are independent of [qaH5]/[Cr(V)], and of increasing [qaH5] and [Cr(V)] at constant [qaH5]/[Cr(V)]and pH values. These signals are consistent with those found with well-characterized Cr(V)-2-hydroxy acidcomplexes and are assigned to two geometric isomers of the [Cr(O)(O1,O7-qaH3)2]- linkage isomer. Both the2-hydroxy acid (O1,O7) and vic-diol (cis-O3,O4; trans-O4,O5) groups of qaH5 are viable Cr(V) donors. In thereduction of Cr(VI) by GSH in the presence of an excess of qaH5, the EPR spectra are similar to that ofK[Cr(O)(qaH3)2]·H2O at low pH values (<4.0). At intermediate pH values (pH 5−7.5) additional signalsappear (giso = 1.9791, giso = 1.9794, giso = 1.9799), which have EPR spectral data consistent with the presenceof Cr(V)-qa linkage isomers, featuring one of each donor type (1 × 2-hydroxy acid; 1 × diol). By using EPRspectral simulation, we deduced that the cis-diol linkage isomer, [Cr(O)(O1,O7-qaH3)(O3,O4-qaH2)]2-, is anorder of magnitude more thermodynamically stable to intramolecular ligand exchange compared to the trans-diol linkage isomer, [Cr(O)(O1,O7-qaH3)(O4,O5-qaH2)]2-. At pH values >7.5, the Cr(V)-qa EPR spectra revealtwo triplets (giso = 1.9800, giso = 1.9802), which are ascribed to geometric isomers of a bis-diol Cr(V)-qacomplex, [Cr(O)(O3,O4-qaH2)2]3-. The concentration of the trans-diol isomer, [Cr(O)(O4,O5-qaH2)2]3-, ispredicted to be negligible. This assignment is supported by the similarity of the EPR spectral data with thoseformed in the Cr(VI) reduction by GSH in the presence of the related polyol (cis-O3,O4; trans-O4,O5) ligand,shikimic acid (3R,4R,5R-3,4,5-trihydroxycyclohexenecarboxylic acid, II), which does not possess a 2-hydroxyacid moiety. The relative intensities of the EPR signals of the Cr(V)-sa species (giso = 1.9800, giso = 1.9801),ascribed to geometric isomers of [Cr(O)(O3,O4-saH)2]3-, are independent of increasing pH and of [saH4] atpH values >4.0. The results show that 2-hydroxy acid ligands are favored with respect to 1,2-diols for stabilizingCr(V) at low pH values relevant to phagocytosis of insoluble chromates (pH ∼4), but the opposite is the casewhen soluble chromates are taken up by the cells at pH = 7.4. Both classes of ligands compete effectively forcomplexation of Cr(V) compared to glutathione at all pH values studied.
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