Competition between 1,2-Diol and 2-Hydroxy Acid Coordination in Cr(V)-Quinic Acid Complexes:  Implications for Stabilization of Cr(V) Intermediates of Relevance to Cr(VI)-Induced Carcinogenesis

Abstract

For the speciation of Cr(V) intermediates formed during the intracellular reduction of Cr(VI) to be understood, the intramolecular competition between 1,2-diol and 2-hydroxy acid coordination to Cr(V) as a function 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 and characterized. In aqueous solutions at pH values <4.0, K[Cr(O)(qaH3)2]·H2O gives two EPR signals (g iso = 1.9787, A iso = 17.2 × 10-4 cm-1; g iso = 1.9791, A iso = 16.4 × 10-4 cm-1). The relative intensities of the signals 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 acid complexes and are assigned to two geometric isomers of the [Cr(O)(O 1,O 7-qaH3)2]- linkage isomer. Both the 2-hydroxy acid (O 1,O 7) and vic-diol (cis-O 3,O 4; trans-O 4,O 5) groups of qaH5 are viable Cr(V) donors. In the reduction of Cr(VI) by GSH in the presence of an excess of qaH5, the EPR spectra are similar to that of K[Cr(O)(qaH3)2]·H2O at low pH values (<4.0). At intermediate pH values (pH 5−7.5) additional signals appear (g iso = 1.9791, g iso = 1.9794, g iso = 1.9799), which have EPR spectral data consistent with the presence of Cr(V)-qa linkage isomers, featuring one of each donor type (1 × 2-hydroxy acid; 1 × diol). By using EPR spectral simulation, we deduced that the cis-diol linkage isomer, [Cr(O)(O 1,O 7-qaH3)(O 3,O 4-qaH2)]2-, is an order of magnitude more thermodynamically stable to intramolecular ligand exchange compared to the trans-diol linkage isomer, [Cr(O)(O 1,O 7-qaH3)(O 4,O 5-qaH2)]2-. At pH values >7.5, the Cr(V)-qa EPR spectra reveal two triplets (g iso = 1.9800, g iso = 1.9802), which are ascribed to geometric isomers of a bis-diol Cr(V)-qa complex, [Cr(O)(O 3,O 4-qaH2)2]3-. The concentration of the trans-diol isomer, [Cr(O)(O 4,O 5-qaH2)2]3-, is predicted to be negligible. This assignment is supported by the similarity of the EPR spectral data with those formed in the Cr(VI) reduction by GSH in the presence of the related polyol (cis-O 3,O 4; trans-O 4,O 5) ligand, shikimic acid (3R,4R,5R-3,4,5-trihydroxycyclohexenecarboxylic acid, II), which does not possess a 2-hydroxy acid moiety. The relative intensities of the EPR signals of the Cr(V)-sa species (g iso = 1.9800, g iso = 1.9801), ascribed to geometric isomers of [Cr(O)(O 3,O 4-saH)2]3-, are independent of increasing pH and of [saH4] at pH values >4.0. The results show that 2-hydroxy acid ligands are favored with respect to 1,2-diols for stabilizing Cr(V) at low pH values relevant to phagocytosis of insoluble chromates (pH ∼4), but the opposite is the case when soluble chromates are taken up by the cells at pH = 7.4. Both classes of ligands compete effectively for complexation of Cr(V) compared to glutathione at all pH values studied.

Reductive activation of hexavalent chromium by human lung epithelial cells: Generation of Cr(V) and Cr(V)-thiol species

“…In cells, Cr(V)-GSH species might participate in ligand exchange reactions with carbohydrates [53]. Cr(V)-carbohydrate species exhibit ESR signals at g = 1.979 [77,78], and it is possible that cellular carbohydrates could extend the half-life of the Cr(V) species [79]. Since thiols are depleted in diamide-treated cells, and since the g = 1.979 signal is not diminished in diamide-treated cells, the g = 1.979 signal is not dependent on Cr(V)-GSH/ carbohydrate ligand exchanges.…”

The chromate resistance phenotype of some yeast mutants correlates with a lower level of Cr(V)-species generated in the extra-cellular medium

“…The formation of paramagnetic Cr(V)-species in the extra-cellular medium was discovered for all tested cultures that supports our hypothesis about the crucial role of extra-cellular Strains: parental (''wild type'') L2 and Crt r -mutants L2-9 and L2-11. EPR spectrometer: Radiopan SE/X2544 Table 2 The comparison of g-factors for Cr(V)-species described in this paper with the data available in literature Reference Substance g-Factor Krumpolc and Rocek (1979) Cr(V)-EHBA complex (EHBA = 2-ethyl-2-hydroxybutyric acid) g = 1.980 Shi and Dalal (1994) Product of reaction of Cr(VI) with vitamin B 2 g = 1.9795 Meyer et al (1998) trans-[Cr V (N)(cyclam)(NCCH 3 )](ClO 4 ) 2 (cyclam = 1,4,8,11-tetraazacyclotetradecane) g \ = 1.997 g k = 1.965 Codd and Lay (1999) K reduction in chromate detoxification. It has been demonstrated for the first time that the chromateresistant phenotype of mutants is related to a lower stationary lever of the extremely toxic intermediate, Cr(V), in the extra-cellular medium.…”

Kinetics and mechanism of the oxidation of disaccharides by CrVI

“…The EPR spectrum of a Cr V -diolato species of sixmembered ring cis-diols yields a doublet, since only one proton is in the plane of the unpaired electron density of the Cr V ion, while the EPR spectrum of a Cr V -diolato species of six-membered ring trans-diols (with no protons lying in the ligand plane) yields a singlet (55). Consequently, the EPR spectral multiplicity of the bis-chelate Cr V -diolato 2 species formed between Cr V and pyranosic cis-and trans-diols will exhibit a triplet and a singlet, respectively, which arise from the orientation of the ring protons of the coordinated diol groups with respect to the basal plane of the complex (1,55,57).…”

“…Consequently, the EPR spectral multiplicity of the bis-chelate Cr V -diolato 2 species formed between Cr V and pyranosic cis-and trans-diols will exhibit a triplet and a singlet, respectively, which arise from the orientation of the ring protons of the coordinated diol groups with respect to the basal plane of the complex (1,55,57). Based on this, the isotropic EPR parameters g iso and A iso were used to deduce the coordination number and nature of the donor groups of the Cr V species formed, either for the reaction of Cr VI with the disaccharides or for the reaction of Cr VI with glutathione in the presence of excess disaccharide, according to a described empirical method (1,57,59,60).…”

“…The five-membered Cr V chelates are the most thermodynamically favoured, and CrO 3+ shows a marked preference for binding to cis-rather than trans-diol groups of cyclic diols (21,22,29,55 -diolato of Lac and Mel.…”