Structure and Reactivity of a Chromium(V) Glutathione Complex1

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Abstract
Chromium(V) glutathione complexes are among the likely reactive intermediates in Cr(VI)-induced genotoxicity and carcinogenicity. The first definitive structure of one such complex, [CrVO(LH2)2]3- (I; LH5 = glutathione = GSH), isolated from the reaction of Cr(VI) with excess GSH at pH 7.0 (O'Brien, P.; Pratt, J.; Swanson, F. J.; Thornton, P.; Wang, G. Inorg. Chim. Acta 1990, 169, 265−269), has been determined by a combination of electrospray mass spectrometry (ESMS), X-ray absorption spectroscopy (XAS), EPR spectroscopy, and analytical techniques. In addition, Cr(V) complexes of GSH ethyl ester (γ-Glu-Cys-GlyOEt) have been isolated and characterized by ESMS, and Cr(III) products of the Cr(VI) + GSH reaction have been isolated and characterized by ESMS and XAS. The thiolato and amido groups of the Cys residue in GSH are responsible for the Cr(V) binding in I. The Cr−ligand bond lengths, determined from multiple-scattering XAFS analysis, are as follows: 1.61 Å for the oxo donor; 1.99 Å for the amido donors; and 2.31 Å for the thiolato donors. A significant electron withdrawal from the thiolato groups to Cr(V) in I was evident from the XANES spectra. Rapid decomposition of I in aqueous solutions (pH = 1−13) occurs predominantly by ligand oxidation with the formation of Cr(III) complexes of GSH and GSSG. Maximal half-lives of the Cr(V) species (40−50 s at [Cr] = 1.0 mM and 25 °C) are observed at pH 7.5−8.0. The experimental data are in conflict with a recent communication (Gaggelli, E.; Berti, F.; Gaggelli, N.; Maccotta, A.; Valensin, G. J. Am. Chem. Soc.2001, 123, 8858−8859) on the formation of a Cr(V) dimer as a major product of the Cr(VI) + GSH reaction, which may have resulted from misinterpretation of the ESMS and NMR spectroscopic data.

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