EPR Spectroscopic Studies of the Reactions of Cr(VI) with L-ASCORBIC ACID, L-Dehydroascorbic Acid, and 5,6-O-Isopropylidene-L
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- 作者：Lianbo Zhang and Peter A. Lay
- 刊名：Journal of the American Chemical Society
- 出版年：1996
- 出版时间：December 18, 1996
- 年：1996
- 卷：118
- 期：50
- 页码：12624 - 12637
- 全文大小：443K
- 年卷期：v.118,no.50(December 18, 1996)
- ISSN：1520-5126
文摘
The characterization of the Cr(V) intermediates inthe reduction of Cr(VI) by L-ascorbic acid(L-AsA) inaqueous solution is described as a function of pH and ascorbateconcentration. In the reaction of Cr(VI) withAsAat least seven EPR-active Cr(V) signals are present in addition tothat of the ascorbate radical. TheCr(V)/ascorbatecomplex at giso = 1.9791(Aiso = 16.4 ×10-4 cm-1) is muchmore stable in moderately acidic solution (pH 3-6)than in neutral and alkaline solutions. The reaction is affectedby aerial O2 producingCr(V)/ascorbate/peroxocomplexes with signals at giso = 1.9818(Aiso = 13.2 ×10-4 cm-1) andgiso = 1.9812 and another atgiso = 1.9824(Aiso = 12.9 ×10-4 cm-1).These signals are most prominent at near physiological pH values(7-8), and all wereabsent when the solutions were preincubated with catalase. TheCr(V)/ascorbate complex is most stabilized in a[AsA]:[Cr(VI)] ratio of 1:2, while theCr(V)/ascorbate/peroxo complexes reach their maximum signalintensities at[AsA]:[Cr(VI)] = 1:1. None of these Cr(V) speciesare stable at pH > 10, and they are reduced rapidly toCr(III)at [AsA]:[Cr(VI)] ratios above 2:1. This is the firstcharacterization of such Cr(V)/ascorbate/peroxocomplexes.They arise from the reduction of O2 during the aerialoxidation of AsA, and their identities were confirmed bystudies on the reductions of Cr(VI) with AsA in the presence ofH2O2. Their presence explains theO2 sensitivityof in vitro DNA damage, and implications forCr(VI)-induced cancers are considered. The reductions ofCr(VI) bydehydroascorbic acid (DHAA) and5,6-O-isopropylidene-L-ascorbic acid (i-p-AsA),in the presence and absence ofH2O2 were also studied using EPR spectroscopy.From these studies, likely solution structures for the Cr(V)complexesformed in the reduction of Cr(VI) by AsA wereassigned.