EPR Spectroscopic Studies of the Reactions of Cr(VI) with <scp>l</scp>-Ascorbic Acid, <scp>l</scp>-Dehydroascorbic Acid, and 5,6-<i>O</i>-Isopropylidene-<scp>l</scp>-ascorbic Acid in Water.¹ Implications for Chromium(VI) Genotoxicity

Abstract

The characterization of the Cr(V) intermediates in the reduction of Cr(VI) by l-ascorbic acid (l-AsA) in aqueous solution is described as a function of pH and ascorbate concentration. In the reaction of Cr(VI) with AsA at least seven EPR-active Cr(V) signals are present in addition to that of the ascorbate radical. The Cr(V)/ascorbate complex at giso = 1.9791 (Aiso = 16.4 × 10-4 cm-1) is much more stable in moderately acidic solution (pH 3−6) than in neutral and alkaline solutions. The reaction is affected by aerial O2 producing Cr(V)/ascorbate/peroxo complexes with signals at giso = 1.9818 (Aiso = 13.2 × 10-4 cm-1) and giso = 1.9812 and another at giso = 1.9824 (Aiso = 12.9 × 10-4 cm-1). These signals are most prominent at near physiological pH values (7−8), and all were absent when the solutions were preincubated with catalase. The Cr(V)/ascorbate complex is most stabilized in a [AsA]:[Cr(VI)] ratio of 1:2, while the Cr(V)/ascorbate/peroxo complexes reach their maximum signal intensities at [AsA]:[Cr(VI)] = 1:1. None of these Cr(V) species are stable at pH > 10, and they are reduced rapidly to Cr(III) at [AsA]:[Cr(VI)] ratios above 2:1. This is the first characterization of such Cr(V)/ascorbate/peroxo complexes. They arise from the reduction of O2 during the aerial oxidation of AsA, and their identities were confirmed by studies on the reductions of Cr(VI) with AsA in the presence of H2O2. Their presence explains the O2 sensitivity of in vitro DNA damage, and implications for Cr(VI)-induced cancers are considered. The reductions of Cr(VI) by dehydroascorbic acid (DHAA) and 5,6-O-isopropylidene-l-ascorbic acid (i-p-AsA), in the presence and absence of H2O2 were also studied using EPR spectroscopy. From these studies, likely solution structures for the Cr(V) complexes formed in the reduction of Cr(VI) by AsA were assigned.