Reaction of Chromium(VI) with Ascorbate Produces Chromium(V), Chromium(IV), and Carbon-Based Radicals

Reaction of potassium dichromate with sodium ascorbate was studied by EPR spectroscopy at room temperature, in 0.10 M N-[2-hydroxyethyl]piperazine-N′-[2-ethanesulfonic acid] (HEPES), phosphate, cacodylate, and tris(hydroxymethyl)aminomethane hydrochloride (Tris-HCl) buffers at pH 7.0, in the presence of 0.10 M spin trap [5,5-dimethyl-1-pyrroline 1-oxide or 2-methyl-N-(4-pyridinylmethylene)-2-propanamine N,N′-dioxide]. Chromium(V), ascorbate radical, CO₂^-, and other carbon-based spin trap–radical adducts were observed. Chromium(V), CO₂^-, and the carbon-based radicals were observed at low ratios of ascorbate to chromium, and ascorbate radical was observed at high ratios of ascorbate to chromium. The presence of Cr(IV) was detected indirectly by reaction with Mn(II) and a subsequent decrease in the Mn(II) EPR signal. More Cr(IV) was found for the higher reaction ratios of ascorbate to Cr(VI). The only buffer effect observed was a relative decrease of the Cr(V) signal in Tris•HCl vs HEPES, phosphate, and cacodylate buffers, no change in the radical adducts was observed. There was no evidence for reactive oxygen species as intermediates in this reaction. Addition of the singlet oxygen trap 2,2,6,6-tetramethyl-4-piperidone hydrochloride showed no 2,2,6,6-tetramethyl-1-piperidinyloxy radical formation. The Cr(V) species did not react with dioxygen, and dioxygen did not affect the formation of carbon-based radicals. A mechanism consistent with these observations is discussed.