The reaction of chromium(VI) with L-ascorbic acid (AsA) in buffer solutions
was investigated by e.p.r. spectroscopy. Chromium(V)/ascorbate complexes
with signals at giso = 1.9791 and chromium(VI)/ascorbate/peroxo
complexes with signals at giso = 1.9819 and
giso = 1.9824 were observed in all buffers. New
signals at giso values of 1.9765 and 1.9781 were
observed in Tris–HCl [tris(hydroxymethyl)aminomethane
hydrochloride] buffer and were assigned to a mixed-ligand
ascorbate/Tris complex of chromium(V),
[CrO(ascorbate)(Tris)]2– , and a
Tris/peroxo species,
[CrO(O2)(Tris)]2– , respectively. The
speciation of the e.p.r.-active chromium(V) complexes detected from solutions
with other buffers, such as HEPES, cacodylate and phosphate, was not
influenced by the buffer type. Preincubation of catalase with ascorbate
solutions inhibited the formation of the chromium(V) peroxo species in all
buffers. Manganese(II) reduces the chromium(V) species produced in the
reaction, which has shown that it is inappropriate to use manganeses(II) as a
selective reagent for monitoring the concentrations of chromium(IV) in such
reactions. In particular, manganeses(II) reacts more efficiently with the
chromium(V) species that are most damaging to DNA
in vitro, viz., the mixed-ligand
chromium(V)/ascorbate/peroxo complexes. The correlation of the present
results with those of in vitro DNA damage experiments
reported in the literature has revealed that the
chromium(V)/ascorbate/peroxo species are the major species responsible
for the in vitro DNA strand breaks in all of the buffer
systems. These species are not expected to be as important
in vivo.
