Increased antioxidant reactivity of vitamin C at low pH in model membranes.

The reactivity of the water-soluble antioxidant vitamin C (l-ascorbic acid) depends on pH. It is generally recognized that the ascorbate monoanion, which predominates at neutral physiological pH, acts as a stronger antioxidant than the protonated form, ascorbic acid. Fluorazophore-L, a long-lived fluorescent probe, was employed as a mimic for lipid peroxyl radicals. The experiments with micellar and bilayer membrane models demonstrate that vitamin C becomes, in fact, a more powerful antioxidant at low pH. This phenomenon may be general for the interception of reactive oxidizing species at the lipid/water interface.