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Interaction of UV Light‐Induced α–Tocopherol Radicals with Lipids Detected by an Electron Spin Resonance Prooxidation Effect

The reaction rate constants of the interaction between light‐induced α–tocopherol radicals with unsaturated lipids in a heterogeneous system compared to a homogeneous system are of the same order of magnitude. The decay rates of compartmentalized ‐α‐tocopherol radicals were significantly reduced by using negatively charged sodium dodecyl sulfate (SDS) micelles. A partially resolved electron spin resonance (ESR) hyperfine structure was observed under the conditions of both high lipid concentrations in comparison to the α‐tocopherol concentration and of a regular distribution of α‐tocopherol molecules inside the heterogeneous lipid structures. Alphα‐to‐copherol radicals have a considerable prooxidation potential at higher concentrations. Ascorbic acid dissolved in the aqueous medium provokes very fast ‐α‐tocopherol radical recycling through the boundary layer between the aqueous medium and micelles. By contrast, very slow reactions such as those of α‐tocopherol radicals with glutathione through this boundary layer are measurable. Despite using the heterogeneous SDS micellar system, the decay kinetics of the α‐tocopherol radical ESR signal is simply compounded. In addition to the known stabilization effect of cholesterol in membrane systems, cholesterol itself acts as a target molecule attacked by free radicals, e.g. ‐α‐tocopherol radicals. Using stratum corneum extracts that contain unsaturated lipids and cholesterol the ‐α‐tocopherol radical can prooxidatively react with these compounds. Using focused UV light generates a high radical yield in a relatively short time compared to the lifetime of the ‐α‐tocopherol radicals. The decay processes after radical induction can be characterized as consecutive reactions. The compartmentalization of radicals induced in SDS micelles and the close proximity of target molecules are essential if very slow one‐electron reductions are to be measured.