Walke M, Beckert D, Lasch J
Institut für Physiologische Chemie, Martin-Luther Universität, Halle, Germany.
Photochem Photobiol. 1998 Oct;68(4):502-10. doi: 10.1111/j.1751-1097.1998.tb02506.x.
The reaction rate constants of the interaction between light-induced alpha-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 alpha-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 alpha-tocopherol concentration and of a regular distribution of alpha-tocopherol molecules inside the heterogeneous lipid structures. Alpha-tocopherol radicals have a considerable prooxidation potential at higher concentrations. Ascorbic acid dissolved in the aqueous medium provokes very fast alpha-tocopherol radical recycling through the boundary layer between the aqueous medium and micelles. By contrast, very slow reactions such as those of alpha-tocopherol radicals with glutathione through this boundary layer are measurable. Despite using the heterogeneous SDS micellar system, the decay kinetics of the alpha-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. alpha-tocopherol radicals. Using stratum corneum extracts that contain unsaturated lipids and cholesterol the alpha-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 alpha-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.
与均相体系相比，非均相体系中光诱导的α-生育酚自由基与不饱和脂质之间相互作用的反应速率常数处于同一数量级。通过使用带负电荷的十二烷基硫酸钠（SDS）胶束，分隔的α-生育酚自由基的衰减速率显著降低。在脂质浓度相对于α-生育酚浓度较高以及α-生育酚分子在非均相脂质结构内呈规则分布的条件下，观察到了部分分辨的电子自旋共振（ESR）超精细结构。在较高浓度下，α-生育酚自由基具有相当大的促氧化潜力。溶解在水介质中的抗坏血酸通过水介质与胶束之间的边界层引发α-生育酚自由基的快速循环。相比之下，诸如α-生育酚自由基通过该边界层与谷胱甘肽的反应等非常缓慢的反应是可测量的。尽管使用了非均相SDS胶束体系，但α-生育酚自由基ESR信号的衰减动力学只是简单地叠加。除了胆固醇在膜系统中已知的稳定作用外，胆固醇本身还作为自由基攻击的靶分子，例如α-生育酚自由基。使用含有不饱和脂质和胆固醇的角质层提取物，α-生育酚自由基可与这些化合物发生促氧化反应。与α-生育酚自由基的寿命相比，使用聚焦紫外光可在相对较短的时间内产生高自由基产率。自由基诱导后的衰减过程可表征为连续反应。如果要测量非常缓慢的单电子还原反应，SDS胶束中诱导的自由基的分隔以及靶分子的紧密接近是必不可少的。
