PHOTOREDUCTION OF ALKYLMETHYLVIOLOGENS WITH ALPHA-TOCOPHEROL IN DIOCTADECYLDIMETHYLAMMONIUM CHLORIDE VESICLES

Electron spin resonance (ESR) spectroscopy is used to detect the photoreduction yield of alkylmethylviologens (AV2+) in rapidly frozen dioctadecyldimethylammonium chloride (DODAC) vesicles containing concentrations of alpha-tocopherol (major component of vitamin E) from 0 to 23 mol %. The observed radicals are alkylmethylviologen cation radicals (AV+) from photoirradiated AV2+ in DODAC vesicles without alpha-tocopherol. For 1-3 mol % alpha-tocopherol, the major radical is AV+ and the minor radical is a neutral free radical of alpha-tocopherol (EO) which is formed by photoinduced conversion from the alpha-tocopherol cation radical (EH+) with DODAC vesicles acting as proton scavengers. The total ESR intensity increases with an increase of the alkyl chain length of AV2+. The AV+ intensity increases slightly with increasing alpha-tocopherol concentration. For over 9 mol % alpha-tocopherol, the major radical becomes EO, and at 17 mol % EO alone is observed. This is explained by acceleration of the photoreduction of AV2+ to AV+ by electrons released from alpha-tocopherol and further photoreduction of AV+ to AV, which is not detected by ESR spectroscopy. The photoyield for 23 mol % alpha-tocopherol in DODAC vesicles without AV2+ is about 2-fold more than that in hexane solution. This enhancement of photoyield suggests that DODAC may act as a proton scavenger and compartmentalize alpha-tocopherol to minimize back electron reaction.