Abstract
Electron spin resonance (ESR) has been applied to measure the photoionization yield of chlorophyll a (Chla) after red-light irradiation with or without electron scavengers in frozen phospholipid vesicles differing in the nature of their headgroups. Along with the Chla monomeric cation, oligomeric cations have been detected. For anionic headgroups the photoionization yield is found to be half or less than that observed with the zwitterionic choline headgroup. A decrease of the yield is also observed when increasing amounts of anionic sodium dihexadecyl phosphate (DHP) are added to vary the surface charge of mixed vesicles. Electron spin echo (ESE) experiments in samples prepared with tetrabromo-P-benzoquinone (TBBQ) showed that the average location of the TBBQ radical anion is not sensitive to DHP addition. At low Chla concentration, the photoionization yield is increased more than 2-fold in dioctadecyldimethylammonium chloride (DODAC) cationic vesicles, prepared in pure water without electron scavengers, compared to the yield in similar dipalmitoylphosphatidylcholine (DPPC), vesicles. An increase in yield is also seen in mixed DPPC/DODAC vesicles at low Chla concentration. The results are discussed in terms of electrostatic barriers influencing electron transfer.
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