TEMPERATURE-DEPENDENCE OF THE PHOTOIONIZATION YIELD OF CHLOROPHYLL-A IN RAPIDLY FROZEN VESICLES

The photoionization yield of chlorophyll-a (Chla) in rapidly frozen dipalmitoylphosphatidylcholine (DPPC) vesicles containing lipophilic and lipophobic electron scavengers has been studied as a function of the temperature by electron spin resonance (ESR) measurements. Samples irradiated at 77 K prepared with lipophilic tetrachloroparabenzoquinone (TCBQ) show an ESR signal intensity which varies as 1/T when measured at various temperatures. Samples prepared with TCBQ or lipophobic potassium ferricyanide (FC) were irradiated at various temperatures. The photoionization yield increases with increasing irradiation temperature. It increases monotonically with temperature with TCBQ while it increases suddenly near 190 K with FC. The results are interpreted as indicating that softening of the matrix enables the reactants to reach an optimal configuration for electron transfer. The different locations of TCBQ inside the vesicle and FC at the vesicle interface can account for their different effects. © 1990.