In one of the photochemical reaction systems of photosynthesis (system I), the primary process is believed to involve an electron transfer between cytochrome and chlorophyll. As an in vitro model for this system, a study has been made of the photochemical reactions between chlorophyll a and hemin, in pyridine solution. A slow dark reduction, undergone by hemin in pyridine, is accelerated by the presence of chlorophyll, and both red and green light further increase the rate of the reaction. A mechanism proposed for this reaction is electron transfer from chlorophyll to hemin, with recovery of oxidized chlorophyll at the expense of solvent. Chlorophyll also sensitizes the photooxidation of reduced hemin in air. The kinetics of this reaction are similar to those of other sensitized photooxidations, indicating a similar mechanism. Quenching of chlorophyll fluorescence by low concentrations of hemin or reduced hemin suggest the existence of a complex between chlorophyll and hemin.
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