The pi-Cation Radical of Chlorophyll a.

Chlorophyll a undergoes reversible one-electron oxidation in dichloromethane and butyronitrile. Removal of the electron by controlled potential electrolysis or by stoichiometric charge transfer to a known cation radical yields a radical (epr line width = 9 gauss, g = 2.0025 +/- 0.0001) whose optical spectrum is bleached relative to that of chlorophyll. Upon electrophoresis this bleached species behaves as a cation. By comparison with the known properties of pi-cation radicals of porphyrins and chlorins, the chlorophyll radical is also identified as a pi-cation. Further correlation of optical and epr properties with published studies on photosynthesis leads to the conclusion that oxidized P700, the first photochemical product of photosystem I in green plants, contains a pi-cation radical of the chlorin component of chlorophyll a. This radical is the likely source of the rapidly-decaying, narrow epr signal of photosynthesis.