AN ATTEMPT AT QUANTITATION OF THE SHARP LIGHT-INDUCED ELECTRON PARAMAGNETIC RESONANCE SIGNAL IN PHOTOSYNTHETIC MATERIALS

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.

Detection of a Free Radical in the Primary Reaction of Chloroplast Photosystem II

Electron paramagnetic resonance (EPR) spectroscopy has revealed a new free-radical signal produced by illumination of spinach chloroplasts at 77 degrees K. This signal is observed only when an oxidant (ferricyanide) is added to the chloroplast suspension in the dark before illumination. The EPR signal is produced at 77 degrees K by illumination with 645-nm monochromatic light capable of activating Photosystem II but not with 715-nm illumination capable of activating Photosystem I. Furthermore, since the signal shows a relative increase in chloroplast fragments enriched in Photosystem II but is absent in chloroplast fragments enriched in Photosystem I, we conclude that this new EPR signal is associated with the primary photoact of Photosystem II in chloroplasts. On the basis of the measured EPR parameters (g = 2.0026 +/- 0.0002, linewidth = 8 G), it is suggested that the signal may be associated with the reaction-center chlorophyll of Photosystem II.