Free radicals in photosynthetic reactions

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.

Differences in the sensitivity to UVB radiation of two cultivars of rice (Oryza sativa L.) based on observation of long-lived radicals

Free radicals with a long lifetime were observed in the leaves of two rice cultivars (Oryza sativa L.), Sasanishiki (UVB resistant) and Norin-1 (UVB sensitive), by electron spin resonance spectroscopy. The leaves of both cultivars grown with visible light show very similar ESR spectra composed of radical 1 (R1) and radical 2 (R2), which may be attributable to P700 cation radicals in the reaction center of photosystem I, and tyrosine cation radicals in the reaction center of photosystem II, respectively. The ESR spectrum composed of R1 and R2 radicals in the leaves of Sasanishiki grown under visible light with supplemental UVB was similar to that in the plant grown without supplemental UVB. On the other hand, the amount of R2 radicals in the leaves of Norin-1 grown under visible light with supplemental UVB was significantly smaller than that in the plant grown without supplemental UVB. It is suggested that the loss of R2 radicals in Norin-1 upon UVB irradiation is related to the instability of the plant.

Photosynthetic Mutants Separate Electron Paramagnetic Resonance Signals of Scenedesmus

Current schemes for the mechanism of the photosynthesis reaction imply a two-part cycle; one part produces a strong photoreductant, the other functions to release molecular oxygen. Wild-type Scenedesmus exhibits the two light-induced electron-paramagnetic resonances typical of green algae and chloroplasts of higher plants. These resonances indicate the presence of unpaired electrons in at least two sites. By means of mutants which are blocked in one or the other part of the dual cycle, the narrow, rapidly decaying signal can be correlated with the photoreductant part, and the broad, slowly decaying signal can be correlated with the part releasing molecular oxygen.