ELECTRON SPIN RESONANCE STUDIES OF FREE RADICAIS DERIVED FROM PLASTOQUINONE, ALPHA-AND GAMMA-TOCOPHEROL AND THEIR RELATION TO FREE RADICALS OBSERVED IN PHOTOSYNTHETIC MATERIALS

Speculation has centered on the semiquinone of plastoquinone as the source of the light induced, broad, structured, slow-decaying free radical (Signal II) generated by O2-evolving photosynthetic systems. However, neither solution nor low-temperature spectra of plastosemiquinone, which appeared in the presence of KOH, showed substantial asymmetry, while Signal II is characteristically asymmetric. Two types of radicals were observed when α-tocopherol was treated with KOH. One of these radicals was identified as a semiquinone; another free radical was observed at a high power. At 77° K this radical and ones generated from α- and γ-tocopherol by ultraviolet irradiation gave lines which closely mimicked Signal II. We propose that these radicals are of the alkoxy-substituted phenoxy type, i.e., that they are chromanoxyl radicals. The consequences of the presence of these radicals for our present conception of photosynthesis is discussed, and it is proposed that Signal II is predominantly due to plastochromanoxyl free radical. © 1969.