In Vitro Photoreactions of Chlorophyll and Photosynthetic Energy Conversion: Chlorophyll-Quinone-Ethanol at Low Temperature as an Analog of Photosystems I and II.

Illumination at low temperatures of ethanol solutions of p-benzoquinone, alone or in the presence of chlorophyll or pheophytin, leads to the reversible formation of radical complexes. Based on g-value determinations and deuterium-substitution effects, the radicals are identified as a semiquinone and an ethoxy radical. In the porphyrin-sensitized system, ternary complexes are produced. Chlorophyll also forms a cation radical upon illumination, whereas pheophytin does not. The generation of this cation radical is independent of the presence of quinone, and proceeds via the chlorophyll singlet state, as opposed to quinone and ethoxy radical production, which involve triplet states. The significance of these reactions to photosynthetic energy conversion is discussed.