Mechanisms of quinol oxidation in photosynthesis

The mechanisms by which para-benzoquinols can be oxidized is reviewed. Emphasis is placed on the information available from chemical and electrochemical studies which may provide insight into the biochemical mechanisms of plastoquinol oxidation in the chloroplast. Three mechanisms of quinol oxidation are possible: (1) The removal of an electron from the quinol, QH_inf2^sup·t, directly to produce the radical cation, QH₂^·+. This may be achieved electrochemically only at very high potential in acidic media. The reaction may be of relevance to D₁, the donor to P-680. (2) The removal of an electron from the anionic quinol. QH^–, formed by quinol deprotonation. It is likely that the catalytic mechanism of the cytochrome bf complex involves this mechanism. (3) The removal of an electron from the dianionic quinol, Q^2–. This route will be dominant only under basic or aprotic conditions and at very low potentials.