### Tocopherol Synthesis
#### 2.1.2 Activity of Isoprenoids towards ROS  
Quinones, both plastoquinone, the mobile electron carrier in the photosynthetic electron-transport chain of the chloroplast, and ubiquinone, the mobile electron carrier in the respiratory electron-transport chain of the mitochondria, possess efficient antioxidant activity. The fully reduced PQ, plastohydrodynamic (PQH\({}_{2}\)), and UQ, bihydroquinone (UQH\({}_{2}\)), neutralize superoxide anion radical (O\({}_{2}\)\({}^{\bullet-}\)), protecting against lipid peroxidation of the membranes during oxidative stress conditions of both abiotic and biotic nature [73; 74; 75; 76; 77]. It is known that O\({}_{2}\)\({}^{\bullet-}\), especially in the protonated state (perhydroxyl radical, HOO\({}^{\bullet}\)) initiates lipid peroxidation [76; 78]. O\({}_{2}\)\({}^{\bullet-}\) is the primary singly reduced product of molecular oxygen reduction. In thylakoids, O\({}_{2}\)\({}^{\bullet-}\) is generated at the level of PS I outside the membrane by F\({}_{A}\)/F\({}_{B}\) terminal clusters and to a high extent inside the thylakoid membranes by phyllosemiaquinone (the singly reduced phyllquinone or vitamin K) [4; 79] presumably in A\({}_{1}\) sites under stress conditions [80]. Moreover, O\({}_{2}\)\({}^{\bullet-}\) can be produced in the plastoquinone pool (PQ pool) [81] as well as in some other complexes of the chain under specific conditions (reviewed in [80]. In the PQ pool, O\({}_{2}\)\({}^{\bullet-}\) is produced in the reaction of singly reduced PQ, plastosemiquinone (PQ\({}^{\bullet-}\)), with molecular oxygen. Presumably, PQ\({}^{\bullet-}\) appears in the PQ pool owing to comproportionation of PQ and PQH\({}_{2}\) molecules rather than as the result of PQH\({}_{2}\) oxidation in the plastohydroquinone-oxidizing site of cytochrome _b6/f_ complex (reviewed in [82]). In the mitochondria, the electron transfer to molecular oxygen with generation of O\({}_{2}\)\({}^{\bullet-}\) can proceed in complex I and complex III. In complex I, such a reducing agent is presumably reduced flavin mononucleotide [83], and at the level of complex III, it is believed to be a singly reduced ubiquinone, ubisemiquinone (UQ\({}^{\bullet-}\)), formed as a result of the oxidation of UQH\({}_{2}\) in the quinol-oxidizing site of complex III [84]. Meanwhile, it is possible to predict that in the UQ pool, as in the PQ pool, UQ\({}^{\bullet-}\) may also be produced owing to comproportionation of UQ and UQH\({}_{2}\). Reactions of the reduced quinones with O\({}_{2}\)\({}^{\bullet-}\) were studied in detail; in an aqueous medium, the constant rate of second order reaction is estimated to be around \(10^{8}\) M\({}^{-1}\) s\({}^{-1}\)[85], and around \(10^{4}\) M\({}^{-1}\) s\({}^{-1}\) in acetonitrile [86]. We propose that the reaction of PQH\({}_{2}\) with O\({}_{2}\)\({}^{\bullet-}\) as well as the reaction of UQH\({}_{2}\) with O\({}_{2}\)\({}^{\bullet-}\) occurs predominantly at the membrane/water phase boundary, where the rate constant of \(10^{8}\) M\({}^{-1}\) s\({}^{-1}\) is applicable. We have previously estimated that the equilibrium constant of the reaction of PQH\({}_{2}\) with O\({}_{2}\)\({}^{\bullet-}\) should be equal to or even higher than \(10^{9}\)[82]. The reaction of the reduced quinone with O\({}_{2}\)\({}^{\bullet-}\) results in the formation of H\({}_{2}\)O\({}_{2}\)[77; 87], which possesses a lower reactivity among ROS. It has now been proven that PQH\({}_{2}\) is also able to react with H\({}_{2}\)O\({}_{2}\)[86; 88; 89]; however, the rate constant of the reaction of hydroquinones with H\({}_{2}\)O\({}_{2}\) is rather low, approximately 10\({}^{3}\) M\({}^{-1}\) s\({}^{-1}\)-10\({}^{4}\) M\({}^{-1}\) s\({}^{-1}\) in the phosphate buffer [88]. Another aspect of the antioxidant activity of the reduced quinones is related to their ability to quench singlet oxygen, \({}^{1}\)O\({}_{2}\), which is known to initiate lipid peroxidation as well. \({}^{1}\)O\({}_{2}\) is formed as a result of the spin inversion of one of the unpaired electrons in the O\({}_{2}\) molecule.