### Tocopherol Synthesis
#### 2.1.2 Activity of Isoprenoids towards ROS
This is especially relevant to chloroplasts, where the main way of \({}^{1}\)O\({}_{2}\) generation is the energy transfer from the chlorophyll in triplet state (\({}^{3}\)Chl) to O\({}_{2}\). That process primarily occurs in PS II of the photosynthetic electron-transport chain in thylakoids [90; 91]. The energy of \({}^{3}\)Chl is approximately 1.3 eV, allowing the molecular oxygen to be converted into \({}^{1}\)O\({}_{2}\) (\(\sim\)1 eV). PQH\({}_{2}\) neutralizes \({}^{1}\)O\({}_{2}\)[86; 92; 93] owing to either chemical or physical mechanisms with a rate constant of approximately 10\({}^{8}\) M\({}^{-1}\) s\({}^{-1}\)[94; 95]. The physical mechanism of quenching is based on the energy transfer from \({}^{1}\)O\({}_{2}\) to PQH\({}_{2}\), resulting in conversion of \({}^{1}\)O\({}_{2}\) back to O\({}_{2}\), while the chemical mechanism of quenching is due to oxidation of PQH\({}_{2}\) (presumably of its isoprenoid chain), resulting in generation of the plastoquinone derivatives, hydroxylastoquinones. The ability to quench \({}^{1}\)O\({}_{2}\) is also typical for other prerylquinols, including tococochromands such as tocopherols, toocotrienols, and plastochromanol [95]. Tocopherols in organic solvents can neutralize both O\({}_{2}\)\({}^{\bullet-}\) with the rate constant of 10\({}^{6}\) M\({}^{-1}\) s\({}^{-1}\)[96] and \({}^{1}\)O\({}_{2}\) with the rate constant of 10\({}^{8}\) M\({}^{-1}\) s\({}^{-1}\)[94], as well as scavenge OH\({}^{\bullet}\)[97] and decompose H\({}_{2}\)O\({}_{2}\)[98]. Although the reaction of UQH\({}_{2}\) with \({}^{1}\)O\({}_{2}\) is less studied, it can be assumed that this activity is also characteristic of ubihydroquinone and both mechanisms, physical and chemical, can be applied. Quinones may not only directly quench \({}^{1}\)O\({}_{2}\) but also quench \({}^{3}\)Chl, therefore preventing the formation of \({}^{1}\)O\({}_{2}\)[99]. Carotenoids, the tetraterpenoid organic pigments, exert an antioxidant activity through the same pathway, i.e., due to their ability to quench both \({}^{1}\)O\({}_{2}\) and \({}^{3}\)Chl. Carotenoids are subdivided into two main groups. The first group is xanthophyls, which are the oxygenated carotenoids. The representatives of this group in thylakoids are violaxanthin, zeaxanthin, lutein, neoxanthin, fucoxanthin, etc. The second group is carotenes; in thylakoids these are \(\beta\)-carotene and lycopene. Quenching of \({}^{3}\)Chl by carotenoids mainly proceeds in the antenna system of PS II [100; 101]. [PERSON] et al. [102] showed that the lack of lutein and zeaxanthin resulted in a higher generation of \({}^{1}\)O\({}_{2}\) in thylakoids. [PERSON] [100] suggested that the main function of \(\beta\)-carotene of the PS II reaction center is the quenching of \({}^{1}\)O\({}_{2}\) if it is generated there. Carotenoids, similar to quinones, quench singlet oxygen by physical and chemical mechanisms, and it has been shown that chemical quenching is much weaker than physical quenching [103]. The rate constants of physical quenching of \({}^{1}\)O\({}_{2}\) by \(\beta\)-carotene and zeaxanthin were estimated to be 7 \(\times\) 10\({}^{9}\) M\({}^{-1}\) s\({}^{-1}\) and of 7 \(\times\) 10\({}^{8}\) M\({}^{-1}\) s\({}^{-1}\) for fucoxanthin, while the rate constants of chemical quenching for \(\beta\)-carotene and zeaxanthin were approximately 4 \(\times\) 10\({}^{6}\) M\({}^{-1}\) s\({}^{-1}\) and 3 \(\times\) 10\({}^{5}\) M\({}^{-1}\) s\({}^{-1}\) for fucoxanthin [103]. Similar results were presented in [104]. However, there is a series of works in which, using biochemical and biophysical methods, researchers have shown that some carotenoids can be involved in the formation of singlet oxygen themselves; for example, [PERSON] et al. [105] provided the evidence that phytofluene, the uncolored C40 carotenoid with a short chain, effectively generated \({}^{1}\)O\({}_{2}\) under UVA conditions, while it was able to quench \({}^{1}\)O\({}_{2}\) in the dark.