Phylloquinones redox potential

In photosystem I, the primary electron acceptor is a monomeric chlorophyll called A. The secondary electron acceptor is a bound phylloquinone (abbreviated as OQ in this book), whose in situ quinone/ semiquinone redox potential has been estimated to be < -0.81 V. Phylloquinone has the same phytol side... 

Fig. 15. Plot ofthe extent of absorbance change due to P7007P430" recombination measured at 695 nm vs. the redox potential of 14 quinones and 7 non-quinone carbonyl compounds, the fluorenones (individual compounds are identified below the plot). The solid curve is the theoretical, one-electron Nernst curve centered near the redox potential of FeS-X in vivo. Data adapted from Itoh and Iwaki (1992) Exchange ofthe acceptor phylloquinone by artificial quinones and fluorenones in green plant photosystem I photosynthetic reaction center. In N Malaga, T Okada and H Masuhara (eds) Dynamics and Mechanism of Photoinduced Transfer and Related Phenomena. p.533. Elsevier,...

Phylloquinone has become a focus of a variety of structure function relationships due to its central role in electron transfer. The PhQ molecules can be extracted from PS I using dry or water-saturated diethyl ether o or using hexane containing 0.3% methanolWater-saturated ether extraction also removes a significant number of antenna chlorophylls, and aU of the carotenoids (and probably Hpids), while leaving the iron-sulfur clusters unaffected. As might be expected, both extraction procedures block room-temperature electron transfer from Aq to the iron-sulfur clusters. - Reconstitution of one PhQ fully restores electron transfer from A to the iron-sulfur clusters. Iwaki and coworkers reported that PhQ could be replaced by a variety of quinones with appropriate redox potentials. Moreover, an assortment of non-native quinones can be inserted into the PhQ binding site in In particular. 

Vitamin K (phylloquinone) structure contains quinone groups that can be electrochenti-cally reduced to hydroquinone counterparts. Therefore, Koivu et al. [110-113] employed isocratic reversed-phase HPLC-ED schemes for vitamin K determination in digested extracts of different foods. They used Coulochem cells in the redox mode with the first cell unit at a large negative reduction potential and the second unit detecting the vitamin K reduction products at very low potential. 

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