Redox potential plastoquinol

Electron Transport Between Photosystem I and Photosystem II Inhibitors. The interaction between PSI and PSII reaction centers (Fig. 1) depends on the thermodynamically favored transfer of electrons from low redox potential carriers to carriers of higher redox potential. This process serves to communicate reducing equivalents between the two photosystem complexes. Photosynthetic and respiratory membranes of both eukaryotes and prokaryotes contain stmctures that serve to oxidize low potential quinols while reducing high potential metaHoproteins (40). In plant thylakoid membranes, this complex is usually referred to as the cytochrome b /f complex, or plastoquinolplastocyanin oxidoreductase, which oxidizes plastoquinol reduced in PSII and reduces plastocyanin oxidized in PSI (25,41). Some diphenyl ethers, eg, 2,4-dinitrophenyl 2 -iodo-3 -methyl-4 -nitro-6 -isopropylphenyl ether [69311-70-2] (DNP-INT), and the quinone analogues,... 

Note that relative to PQ, plastoquinol [PQH2] has 2 extra H" and 2 extra e s, each of which can be given up one at a time. Furthermore, the two electrons produced by oxidation of plastoquinol at the Qq site do not take the same reaction pathway. The first electron released goes to reduce the high-potential Rieske R-[2Fe 2S] in Fig. 11 (C), while the plastosemiquinone [PQH] that results has a redox potential capable of reducing the low-potential Cyt b(,(JJP) nearby [see the reaction (a)- (b) in Fig. 11 (C)]. Thus,... 

Fig. 13 (B) shows flash-induced absorption-change signals in Rb. sphaeroides chromatophores in the absence and presence of known inhibitors to the Cyt bc complex. Prior to flash excitation, the redox potential of the sample was poised at 100 mV in order to keep R-[2Fe 2S] and the Cyts Cj and C2 in the reduced state and both Cyts (HP) and G P) oxidized [see Fig. 13 (A)]. Redox reactions of Cyt (HP) and Cyts c, and C2 collectively are monitored by absorbance changes induced by flash excitation. At 100 mV, about half of the quinone pool is estimated to be in the reduced state before flash excitation. As shown by the signals in Fig. 13 (B, a), in the absence of inhibitors, flash excitation immediately elicits a rapid, initial oxidation of both Cjds Cj and C2 by the reaction center [Fig. 13 (A, a)]. Cyts c, and C2 are then reduced directly by R-[2Fe 2S] as a result of plastoquinol oxidation at the site. In the meantime, Cyt fe(LP) and Cyt Z G3P) are rapidly reduced and re-oxidized in turn following flash excitation, as a result of turnover at the Qj site.

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