Kinetics of Electron Transfer in RC Protein from Rhodobacter sphaeroides

3 Kinetics of Electron Transfer in RC Protein from Rhodobacter Sphaeroides 

Fio 12 Flash induced fBChl)J formation and decay in Rhodopseudomonas sphaeroides reaction centre protein (A) - for native RC, (B) - for RC, reconstituted with ubiquinone-7. T = 14 K. From Ref. [258] 

Eleven 9,10-anthraquinones with various substituents, seven 1,4-naphthoquinones, 1,2-naphthaquinone and five 1,4-benzoquinones were used as QA. These quinones provide a series of RCs with a variation of the reaction exothermicity, - AG , from 0.11 to 0.9 eV. The rates of intraprotein electron transfer from various Qa to (BChl)J were found to be virtually temperature independent from 5 to 100 K and to decrease severalfold from 100 to 300 K. Only a small change of the rate upon the — AG° variation was found when reaction was made more exothermic than in the native RC. As the reaction was made less exothermic, the rate decreased notably without becoming temperature dependent. 

A study of long-range electron transfer from BPh to QA in both the native and reconstructed RCs was reported in Ref. [259]. The rate of electron transfer from BPh to QA was determined at 14 K, 35 K, 113 K and 298 K. The majority of quinones used for reconstruction has in situ polarographic midpoints lower than that for UQ10. The electron transfer rate was determined from EPR measurements of the quantum yield of (BChl)2 QA radical-ion pair. 

The character of the rate constant vs — AG° dependence observed in Ref. [259] for electron transfer from BPh to QA was quantitatively explained in terms of Eq. (23) assuming that (1) vibrations with ha 100 meV are coupled to electron transfer, the total reorganization energy, Er for these vibrations being 600 + 100 eV (2) vibrations with fico 15 meV are coupled to electron transfer (3) Er for the vibrations with ha 1 meV being less than 300 meV. 

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