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Kinetic Phases in the Electron Transfer from P + Q A - Q B to P + Q A Q B - and the Associated Processes in Rhodobacter sphaeroides R-26 Reaction Centers

1998; American Chemical Society; Volume: 37; Issue: 9 Linguagem: Inglês

10.1021/bi971699x

1943-295X

Jiali Li, Dan Gilroy, David M. Tiede, M. R. Gunner,

Porphyrin and Phthalocyanine Chemistry

Electron transfer from P+QA-QB to form P+QAQB- was measured in Rhodobacter sphaeroides R-26 reaction centers (RCs) where the native primary quinone, ubiquinone-10 (UQA), was replaced by 2-methyl-3-phytyl-1,4-naphthoquinone (MQA). The native secondary quinone, UQ-10, was retained as UQB. The difference spectrum of the semiquinone MQA- minus UQB- absorption is very similar to that of MQ- minus UQ- in solution (398−480 nm). Thus, the absorption change provides a direct monitor of the electron transfer from MQA- to UQB. In contrast, when both QA and QB are UQ-10 the spectral difference between UQA- and UQB- arises from electrochromic responses of RC chromophores. Three kinetic processes are seen in the near UV (390−480 nm) and near-IR (740−820 nm). Analysis of the time-correlated spectra support the conclusion that the changes at τ1 ≈ 3 μs are mostly due to electron transfer, electron transfer and charge compensation are mixed in τ2 ≈ 80 μs, while little or no electron transfer occurs at 200−600 μs (τ3) in MQAUQB RCs. The 80-μs rate has been previously observed, while the fast component has not. The fast phase represents 60% of the electron-transfer reaction (398 nm). The activation energy for electron transfer is ΔG ≈ 3.5 kcal/mol for both τ1 and τ2 between 0 and 30 °C. In isolated RCs with UQA, if there is any fast component, it appears to be faster and less important than in the MQA reconstituted RCs.