Using 1‐, 2‐, 3‐ and 4‐13C site‐specifically labelled ubiquinone‐10, reconstituted at the QA site of Rhodobacter sphaeroides R26 reaction centres, the infra‐red bands dominated by the 1‐ and 4‐C = O vibration of QA are assigned in the QA(‐)‐QA difference spectra. The mode dominated by the 4‐C = O vibration is drastically downshifted in the reaction centres as compared with its absorption frequency in free ubiquinone‐10. In contrast, the mode dominated by the 1‐C = O vibration absorbs at similar frequencies in the free and the bound forms. The frequency shift of the 4‐C = O vibration is due to a large decrease in bond order and indicates a strong interaction with the protein microenvironment in the ground state. In the charge‐separated state the mode dominated by the semiquinone 4‐C = O vibration is characteristic of strong hydrogen bonding to the microenvironment, whereas the mode dominated by the 1‐C = O vibration indicates a weaker interaction. The asymmetric binding of the 1‐ and 4‐C = O groups to the protein might contribute to the factors governing different redox reactions of ubiquinone‐10 at the QA site as compared with its reactions at the QB site.

The reaction center (RC)-bound primary acceptor quinone QA of the photosynthetic bacterium Rhodobacter sphaeroides R26 functions as a one-electron gate. The radical anion Q~-is proposed to have an asymmetric electron distribution, induced by the protein environment. We replace the native ubiquinone-10 (UQ10) with specifically 13C-labelled UQ10, and use Q-band (35 GI-Iz) EPR spectroscopy to investigate this phenomenon in closer detail. The direct observation of the ~3C-hyperfine splitting of the gz-COmponent of UQ I 0~,-in the RC and in frozen isopropanol shows that the electron spin distribution is symmetric in the isopropanol glass, and asymmetric in the RC. Our results allow qualitative assessment of the spin and charge distribution for Q~-in the RC. The carbonyl oxygen of the semiquinone anion nearest to the S = 2 Fe2+-ion and QB is shown to acquire the highest (negative) charge density.

No abstract

No abstract

Abstract.(1-',C)-, (2-"C)-, (3-"C)-, (3-I3CH3)-, (4-I3C), and ("CH,O),-ubiquinone-10 and the corresponding (1-"C)-, (6-"C)-, (5-I")-, (5-I3CH,)-, (4-"C)-, and (13CH30)2-ubiquinone-0 have been synthesised from simple labelled starting materials via a single reaction scheme. The ubiquinones have been characterised using mass spectrometry, IH NMR and I3C NMR. The spectroscopic results indicate that, within experimental error, the syntheses have been accomplished without scrambling or dilution of label. All labelled ubiquinones have been synthesised on a decigram scale.