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.

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.