ASYMMETRIC BINDING OF THE 1-C=0 AND 4-C=0 GROUPS OF Q(A) IN RHODOBACTER-SPHAEROIDES-R26 REACTION CENTERS MONITORED BY FOURIER-TRANSFORM INFRARED-SPECTROSCOPY USING SITE-SPECIFIC ISOTOPICALLY LABELED UBIQUINONE-10

Using 1-, 2-, 3- and 4-C-13 site-specifically labelled ubiquinone-10, reconstituted at the Q(A) site of Rhodobacter sphaeroides R26 reaction centres, the infra-red bands dominated by the 1- and 4-C=O vibration of Q(A) are assigned in the Q(A)(-)-Q(A) 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 Q(A) site as compared with its reactions at the Q(B) site.