FTIR spectroscopic characterization of the QAand QB binding sites in Rhodobacter sphaeroides R26 reaction centres

Abstract

The QA and the QB binding sites of Rhodobacter sphaeroides R26 reaction centres have been investigated by FTIR spectroscopy, using site specifically 13C-labelled ubiquinone-10. The C=O and C=C (neutral state) and the (Math) and (Math) (semiquinone state) stretching vibrations of QA and QB have been assigned. At the QA site asymmetric binding of the 1- and the 4-C=O groups has been detected. The 4-C=O group is strongly bound to the RC, most probably via a hydrogen bond to His M219. The 1-C=O group is only weakly hydrogen bonded. In the charge separated state the asymmetry is largely maintained. At the QB site two different fractions of ubiquinone-10 have been observed. One fraction (∼25%) has no specific interactions with the protein, the other (∼75%) shows distinct but less strong hydrogen bonding to the RC than the 4-C=O group of QA. Both carbonyl groups are symmetrically bound to the RC at the QB site. In the charge separated state the carbonyl groups of QB - are nearly equivalent and equally hydrogen bonded to the protein. The different C=O protein interactions contribute to the factors determining different functions of ubiquinone-10 at the QA and the QB site, respectively.