The absorption frequencies of the C = O and C = C (neutral state) and of the C̲⋯O (semiquinone state) stretching vibrations of QB have been assigned by FTIR spectroscopy, using native and site-specifically 1-, 2-, 3- and 4-13C-labelled ubiquinone-10 (UQ10) reconstituted at the QB binding site of Rhodobacter sphaeroides R26 reaction centres. Besides the main C = O band at 1641 cm−1, two smaller bands are observed at 1664 and 1651 cm−1. The smaller bands at 1664 and 1651 cm−1 agree in frequencies with the 1- and 4-C = O vibrations of unbound UQ10, showing that a minor fraction is loosely and symmetrically bound to the protein. The larger band at 1641 cm−1 indicates symmetric H-bonding of the 1- and 4-C = O groups for the layer fraction of UQ10 but much weaker interaction as for the 4-C = O group of QA The FTIR experiments show that different C = O protein interactions contribute to the factors determining the different functions of UQ10 at the QA and the QB binding sites.
