FTIR spectroscopy shows weak symmetric hydrogen bonding of the Q B carbonyl groups in Rhodobacter sphaeroides R26 reaction centres

The absorption frequencies of the C = O and C = C (neutral state) and of the C̲⋯O (semiquinone state) stretching vibrations of Q B have been assigned by FTIR spectroscopy, using native and site‐specifically 1‐, 2‐, 3‐ and 4‐ 13 C‐labelled ubiquinone‐10 (UQ 10 ) reconstituted at the Q B 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 UQ 10 , 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 UQ 10 but much weaker interaction as for the 4‐C = O group of Q A The FTIR experiments show that different C = O protein interactions contribute to the factors determining the different functions of UQ 10 at the Q A and the Q B binding sites.