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

Abstract

The absorption frequencies of the C  O and C  C (neutral state) and of the CO (semiquinone state) stretching vibrations of Q B have been assigned by FTIR spectroscopy, using native and site-specifically 1-, 2-, 3- and 4- 13C-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.