FTIR SPECTROSCOPY SHOWS WEAK SYMMETRICAL HYDROGEN-BONDING OF THE Q(B)CARBONYL GROUPS IN RHODOBACTER-SPHAEROIDES R26 REACTION CENTERS

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 be en assigned by FTIR spectroscopy, using native and site-specifically 1 -, 2-, 3- and 4-C-13-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 obs erved 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 bou nd to the protein, The larger band at 1641 cm(-1) indicates symmetric H-bonding of the 1- and 4-C = O groups for the larger fraction of UQ(1 0) but much weaker interaction as for the 4-C = O group of Q(A). The F TIR experiments show that different C = O protein interactions contrib ute to the factors determining the different functions of UQ(10) at th e Q(A) and the Q(B) binding sites.