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

Abstract

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 4‐¹³C‐labelled ubiquinone‐10 (UQ₁₀) 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 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 Q_A The FTIR experiments show that different C = O protein interactions contribute to the factors determining the different functions of UQ₁₀ at the Q_A and the Q_B binding sites.