Characterization of the photoreduction of the secondary quinone QB in the photosynthetic reaction center from Rhodobacter capsulatus with FTIR spectroscopy

AbstractThe photoreduction of the secondary quinone acceptor QB in reaction centers (RCs) of the photosynthetic bacteria Rhodobacter (Rb.) capsulatus has been investigated by light-induced FTIR difference spectroscopy in 1H2O and 2H2O. The Q−B/QB FTIR spectra reflect reorganization of the protein upon electron transfer, changes of protonation state of carboxylic acid groups, and (semi)quinone–protein interactions. As expected from the conservation of most of the amino acids near QB in the RCs from Rb. capsulatus and Rb. sphaeroides, several protein and quinone IR bands are common to both spectra, e.g., the 1728 cm−1 band is assigned to proton uptake by a carboxylic acid residue, most probably Glu L212 as previously proposed for Rb. sphaeroides RCs. However, noticeable changes are observed at 1709 cm−1 (deprotonation of a Glu or Asp residue), 1674 and 1659 cm−1 (side chain and/or backbone), around 1540 cm−1 (amide II), and in the semiquinone absorption range. This FTIR study demonstrates that the environment of the secondary quinone in Rb. capsulatus is close but not identical to that in Rb. sphaeroides suggesting slight differences in the structural organization of side chains and/or ordered water molecules near QB