Characterization of the photoreduction of the secondary quinone Q(B) in the photosynthetic reaction center from Rhodobacter capsulatus with FTIR spectroscopy

The photoreduction of the secondary quinone acceptor Q(B) in reaction cente rs (RCs) of the photosynthetic bacteria Rhodobacter (Rb.) capsulatus has be en investigated by light-induced FTIR difference spectroscopy in (H2O)-H-1 and (H2O)-H-2. The Q(B)(-)/Q(B) 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 c onservation of most of the amino acids near Q(B) in the RCs from Rb, capsul atus and Rb. sphaeroines, several protein and quinone IR bands are common t o 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 fo r 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 cha in 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 R b. sphaeroides suggesting slight differences in the structural organization of side chains and/or ordered water molecules near Q(B) (C) 1999 Elsevier Science B.V. All rights reserved.