A comparison is made between the PQA → P+Q− A and PQAQB → P+QAQ− B transitions in Rps. viridis and Rb. sphaeroides reaction centers (RCs) by the use of light-induced Fourier transform infrared (FTIR) difference spectroscopy. In Rb. sphaeroides RCs, we identify a signal at 1650 cm−1 which is present in the P+QA-minus-PQA spectrum and not in the P+QAQ− B-minus-PQAQB spectrum. In contrast, this signal is present in both P+Q− A-minus-PQ− A and P+QAQ− B-minus-PQAQB spectra of Rps. viridis RCs. These data are interpreted in terms of a conformational change of the protein backbone near QA (possible at the peptide C≡O of a conserved alanine residue in the QA pocket) and of the different bonding interactions of QB with the protein in the RC of the two species.
