* To whom correspondence should be addressed.
Received September 14, 2011; Revision received October 6, 2011
Photochemical oxidation of the primary electron donor P in reaction centers (RCs) of the filamentous anoxygenic phototrophic bacterium Chloroflexus (C.) aurantiacus was examined by light-induced Fourier transform infrared (FTIR) difference spectroscopy at 95 K in the spectral range of 4000-1200 cm–1. The light-induced P+QA–/PQA IR spectrum of C. aurantiacus RCs is compared to the well-characterized FTIR difference spectrum of P photooxidation in the purple bacterium Rhodobacter (R.) sphaeroides R-26 RCs. The presence in the P+QA–/PQA FTIR spectrum of C. aurantiacus RCs of specific low-energy electronic transitions at ~2650 and ~2200 cm–1, as well as of associated vibrational (phase-phonon) bands at 1567, 1481, and 1294-1285 cm–1, indicates that the radical cation P+ in these RCs has dimeric structure, with the positive charge distributed between the two coupled bacteriochlorophyll a molecules. The intensity of the P+ absorbance band at ~1250 nm (upon chemical oxidation of P at room temperature) in C. aurantiacus RCs is approximately 1.5 times lower than that in R. sphaeroides R-26 RCs. This fact, together with the decreased intensity of the absorbance band at ~2650 cm–1, is interpreted in terms of the weaker coupling of bacteriochlorophylls in the P+ dimer in C. aurantiacus compared to R. sphaeroides R-26. In accordance with the previous (pre)resonance Raman data, FTIR measurements in the carbonyl stretching region show that in C. aurantiacus RCs (i) the 131-keto C=O groups of PA and PB molecules constituting the P dimer are not involved in hydrogen bonding in either neutral or photooxidized state of P and (ii) the 31-acetyl C=O group of PB forms a hydrogen bond (probably with tyrosine M187) absorbing at 1635 cm–1. Differential signals at 1757(+)/1749(–) and 1741(+)/1733(–) cm–1 in the FTIR spectrum of C. aurantiacus RCs are attributed to the 133-ester C=O groups of P in different environments.
KEY WORDS: Chloroflexus aurantiacus, reaction center, primary electron donor, photooxidation, FTIR spectroscopy, Rhodobacter sphaeroides