FTIR Spectroscopy of the Reaction Center of Chloroflexus aurantiacus: Photooxidation of the Primary Electron Donor

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A. A. Zabelin, V. A. Shkuropatova, V. A. Shuvalov, and A. Ya. Shkuropatov^*

Institute of Basic Biological Problems, Russian Academy of Sciences, ul. Institutskaya 2, 142290 Pushchino, Moscow Region, Russia; fax: (4967) 330-532; E-mail: ashkur@mail.ru

^* To whom correspondence should be addressed.

Received September 14, 2011; Revision received October 6, 2011

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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⁺Q_A^–/PQ_A 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⁺Q_A^–/PQ_A 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 P_A and P_B­ 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 P_B 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.

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KEY WORDS: Chloroflexus aurantiacus, reaction center, primary electron donor, photooxidation, FTIR spectroscopy, Rhodobacter sphaeroides

DOI: 10.1134/S000629791202006X

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