FTIR spectroscopy of the reaction center of Chloroflexus aurantiacus: Photoreduction of the bacteriopheophytin electron acceptor

Abstract

Mid-infrared spectral changes associated with the photoreduction of the bacteriopheophytin electron acceptor H A in reaction centers (RCs) of the filamentous anoxygenic phototrophic bacterium Chloroflexus ( Cfl.) aurantiacus are examined by light-induced Fourier transform infrared (FTIR) spectroscopy. The light-induced H A −/H A FTIR (1800–1200 cm − 1 ) difference spectrum of Cfl. aurantiacus RCs is compared to that of the previously well characterized purple bacterium Rhodobacter ( Rba.) sphaeroides RCs. The most notable feature is that the large negative IR band at 1674 cm − 1 in Rba. sphaeroides R-26, attributable to the loss of the absorption of the 13 1-keto carbonyl of H A upon the radical anion H A − formation, exhibits only a very minor upshift to 1675 cm − 1 in Cfl. aurantiacus. In contrast, the absorption band of the 13 1-keto C = O of H A − is strongly upshifted in the spectrum of Cfl. aurantiacus compared to that of Rba. sphaeroides (from 1588 to 1623 cm − 1 ). The data are discussed in terms of: (i) replacing the glutamic acid at L104 in Rba. sphaeroides R-26 RCs by a weaker hydrogen bond donor, a glutamine, at the equivalent position L143 in Cfl. aurantiacus RCs; (ii) a strengthening of the hydrogen-bonding interaction of the 13 1-keto C = O of H A with Glu L104 and Gln L143 upon H A − formation and (iii) a possible influence of the protein dielectric environment on the 13 1-keto C = O stretching frequency of neutral H A. A conformational heterogeneity of the 13 3-ester C = O group of H A is detected for Cfl. aurantiacus RCs similar to what has been previously described for purple bacterial RCs.