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

Mid-infrared spectral changes associated with the photoreduction of the bacteriopheophytin electron acceptor HA 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 HA−/HA 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 131-keto carbonyl of HA upon the radical anion HA− formation, exhibits only a very minor upshift to 1675 cm− 1 in Cfl. aurantiacus. In contrast, the absorption band of the 131-keto C = O of HA− 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 131-keto C = O of HA with Glu L104 and Gln L143 upon HA− formation and (iii) a possible influence of the protein dielectric environment on the 131-keto C = O stretching frequency of neutral HA. A conformational heterogeneity of the 133-ester C = O group of HA is detected for Cfl. aurantiacus RCs similar to what has been previously described for purple bacterial RCs.

► FTIR spectroscopy studies of the bacteriopheophytin electron acceptor HA in reaction centers from Cfl. aurantiacus.
► A weak hydrogen bond between the anionic state HA− and Gln L143 is shown.
► A strengthening of the hydrogen bond in the HA− state is proposed.
► A possible influence of a dielectric environment on the frequency of the keto carbonyl of neutral HA is discussed.
► Comparisons are made to purple bacteria and Photosystem II.