FTIR SPECTROSCOPY OF THE PHOTOREDUCTION OF THE BACTERIOPHEOPHYTIN ELECTRON-ACCEPTOR IN REACTION CENTERS OF RHODOBACTER-SPHAEROIDES AND RHODOPSEUDOMONAS-VIRIDIS

The photoreduction of the bacteriopheophytin electron acceptor H-A in reaction centers from Rbodobacter sphaeroides and Rhodopseudomonas viridis has been monitored by light-induced FTIR difference spectroscopy at 10 degrees C, in the presence of reductant and mediator. The striking similarity of the H-A(-)/H-A spectra obtained for Rb. sphaeroides and Rps. viridis reflects comparable interactions of the bacteriopheophytin electron acceptor with the protein in both reaction centers and implies that the photoreduction of H-A affects conserved amino acid residues. The H-A(-)/H-A spectra are interpreted by comparison with model compound spectra of the anion radicals of bacteriopheophytin a and b, and by analysis of H-1/H-2 isotope effects. The downshift of the 1677 cm(-1) mode in Rb. sphaeroides (1681 cm(-1) in Rps. viridis) reaction centers with respect to the model compound is interpreted in terms of a strongly perturbed 9-keto carbonyl of H-A. This perturbation most probably originates from hydrogen bonding to Glu L104. At least part of the positive signal at 1591 cm(-1) in Rb. sphaeroides and at 1601 cm(-1) in Rps. viridis is assigned to the 9-keto carbonyl mode of H-A(-). From H-1/H-2 exchange experiments, it is proposed that the (COOH)-H-1 side chain of Glu L104 contributes to the 1745-1735 cm(-1) spectral range with the corresponding (COOH)-H-2 signal displaced to lower frequencies and partly hidden under the 1732 cm(-1) band.