FTIR spectroscopy of the photoreduction of the bacteriopheophytin electron acceptor in reaction centers of Rb. sphaeroides and Rps. viridis. Original Research Article

The photoreduction of the bacteriopheophytin electron acceptor HA in reaction centers from Rhodobacter sphaeroides and Rhodopseudomonas viridis has been monitored by light-induced FTIR difference spectroscopy at 10°C, in the presence of reductant and mediator. The striking similarity of the HA−HA 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 HA affects conserved amino acid residues. The HA−HA spectra are interpreted by comparison with model compound spectra of the anion radicals of bacteriopheophytin a and b,a nd by analysis of 1H2H 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 HA. 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 HA−. From 1H2H exchange experiments, it is proposed that the COO1H side chain of Glu L104 contributes to the 1745-1735 cm−1 spectral range with the corresponding COO2H signal displaced to lower frequencies and partly hidden under the 1732 cm−1 band.