Fourier transform Raman investigation of the electronic structure and charge localization in a bacteriochlorophyll-bacteriopheophytin dimer of reaction centers from Rhodobacter sphaeroides

Abstract

The Raman spectra of a bacteriochlorophyll (BChl)-bacteriopheophytin (BPhe) heterodimeric primary electron donor from mutant Rhodobacter sphaeroides reaction centers, where histidine M202 has been replaced by leucine, have been obtained in (pre)resonance with its lowest electronic Q y state using 1064 nm excitation. For reaction centers where the heterodimer is in its reduced, neutral state, the dominant Raman scattering is from the BPhe constituent of the ground state heterodimer, and indicates that the 1064 nm-excitation wavelength is interacting with an electronic state or transition which is largely BPhe in character, such as a charge transfer state with a dominant (BChl +BPhe −) configuration. Previous electron paramagnetic studies have established that the unpaired electron spin density on the oxidized, cation radical heterodimer resides almost totally on the BChl constituent. Near infrared electronic absorption spectra of the oxidized heterodimer exhibit a weak band at 900 nm, characteristic of a BChl a ·+ species. The (pre)resonance Raman spectrum of this cation radical, excited at 1064 nm, exhibits a 1723 cm −1 band attributable to the C 9 keto carbonyl group of the BChl constituent whose corresponding band in wild type is observed at 1715–1717 cm −1. This 1723 cm −1 frequency for BChl in the oxidized heterodimer, compared to 1691 cm −1 for the reduced state, represents an oxidation-induced increase in frequency of +32 cm −1, similar to what is observed for the one-electron oxidation of chlorophylls in non-protic solvents. The results presented here indicate that the oxidation-induced change in vibrational frequency of the C 9 keto carbonyl group of the BChl in the reaction center is not significantly perturbed by the protein.