Near-infrared-excitation resonance Raman spectra of bacterial photosynthetic reaction centers. Implications for path-specific electron transfer.

The first near-infrared (Qy)-excitation resonance Raman spectrum of photosynthetic reaction centers (Rb. sphaeroides wild type) is reported. This spectrum exhibits features which are not observed with excitation into either the Qx or Soret absorption bands. The spectral data indicate that the partial double-bond character is induced in the C9C10 bond of the isocyclic ring of one of the pigments via interactions with the protein. It is proposed that this modified pigment is the L-subunit bacteriopheophytin and that the preference for electron transfer to this molecule could be in part due to the change in electronic structure induced by the site-specific pigment-protein interaction.