Stability of Isolated Bacterial and Photosystem II Reaction Center Complexes on Ag Electrode Surfaces, A Surface-Enhanced Resonance Raman Study

Surface-enhanced resonance Raman scattering (SERRS) spectra from isolated Rhodospirillum rubrum bacterial reaction center complexes indicate that the carotenoid, spirilloxanthin, resides in the cis conformation, while SERRS spectra from isolated photosystem II (PSII) D1-D2-cytochrome b-559 reaction center complexes show that β-carotene is in the all-trans conformation. The fact that the cis conformation of the native bacterial reaction center carotenoid is maintained during SERRS experiments suggests that significant denaturation of the protein matrix does not occur as a result of contact with the anodized Ag metallic surface required for observation of SERRS spectra. Although a similar conclusion cannot be drawn from observations of β-carotene in PSII reaction centers, no new spectral bands previously ascribed to denatured cytochrome (Cyt) heme groups are observed with Cyt 6-559 during SERRS experiments. Specific enhancement of SERRS signals from Cyt b-559 compared to the other chromophores of the PSII reaction center and in particular observation of a strong 1360-cm−1 band indicate that the heme group is located on or very close to the surface of the PSII reaction center complex and can be reduced by the Ag electrode.