SURFACE-ENHANCED RESONANCE RAMAN-SCATTERING SPECTROSCOPY OF PHOTOSYSTEM-II PIGMENT-PROTEIN COMPLEXES

Three different photosystem II (PSII) pigment-protein complexes (D1-D2 -Cyt b(559)-CP47, D1-D2-Cyt b(559), and CP47) isolated from spinach we re studied by surface-enhanced resonance Raman scattering (SERRS) spec troscopy. Surface-enhanced Raman scattering (SERS) is a distance sensi tive (on a 5-10-Angstrom scale) spectroscopic tool that can be used to examine structural properties of large biological molecules. It is de monstrated here that SERS can also be used to determine organizational relationships between different pigment-protein complexes. Strong SER RS spectra from the above PSII complexes before and after treatment wi th sodium dithionite were obtained on roughened Ag electrodes and in c itrate-reduced Ag colloids. The D1-D2-Cyt b(559) complex adsorbs with the Cyt b(559) heme close to the surface in the colloid, whereas the c omplex adsorbs differently on the Ag electrode due to the differing su rface properties of the two types of substrates. An analysis of the SE RRS spectra led to the following conclusions: CP47 binds next to Cyt b (559) in the D1-D2-Cyt b(559)-CP47 complex and covers the heme, the Cy t b(559) heme is located closer to one side of the complex (the stroma l side in the intact thylakoid membrane), and both Chl and beta-carote ne molecules ace located closer to the opposite side of the complex. A ssuming a barrel-shape structure for the pigment-protein complexes, th e results imply that the complexes are oriented with the central axis perpendicular to the metal surface both in the colloids and on the ele ctrodes.