Surface-Enhanced Resonance Raman Scattering Spectroscopy of Photosystem II Pigment-Protein Complexes

Three different photosystem II (PSII) pigment-protein complexes (D1-D2-Cyt b₅₅₉-CP47, D1-D2-Cyt b₅₅₉, and CP47) isolated from spinach were studied by surface-enhanced resonance Raman scattering (SERRS) spectroscopy. Surface-enhanced Raman scattering (SERS) is a distance sensitive (on a 5-10-A scale) spectroscopic tool that can be used to examine structural properties of large biological molecules. It is demonstrated here that SERS can also be used to determine organizational relationships between different pigment-protein complexes. Strong SERRS spectra from the above PSII complexes before and after treatment with sodium dithionite were obtained on roughened Ag electrodes and in citrate-reduced Ag colloids. The D1-D2-Cyt b₅₅₉ complex adsorbs with the Cyt b₅₅₉ heme close to the surface in the colloid, whereas the complex adsorbs differently on the Ag electrode due to the differing surface properties of the two types of substrates. An analysis of the SERRS spectra led to the following conclusions: CP47 binds next to Cyt b₅₅₉ in the D1-D2-Cyt b₅₅₉-CP47 complex and covers the heme, the Cyt b₅₅₉ heme is located closer to one side of the complex (the stromal side in the intact thylakoid membrane), and both Chl and β-carotene molecules are located closer to the opposite side of the complex. Assuming a barrel-shape structure for the pigment-protein complexes, the results imply that the complexes are oriented with the central axis perpendicular to the metal surface both in the colloids and on the electrodes.