Structural Dynamics of the Oxygen-Evolving Complex of Photosystem II in Water-Splitting Action

Oxygenic photosynthesis in nature occurs via water splitting catalyzed by the oxygen-evolving complex (OEC) of photosystem II. To split water, the OEC cycles through a sequence of oxidation states (S _i, i = 0-4), the structural mechanism of which is not fully understood under physiological conditions. We monitored the OEC in visible-light-driven water-splitting action by using in situ, aqueous-environment surface-enhanced Raman scattering (SERS). In the unexplored low-frequency region of SERS, we found dynamic vibrational signatures of water binding and splitting. Specific snapshots in the dynamic SERS correspond to intermediate states in the catalytic cycle, as determined by density functional theory and isotopologue comparisons. We assign the previously ambiguous protonation configuration of the S₀-S₃ states and propose a structural mechanism of the OEC's catalytic cycle. The findings address unresolved questions about photosynthetic water splitting and introduce spatially resolved, low-frequency SERS as a chemically sensitive tool for interrogating homogeneous catalysis in operando.

Langmuir-Blodgett and X-ray diffraction studies of isolated photosystem II reaction centers in monolayers and multilayers: physical dimensions of the complex

The photosystem II (PSII) reaction center (RC) is a hydrophobic intrinsic protein complex that drives the water-oxidation process of photosynthesis. Unlike the bacterial RC complex, an X-ray crystal structure of the PSII RC is not available. In order to determine the physical dimensions of the isolated PSII RC complex, we applied Langmuir techniques to determine the cross-sectional area of an isolated RC in a condensed monolayer film. Low-angle X-ray diffraction results obtained by examining Langmuir-Blodgett multilayer films of alternating PSII RC/Cd stearate monolayers were used to determine the length (or height; z-direction, perpendicular to the plane of the original membrane) of the complex. The values obtained for a PSII RC monomer were 26 nm2 and 4.8 nm, respectively, and the structural integrity of the RC in the multilayer film was confirmed by several approaches. Assuming a cylindrical-type RC structure, the above dimensions lead to a predicted volume of about 125 nm3. This value is very close to the expected volume of 118 nm3, calculated from the known molecular weight and partial specific volume of the PSII RC proteins. This same type of comparison was also made with the Rhodobacter sphaeroides RC based on published data, and we conclude that the PSII RC is much shorter in length and has a more regular solid geometric structure than the bacterial RC. Furthermore, the above dimensions of the PSII RC and those of PSII core (RC plus proximal antenna) proteins protruding outside the plane of the PSII membrane into the lumenal space as imaged by scanning tunneling microscopy (Seibert, Aust. J. Pl. Physiol. 22, 161-166, 1995) fit easily into the known dimensions of the PSII core complex visualized by others as electron-density projection maps. From this we conclude that the in situ PSII core complex is a dimeric structure containing two copies of the PSII RC.