Primary photophysical processes in photosystem II: bridging the gap between crystal structure and optical spectra.

This Minireview summarizes our current knowledge of the optical properties of photosystem II (PS-II) and how these properties are related to the photosynthetic function, that is, excitation energy transfer from the antenna complexes to the reaction center (RC) and the subsequent transmembrane charge separation in the latter. Interpretation of the optical spectra of PS-II is much more difficult than for the RC of purple bacteria, due to the "spectral congestion" problem, namely, the strong spectral overlap of optical bands in PS-II. Recent developments in deciphering the optical properties of the pigments in PS-II, the identification of functional states, and the kinetic details of the primary excitation energy and charge-transfer reactions are summarized. The spectroscopic term P(680) that is generally used in the literature no longer indicates the same entity in its cationic and singlet excited form but different subsets of the six innermost pigments of the RC. The accessory chlorophyll Chl(D1) forms a sink for singlet excitation and triplet energy and most likely represents the primary electron donor in PS-II. In this respect, a special chlorophyll monomer in PS-II plays the role of the special pair in purple bacteria. Evidence that exciton transfer between the core antenna complexes CP43 and CP47 and the RC is the bottleneck for the overall photochemical trapping of excitation energy in PS-II is discussed. A short summary is provided of PS-II of Acaryochloris marina, which mainly contains chlorophyll d instead of the usual chlorophyll a. This system does not suffer from the spectral congestion problem and, therefore, represents an interesting model system. The final part of this Minireview provides a discussion of challenging problems to be solved in the future.