Primary Photophysical Processes in Photosystem II: Bridging the Gap between Crystal Structure and Optical Spectra

Abstract

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.

Photosynthetic reaction center as a quantum heat engine

“…At the moment, there is much evidence that two main pathways make significant contributions under ambient conditions and the lowest energy states depend on disorder (44)(45)(46)(47). Whereas in bacterial reaction centers the primary charge separation takes place at the special pair (as used in this work), the reaction centers of Photosystem II also use an additional pathway which starts at the accessory chlorophyll of the D1 branch (48,49). In this work we discuss only the first pathway, which is present in both types of reaction centers and plays an important role in optimizing the electron transfer efficiency.…”

Quantum Chemical Description of Absorption Properties and Excited‐State Processes in Photosynthetic Systems

“…[468,[470][471][472] In purple bacteria the strong coupling within the special pair leads to an excitation mainly localized on that dimer, while the lowest excited state in PSII is delocalized over all six pigments with significant excitonic coupling within the special pair and between each pigment of the special pair and the neighboring pigment. [471][472][473][474] Other studies indicate that all excited states except those of the special pair can be described as monomeric, [475] or that delocalization occurs mainly in the active branch, whereas the excited states in branch B are mainly localized on individual pigments. [470] Also, the reported relative excitonic couplings for the reaction center in PSII are diverse.…”

Light-harvesting and structural organization of Photosystem II: From individual complexes to thylakoid membrane

“…We will end this review with some open questions and suggestions for future research. We would also like to refer to various other reviews that have appeared in recent years discussing more/other aspects of PSII [2,3,[33][34][35][36][37][38].…”