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Abstract

Photosystem II (PSII) is an essential protein complex involved in the photosynthetic processes that depend on sunlight. The function of photosystem II is greatly influenced by the redox characteristics of its cofactors. However, it might be difficult to undertake experimental studies
of these features due to the intricacy of the protein matrix. In this computational study, the density functional theory (DFT) with QM/MM technique was used to examine the redox characteristics of cofactors in PSII. The calculated parameters included vertical electron affininity, vertical ionisation energy and first excitation energy of cofactors. All calculations made use of ORCA 5.0’s stable version. The range-separated functional was used for these calculations.
The Hartree–Fock (HF) exchange was also observed to affect the redox characteristics. The results demonstrated that the redox characteristics are significantly influenced by the electrostatic environment of protein of the PSII reaction centre. Only the D1 branch of the reaction centre is redox active because of the asymmetry in redox characteristics between the D1 and
D2 branches driven on by the intrinsic protein environment. The redox characteristics observed provide strong support for the idea that accessory chlorophyll of the D1 branch is the principle
electron donor and pheophytin of the D1 branch is the primary electron acceptor. It was found that the change in oxidation state of the oxygen evolving complex (OEC) and plastoquinone (QA) tuned the redox characteristics to prevent charge recombination of reaction centre “P680”.
The trends of vertical ionisation energy and electron affinity were shown to be consistent with published values of the site energies and redox potential.