A deeper understanding of the mechanisms involved in natural photosynthesis could inspire new artificial photosynthetic systems and bio-engineered organisms. Study of photosystem II, a pigment-protein complex responsible for the first step of solar energy conversion in oxygenic photosynthesis, is therefore driven by the need for clean energy and robust crops.

Following excitation of the reaction centre of photosystem II, electron transfer proceeds selectively up one branch (the D1 branch) of the pseudo-C2-symmetric reaction centre producing a highly oxidative charge separated state, capable of abstracting electrons from substrate water. The structural reasons for the D1 branch preference are unclear, and remain difficult to probe due to the complexity of the system and its spectral features. Finding these will necessarily rely on detailed modelling, the challenge of which can finally be met with sufficient computational resource using Density Functional Theory.

Results of Time Dependent Density Functional Theory calculations of excited states of the Photosystem II reaction centre are presented. It was found that pigment phytol chains and surrounding amino acid residues must be explicitly included in any model in order for the D1 branch preference to be reproduced. Building on this insight, results of calculations on mutant models of the same size are presented. A qualitative agreement with experimental shifts in simulated absorption difference spectra is achieved for seven out of ten mutant reaction centre models. Including further regions of the reaction centre was found to exaggerate the D1 branch preference further but overall larger models were in agreement with the benchmark model results.

This work takes steps towards a working model of the reaction centre that could be used to investigate the individual structural features which lead to the D1 branch preference and as a predictive tool for the action of specific mutations.