Photosystem II oxidises water during photosynthesis, via a powerfully oxidising radical-pair state generated in its reaction centre. A better understanding of this natural process will aid in the design of future solar technology and bio-engineered reaction centres. The reaction centre comprises six pigments, arranged pseudo-symmetrically in two branches and embedded in a protein matrix. Following initial excitation of the reaction centre, charge transfer occurs preferentially down one branch (the ‘active’ branch) resulting in a highly-oxidising charge-separated state. Identifying the excited states and subsequent electron-transfer intermediates of this process using traditional spectroscopic methods has been a significant challenge due to large spectral overlap. In an effort to overcome this, many computational models of the reaction centre have been made. However, existing models use large approximations for the reaction centre pigments, or have treated too small a region of the system, yielding an incomplete picture of the excited states. We present the results of robust time-dependent density functional theory calculations on the largest model of the reaction centre to date. Our model is built from the crystal structure of T. Vulcanus and uses long-range corrected functionals wB97X-D and CAM-b3LYP. Both wild-type and mutant reaction centres have been modelled and our results have been validated through comparison to mutant and wild-type absorption spectra. These results allow us to closely observe three different low-lying excited states with charge-transfer character, each in a configuration that would lead to successful charge-separation. We find that our model mimics the behaviour of the natural reaction centre in which charge transfer is directed preferentially down the ‘active’ branch. Furthermore, experimentally observed shifts in mutant absorption bands are successfully reproduced. This work provides new insight into the nature of the excited states in the Photosystem II reaction centre which are precursors to charge separation.

Excited states of the photosystem II reaction center

2019 Fall National Meeting

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