Abstract
Photosystem II (PS II) captures solar energy and directs charge separation (CS) across the thylakoid membrane during photosynthesis. The highly oxidizing, charge-separated state generated within its reaction center (RC) drives water oxidation. Spectroscopic studies on PS II RCs are difficult to interpret due to large spectral congestion, necessitating modeling to elucidate key spectral features. Herein, we present results from time-dependent density functional theory (TDDFT) calculations on the largest PS II RC model reported to date. This model explicitly includes six RC chromophores and both the chlorin phytol chains and the amino acid residues <6 Å from the pigments' porphyrin ring centers. Comparing our wild-type model results with calculations on mutant D1-His-198-Ala and D2-His-197-Ala RCs, our simulated absorption-difference spectra reproduce experimentally observed shifts in known chlorophyll absorption bands, demonstrating the predictive capabilities of this model. We find that inclusion of both nearby residues and phytol chains is necessary to reproduce this behavior. Our calculations provide a unique opportunity to observe the molecular orbitals that contribute to the excited states that are precursors to CS. Strikingly, we observe two high oscillator strength, low-lying states, in which molecular orbitals are delocalized over ChlD1 and PheD1 as well as one weaker oscillator strength state with molecular orbitals delocalized over the P chlorophylls. Both these configurations are a match for previously identified exciton-charge transfer states (ChlD1+PheD1-)* and (PD2+PD1-)*. Our results demonstrate the power of TDDFT as a tool, for studies of natural photosynthesis, or indeed future studies of artificial photosynthetic complexes.
Highlights
Photosystem II (PS II) captures solar energy and directs charge separation (CS) across the thylakoid membrane during photosynthesis
excited state (ES) are characterized by a weighted combination of transitions, represented by pairs of occupied and virtual molecular orbitals (MOs), where the weights are given by the computed configuration-interaction coefficients
When comparing models 3 and 4, it can be seen that the energies of the Phe excitations are shifted by addition of phytol chains when the 23 amino acids are present, including the phytol chains raises the energy of PheD2 excitation above PheD1 (Table 1)
Summary
Photosystem II (PS II) captures solar energy and directs charge separation (CS) across the thylakoid membrane during photosynthesis. A photon of sunlight may be absorbed by any of the >250 chlorophylls or carotenoids in the Photosystem II (PS II) complex [1] This energy is rapidly transferred to the reaction center (RC), where a remarkable charge separation (CS) process takes place, resulting in a radical pair species with oxidative ability, which is unparalleled in nature (+1.1 eV) [2]. Histidine (His) residues ligate the two P chlorophylls in the wild-type (W-T) crystal structure [3] These ligands have high polarizability and will stabilize the positive charge on the P chlorophylls during the formation of the primary radical pair state P+PheD1− [11]. The powerfully oxidizing cation of the P+ PheD1− radical pair proceeds to abstract an electron from substrate water, which is Significance
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