Quantum mechanical analysis of excitation energy transfer couplings in photosystem II

We evaluated excitation energy transfer (EET) coupling (J) between all pairs of chlorophylls (Chls) and pheophy-tins (Pheos) in the protein environment of photosystem II based on the time-dependent density functional theory with a quantum mechanical/molecular mechanics approach. In the reaction center, the EET coupling between Chls PD1 and PD2 is weaker (|J(PD1/PD2)| = 79 cm-1), irrespective of a short edge-to-edge distance of 3.6 A (Mg-to-Mg distance of 8.1 A), than the couplings between PD1 and the accessory ChlD1 (|J(PD1/ChlD2)| = 104 cm-1) and between PD2 and ChlD2 (|J(PD2/ChlD1)| = 101 cm-1), suggesting that PD1 and PD2 are two monomeric Chls rather than a special pair. There exist strongly coupled Chl pairs (|J| >-100 cm-1) in the CP47 and CP43 core antennas, which may be candidates for the red-shifted Chls observed in spec-troscopic studies. In CP47 and CP43, Chls ligated to CP47-His26 and CP43-His56, which are located in the middle layer of the thylakoid membrane, play a role in the hubthat mediates the EET from the lumenal to stromal layers. In the stromal layer, Chls ligated to CP47-His466, CP43-His441, and CP43-His444 mediate the EET from CP47 to ChlD2/PheoD2 and from CP43 to ChlD1/ PheoD1 in the reaction center. Thus, the excitation energy from both CP47 and CP43 can always be utilized for the charge -sep-aration reaction in the reaction center.

Automated summary

Based on time-dependent density functional theory and quantum mechanical/molecular mechanics approach, we evaluated the excitation energy transfer (EET) coupling (J) between chlorophylls (Chls) and pheophytins (Pheos) in the protein environment of photosystem II. The results showed that the EET coupling between Chls PD1 and PD2 in the reaction center is weaker than the couplings between PD1 and the accessory ChlD1, and between PD2 and ChlD2, suggesting that PD1 and PD2 are monomeric Chls rather than a special pair. Strongly coupled Chl pairs were found in the CP47 and CP43 core antennas. Chls ligated to specific histidine residues play a role in mediating the EET from the lumenal to stromal layers.