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 pheophytins (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 P_D1 and P_D2 is weaker (|J(P_D1/P_D2)| = 79 cm^-1), irrespective of a short edge-to-edge distance of 3.6 Å (Mg-to-Mg distance of 8.1 Å), than the couplings between P_D1 and the accessory Chl_D1 (|J(P_D1/Chl_D2)| = 104 cm^-1) and between P_D2 and Chl_D2 (|J(P_D2/Chl_D1)| = 101 cm^-1), suggesting that P_D1 and P_D2 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 spectroscopic 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 "hub" that 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 Chl_D2/Pheo_D2 and from CP43 to Chl_D1/Pheo_D1 in the reaction center. Thus, the excitation energy from both CP47 and CP43 can always be utilized for the charge-separation reaction in the reaction center.