Nature of Asymmetric Electron Transfer in the Symmetric Pathways of Photosystem I

Photosystem I has two active electron-transfer pathways. However, electron transfer occurs primarily along one of the two branches (A-branch) irrespective of the similar protein environments. Here, we report the origin of the A-branch electron transfer, considering the electronic coupling of the pigments and the electrostatic interaction with the protein environments. In the chlorophyll pair [P_AP_B], the electronic coupling between P_A and P_B is large (85 meV) for the highest occupied molecular orbital, forming the electronically coupled dimer [P_AP_B] and serving as an initial electron donor. In contrast, the coupling for the lowest unoccupied molecular orbital is small (15 meV), leading to charge transfer from P_B to P_A upon the [P_AP_B] excitation. The electronic coupling between [P_AP_B] and the accessory chlorophyll in the A-branch is significantly larger than that in the B-branch. These results indicate that the asymmetry of the electron-transfer activity originates from P_A as a chlorophyll epimer.