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 [PP], the electronic coupling between P and P is large (85 meV) for the highest occupied molecular orbital, forming the electronically coupled dimer [PP] 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 to P upon the [PP] excitation. The electronic coupling between [PP] 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 as a chlorophyll epimer.