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