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 P-A and P-B 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 P-B to P-A 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 P-A as a chlorophyll epimer.

Automated summary

This study reports the origin of A-branch electron transfer in Photosystem I, focusing on the electronic coupling between pigments and the electrostatic interaction with protein environments. The asymmetry of electron-transfer activity is found to originate from P-A as a chlorophyll epimer.