Intramolecular Vibrations Complement the Robustness of Primary Charge Separation in a Dimer Model of the Photosystem II Reaction Center

TL;DR

This study combines quantum theories and chemical calculations to show that intramolecular vibrations, while individually minor, collectively enhance the robustness and speed of primary charge separation in photosystem II, despite environmental disorder.

Contribution

It reveals that intramolecular vibrations collectively support rapid and robust charge separation in photosystem II, challenging previous views on their role.

Findings

Vibrational modes have a minor individual impact on charge separation.

Small vibrational contributions accumulate to significantly influence the rate.

Charge separation occurs in sub-picoseconds, largely independent of external conditions.

Abstract

The energy conversion of oxygenic photosynthesis is triggered by primary charge separation in proteins at the photosystem II reaction center. Here, we investigate the impacts of the protein environment and intramolecular vibrations on primary charge separation at the photosystem II reaction center. This is accomplished by combining the quantum dynamic theories of condensed phase electron transfer with quantum chemical calculations to evaluate the vibrational Huang-Rhys factors of chlorophyll and pheophytin molecules. We report that individual vibrational modes play a minor role in promoting the charge separation, contrary to the discussion in recent publications. Nevertheless, these small contributions accumulate to considerably influence the charge separation rate, resulting in sub-picosecond charge separation almost independent of the driving force and temperature. We suggest that the intramolecular vibrations complement the robustness of the charge separation in the photosystem II reaction center against the inherently large static disorder of the involved electronic energies.