The initial charge separation step in oxygenic photosynthesis

TL;DR

This study uses advanced spectroscopic techniques to clarify the initial charge separation process in Photosystem II, revealing the role of specific radical pairs and the initial electron acceptor, advancing understanding of oxygenic photosynthesis.

Contribution

The paper provides direct spectroscopic evidence identifying the mixed exciton-charge transfer state and the initial electron acceptor in PSII-RC, clarifying longstanding debates.

Findings

The mixed exciton-charge transfer state involves Chl_D1^+Phe^- radical pair.

The initial electron acceptor in PSII-RC is Phe, not P_D1.

Photoexcitation directly prepares the radical pair state.

Abstract

Photosystem II is crucial for life on Earth as it provides oxygen as a result of photoinduced electron transfer and water splitting reactions. The excited state dynamics of the photosystem II-reaction center (PSII-RC) has been a matter of vivid debate because the absorption spectra of the embedded chromophores significantly overlap and hence it is extremely difficult to distinguish transients. Here, we report the two-dimensional electronic-vibrational spectroscopic study of the PSII-RC. The simultaneous resolution along both the visible excitation and infrared detection axis is crucial in allowing for the character of the excitonic states and interplay between them to be clearly distinguished. In particular, this work demonstrates that the mixed exciton-charge transfer state, previously proposed to be responsible for the far-red light operation of photosynthesis, is characterized by the Chl$_{\rm D1}^+$Phe$^-$ radical pair and can be directly prepared upon photoexcitation. Further, we find that the initial electron acceptor in the PSII-RC is Phe, rather than P$_{\rm D1}$, regardless of excitation wavelength.