Two-Dimensional Electronic Spectroscopy of the Far-Red-Light Photosystem II Reaction Center.

Understanding the role of specific pigments in primary energy conversion in the photosystem II (PSII) reaction center has been impeded by the spectral overlap of its constituent pigments. When grown in far-red light, some cyanobacteria incorporate chlorophyll- and chlorophyll- into PSII, relieving the spectral congestion. We employ two-dimensional electronic spectroscopy to study PSII at 77 K from sp. PCC 7335 cells that were grown in far-red light (FRL-PSII). We observe the formation of a radical pair within ∼3 ps that we assign to ChlP. While Pheo is thought to act as the primary electron acceptor in PSII from cells grown in visible light, we see no evidence of its involvement, which we attribute to its reduction by dithionite treatment in our samples. Our work demonstrates that primary charge separation occurs between Chl and P in FRL-PSII, suggesting that P/P may play an underappreciated role in PSII's charge separation mechanism.