Primary processes in the bacterial reaction center probed by two-dimensional electronic spectroscopy

Abstract

In the initial steps of photosynthesis, reaction centers convert solar energy to stable charge-separated states with near-unity quantum efficiency. The reaction center from purple bacteria remains an important model system for probing the structure-function relationship and understanding mechanisms of photosynthetic charge separation. Here we perform 2D electronic spectroscopy (2DES) on bacterial reaction centers (BRCs) from two mutants of the purple bacterium , spanning the Q absorption bands of the BRC. We analyze the 2DES data using a multiexcitation global-fitting approach that employs a common set of basis spectra for all excitation frequencies, incorporating inputs from the linear absorption spectrum and the BRC structure. We extract the exciton energies, resolving the previously hidden upper exciton state of the special pair. We show that the time-dependent 2DES data are well-represented by a two-step sequential reaction scheme in which charge separation proceeds from the excited state of the special pair (P*) to PH via the intermediate PB When inhomogeneous broadening and Stark shifts of the B* band are taken into account we can adequately describe the 2DES data without the need to introduce a second charge-separation pathway originating from the excited state of the monomeric bacteriochlorophyll B*.

Why are photosynthetic reaction centres dimeric?

“…P is the dominant acceptor of excitons from antenna complexes and the primary charge donor. [12][13][14] From P, the charges are transferred down either branch in homodimeric RCs (such as heliobacteria 11,15 ) or down only one branch in heterodimers. The pigments involved in the initial chargetransfer step always come from a small set of closely related tetrapyrroles (Chl, BChl, or (bacterio)phaeophytin, (B)Phe).…”

“…Intriguingly, the constituent pigments within P are the only ones in the RC close enough to inuence each other, meaning that the part of the RC most affected by dimerisation is also both the ultimate acceptor of excitonic energy and the location of charge separation. [12][13][14] Therefore, to understand the role of dimerisation, we must consider how both exciton transfer to P and charge transfer out of it would have been affected by dimerisation.…”

Theory of delocalised charge transfer​​​​​​​

“…All RCs are dimeric, with two branches that only interact at a strongly coupled special pair (P) of chlorophyll (Chl) or bacteriochlorophyll (BChl) molecules. P is the dominant acceptor of excitons from antenna complexes and the primary charge donor [142][143][144] . From P, the charges are transferred down either branch in homodimeric RCs (such as heliobacteria 74,83 ) or down only one branch in heterodimers.…”

“…Analysing Explanation 6 requires understanding how the dimerisation affected the exciton-and charge-transfer functions in the RC. Intriguingly, the constituent pigments within P are the only ones in the RC close enough to influence each other, meaning that the part of the RC most affected by dimerisation is also both the ultimate acceptor of excitonic energy and the location of charge separation [142][143][144] . Therefore, to understand the role of dimerisation, we must consider how both exciton transfer to P and charge transfer out of it would have been affected by dimerisation.…”

“…We determined the limits of the parameter space by considering modern RCs. We assume that the monomer was no faster at EET or CT than modern RCs, implying that the monomer EET and CT rates are less than the natural rates: k mon EET ≤ 10 11 s −1 and k mon CT ≤ 10 12 s −1 34,143,144,154 . We also assume that these rates are larger than the recombination rate k mon rec = 10 9 s −1 , which is assumed constant, essentially setting the timescale.…”