Mixed Upper Exciton State of the Special Pair in Bacterial Reaction Centers

Abstract

Excitonic interactions between two closely separated bacteriochlorophyll a molecules (BChls) in the special pair of the reaction center (RC) of purple bacteria determine the positions and relative oscillator strengths of its two excitonic components. While the absorption of the lower excitonic band is well-defined, the position and the intensity of the upper excitonic band (PY+ ) are still under debate. Recent 77 K two-dimensional electronic spectroscopy data on Rba. capsulatus suggested that the PY+ component absorbs at ∼840 nm, i.e., at a significantly lower energy than previously suggested. In the present work, we argue that the PY+ state is mixed with the excited states of the accessory BChls (B*/P Y+) leading to excitons contributing to the 785–825 nm spectral region which is consistent with previously published data. This conclusion is based on hole-burning/linear dichroism data and modeling studies of the excitonic structure of the RC using a non-Markovian reduced density matrix approach.

Exact Simulation of Pigment-Protein Complexes Unveils Vibronic Renormalization of Electronic Parameters in Ultrafast Spectroscopy

“…We will demonstrate that by accounting for the full environmental spectral density, involving more than 50 intra-pigment modes per site in addition to a broad background, the presence of high-frequency long-lived vibrational modes can lead to quantitatively significant modification of the calculated linear spectra of PPCs and consequently the estimated values of electronic parameters to recover a best fit with actual measurements. These corrections do not appear when considering only selected resonant modes and go well beyond predictions that can be obtained by using conventional lineshape theory [48][49][50][51][52][53][54][55][56][57].…”

“…[65]). Since conventional lineshape theories [48][49][50][51][52][53][54][55][56][57] do not include the renormalisation of the excitonic coupling by the intra-pigment electron-vibrational coupling, the absorption spectrum calculated with the bare excitonic coupling V = 140 cm −1 cannot describe the numerically exact absorption spectrum, as shown in blue in Fig. 4C, where the energy-gap between absorption peaks in 0-0 line region is close to a bare excitonic splitting ∆ ≈ 2V ≈ 280 cm −1 .…”

“…(8). In conventional lineshape theory [48][49][50][51][52][53][54][55][56][57], the diagonal vibronic couplings in the exciton basis, described (ω) + J h (ω), J B777 l (ω) + J h (ω)), numerically exact absorption spectra of WSCP are shown in black, and approximate results obtained by reduced vibronic models, taking into account all Vmn couplings or only V00, are shown in red and green, respectively, where the Lamb shift induced by protein modes is included (see section X of the SI). (C) For J B777 l (ω) + J h (ω), numerical results obtained by conventional lineshape theory with V = 140 cm −1 and V = 140 cm −1 exp(− 55 k=1 s k ) ≈ 70 cm −1 are shown in blue and green solid lines, respectively.…”

“…The SP is a strongly coupled dimeric unit with an electronic coupling estimated to be V = 625 cm −1 , a difference in mean site energies of ε 1 − ε 2 = 315 cm −1 and consequently a bare excitonic splitting of ∆ ≈ 1290 cm −1 . These electronic parameters have been estimated based on a best fit to absorption, linear dichroism and hole burning spectra of bacterial reaction centers using conventional lineshape theory [57]. In what follows, we neglect the order of magnitude weaker electronic coupling of the SP to the four additional pigments and do not aim to reproduce exper- imentally measured absorption spectra of the whole bacterial reaction centers and re-estimate electronic parameters.…”

“…For SP heterodimers, it has been estimated that the difference in mean site energies is ε 1 − ε 2 = 315 cm −1 , electronic coupling is V = 625 cm −1 , and the angle between transition dipole moments of monomers [57] is 143 • . Experimentally estimated spectral density consists of a log-normal distribution function [68]…”