Band Structure of the Rhodobacter sphaeroides Photosynthetic Reaction Center from Low-Temperature Absorption and Hole-Burned Spectra

Abstract

Persistent/transient spectral hole burning (HB) and computer simulations are used to provide new insight into the excitonic structure and excitation energy transfer of the widely studied bacterial reaction center (bRC) of Rhodobacter (Rb.) sphaeroides. We focus on site energies of its cofactors and electrochromic shifts induced in the chemically oxidized (P(+)) and charge-separated (P(+)QM(-)) states. Theoretical models lead to two alternative interpretations of the H-band. On the basis of our experimental and simulation data, we suggest that the bleach near 813-825 nm in transient HB spectra in the P(+)QM(-) state, often assigned to the upper exciton component of the special pair, is mostly due to different electrochromic shifts of the BL/M cofactors. From the exciton compositions in the charge-neutral (CN) bRC, the weak fourth excitonic band near 780 nm can be denoted PY+, that is, the upper excitonic band of the special pair, which in the CN bRC behaves as a delocalized state over PM and PL pigments that weakly mixes with accessory BChls. Thus, the shoulder in the absorption of Rb. sphaeroides near 813-815 nm does not contain the PY+ exciton band.

On uncorrelated inter-monomer Förster energy transfer in Fenna–Matthews–Olson complexes

“…When the system relaxes the chromophore has a different transition energy due to the altered environment. In general, non-photochemical HB offers more direct information on the Q y absorbing states (as has been demonstrated in [54,55]). Formation of charge-separated states in photochemical HB spectra produces additional electrochromic shifts of the zero-order state energies and, possibly, protein structural changes that might also affect these energies.…”

“…Formation of charge-separated states in photochemical HB spectra produces additional electrochromic shifts of the zero-order state energies and, possibly, protein structural changes that might also affect these energies. The latter complicates the theoretical description of photochemical HB spectra [55].…”

Hidden vibronic and excitonic structure and vibronic coherence transfer in the bacterial reaction center

“…In contrast, the location of P + has been historically difficult to assign given its weak oscillator strength and proximity to the strong ~800-nm absorption of the B pigments (49). Several studies have developed excitonic models for the BRC from Rhodobacter sphaeroides (49)(50)(51) and Rhodobacter viridis (49). We previously proposed exciton energies for the R. capsulatus BRC studied here based on a multiexcitation 2D global analysis of 2DES data by Niedringhaus et al (45).…”

On the Conflicting Estimations of Pigment Site Energies in Photosynthetic Complexes: A Case Study of the CP47 Complex

“…Thus, these absorption measurements can significantly modify site energies, and as a result, optical spectra. 33 …”

“…Another important issue, as discussed in Refs. 3 and 33 , is using a proper shape of J ph ( ω ), which also affects the extracted site energies and population dynamics. For example, it is not clear: (i) whether an experimentally determined spectral density obtained via the delta FLN methodology, within the lowest energy state, is the same for all pigments; (ii) whether the el–ph coupling strength is sufficiently similar for all chromophores; (iii) to what extent inhomogeneous broadening (Γ inh ) varies from pigment to pigment; (iv) if the protein dielectric constant varies for different binding sites; and (v) how one should determine excitonic domains using a single coupling cutoff value.…”