Solvation Effect of Bacteriochlorophyll Excitons in Light-Harvesting Complex LH2

Abstract

We have characterized the influence of the protein environment on the spectral properties of the bacteriochlorophyll (Bchl) molecules of the peripheral light-harvesting (or LH2) complex from Rhodobacter sphaeroides. The spectral density functions of the pigments responsible for the 800 and 850 nm electronic transitions were determined from the temperature dependence of the Bchl absorption spectra in different environments (detergent micelles and native membranes). The spectral density function is virtually independent of the hydrophobic support that the protein experiences. The reorganization energy for the B850 Bchls is 220 cm(-1), which is almost twice that of the B800 Bchls, and its Huang-Rhys factor reaches 8.4. Around the transition point temperature, and at higher temperatures, both the static spectral inhomogeneity and the resonance interactions become temperature-dependent. The inhomogeneous distribution function of the transitions exhibits less temperature dependence when LH2 is embedded in membranes, suggesting that the lipid phase protects the protein. However, the temperature dependence of the fluorescence spectra of LH2 cannot be fitted using the same parameters determined from the analysis of the absorption spectra. Correct fitting requires the lowest exciton states to be additionally shifted to the red, suggesting the reorganization of the exciton spectrum.

Gaussian decomposition of absorption spectra of peripheral light-harvesting complexes of photosynthetic bacteria

“…The slight temperature dependence of the resonance interaction determined by our estimations most probably reflects the temperature dependence of the dielectric constant of the solvent as has already been outlined recently [7]. According to the subband attribution the absorption above the U-maximum (see Fig.…”

“…All main spectroscopic features of the absorption spectra in a wide temperature range (from 4 K to room temperature) and for different conditions of sample preparation can be understood in terms of the exciton model [5][6][7]. In order to explain these spectroscopic data, the static disorder of pigment site energies, and the coupling of the electronic excitations to intra-and intermolecular vibrations / phonons originating in the so-called dynamic disorder have to be taken into account [1,8].…”

Davydov Splitting of Excitons in Cyclic Bacteriochlorophyll a Nanoaggregates of Bacterial Light‐Harvesting Complexes between 4.5 and 263 K

“…[22,23,33,34,40,[46][47][48][49][50][51][52][53][54][55][56] For theory, when aiming at simultaneously description of the steady state absorption and fluorescence emission spectra in wide temperature range, the greatest challenge is dealing with the exciton and excitonbath couplings of similar magnitude. [35,37] As follows, the simplest model of absorbing and emitting states in the B850 aggregate is introduced. It assumes identical dynamic broadening of all the absorbing exciton states caused by diagonal exciton-phonon bath coupling.…”

“…It assumes identical dynamic broadening of all the absorbing exciton states caused by diagonal exciton-phonon bath coupling. The required extra lowering of the fluorescing states [35] is introduced by the Su-Schrieffer-Heeger (SSH) or nonlocal polaronic coupling mechanism (reviewed in ref. [57]).…”

Spectral properties of LH2 exhibit very similar even when heterologously express LH2 with <i>β</i>-subunit fusion protein in <i>Rhodobacter</i> <i>sphaeroides</i>

“…Interactions between the BChls and within the protein binding pocket studies indicate the structural arrangement of the BChls and the central Mg 2+ atoms plays important roles in the binding of the pigment to α/β-subunits [11]. It has been proposed that protein environment exerts significant effects on the spectral properties of the BChl molecules of the LH2 [12]. Moreover, protein-induced macrocycle distortion will contribute significantly to spectral properties of the BChl bound in purple bacterial lightharvesting complex [13].…”