Insight into the Structure of Photosynthetic LH2 Aggregate from Spectroscopy Simulations

Using the electrostatic model of intermolecular interactions, we obtain the Frenkel exciton Hamiltonian parameters for the chlorophyll <i>Q</i>_<i>y</i> band of a photosynthetic peripheral light harvesting complex LH2 of a purple bacteria Rhodopseudomonas acidophila from structural data. The intermolecular couplings are mostly determined by the chlorophyll relative positions, whereas the molecular transition energies are determined by the background charge distribution of the whole complex. The protonation pattern of titratable residues is used as a tunable parameter. By studying several protonation state scenarios for distinct protein groups and comparing the simulated absorption and circular dichroism spectra to experiment, we determine the most probable configuration of the protonation states of various side groups of the protein

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

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 (<i>P</i>⁺) and charge-separated (<i>P</i>⁺<i>Q</i>_<i>M</i>^–) states. Theoretical models lead to two alternative interpretations of the <i>H</i>-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 <i>P</i>⁺<i>Q</i>_<i>M</i>^– state, often assigned to the upper exciton component of the special pair, is mostly due to different electrochromic shifts of the <i>B</i>_{<i>L</i>/<i>M</i>} cofactors. From the exciton compositions in the charge-neutral (CN) bRC, the weak fourth excitonic band near 780 nm can be denoted <i>P</i>_<i>Y</i>+, that is, the upper excitonic band of the special pair, which in the CN bRC behaves as a delocalized state over <i>P</i>_<i>M</i> and <i>P</i>_<i>L</i> pigments that weakly mixes with accessory BChls. Thus, the shoulder in the absorption of Rb. sphaeroides near 813–815 nm does not contain the <i>P</i>_<i>Y</i>+ exciton band

A Unified Picture of S* in Carotenoids

In π-conjugated chain molecules such as carotenoids, coupling between electronic and vibrational degrees of freedom is of central importance. It governs both dynamic and static properties, such as the time scales of excited state relaxation as well as absorption spectra. In this work, we treat vibronic dynamics in carotenoids on four electronic states (|S₀⟩, |S₁⟩, |S₂⟩, and |S_n⟩) in a physically rigorous framework. This model explains all features previously associated with the intensely debated S* state. Besides successfully fitting transient absorption data of a zeaxanthin homologue, this model also accounts for previous results from global target analysis and chain length-dependent studies. Additionally, we are able to incorporate findings from pump-deplete-probe experiments, which were incompatible to any pre-existing model. Thus, we present the first comprehensive and unified interpretation of S*-related features, explaining them by vibronic transitions on either S₁, S₀, or both, depending on the chain length of the investigated carotenoid

Excitons in the LH3 Complexes from Purple Bacteria

The noncovalently bound and structurally identical bacteriochlorophyll <i>a</i> chromophores in the peripheral light-harvesting complexes LH2 (B800–850) and LH3 (B800–820) from photosynthetic purple bacteria ensure the variability of the exciton spectra in the near-infrared (820–850 nm) wavelength region. As a result, the spectroscopic properties of the antenna complexes, such as positions of the maxima in the exciton absorption spectra, give rise to very efficient excitation transfer toward the reaction center. In this work, we investigated the possible molecular origin of the excitonically coupled B820 bacteriochlorophylls in LH3 using femtosecond transient absorption spectroscopy, deconvolution of steady-state absorption spectra, and modeling of the electrostatic intermolecular interactions using a charge density coupling approach. Compared to LH2, the upper excitonic level is red-shifted from 755 to 790 nm and is associated with an approximate 2-fold decrease of B820 intrapigment coupling. The absorption properties of LH3 cannot be reproduced by only changing the B850 site energy but also require a different scaling factor to be used to calculate interpigment couplings and a change of histidine protonation state. Several protonation patterns for distinct amino acid groups are presented, giving values of 162–173 cm^–1 at 100 K for the intradimer resonance interaction in the B820 ring

Mutation-Induced Changes in the Protein Environment and Site Energies in the (M)L214G Mutant of the <i>Rhodobacter sphaeroides</i> Bacterial Reaction Center

This work focuses on the low-temperature (5 K) photochemical (transient) hole-burned (HB) spectra within the P870 absorption band, and their theoretical analysis, for the (M)­L214G mutant of the photosynthetic <i>Rhodobacter sphaeroides</i> bacterial reaction center (bRC). To provide insight into system–bath interactions of the bacteriochlorophyll <i>a</i> (BChl <i>a</i>) special pair, i.e., P870, in the mutated bRC, the optical line shape function for the P870 band is calculated numerically. On the basis of the modeling studies, we demonstrate that (M)­L214G mutation leads to a heterogeneous population of bRCs with modified (increased) total electron–phonon coupling strength of the special pair BChl <i>a</i> and larger inhomogeneous broadening. Specifically, we show that after mutation in the (M)­L214G bRC a large fraction (∼50%) of the bacteriopheophytin (<i>H</i>_<i>A</i>) chromophores shifts red and the 800 nm absorption band broadens, while the remaining fraction of <i>H</i>_<i>A</i> cofactors retains nearly the same site energy as <i>H</i>_<i>A</i> in the wild-type bRC. Modeling using these two subpopulations allowed for fits of the absorption and nonresonant (transient) HB spectra of the mutant bRC in the charge neutral, oxidized, and charge-separated states using the Frenkel exciton Hamiltonian, providing new insight into the mutant’s complex electronic structure. Although the average (M)­L214G mutant quantum efficiency of <i>P</i>^<i>+</i><i>Q</i>_<i>A</i>^<i>–</i> state formation seems to be altered in comparison with the wild-type bRC, the average electron transfer time (measured via resonant transient HB spectra within the P870 band) was not affected. Thus, mutation in the vicinity of the electron acceptor (<i>H</i>_<i>A</i>) does not tune the charge separation dynamics. Finally, quenching of the (M)­L214G mutant excited states by <i>P</i>^<i>+</i> is addressed by persistent HB spectra burned within the <i>B</i> band in chemically oxidized samples