Static and dynamic protein impact on electronic properties of light-harvesting complex LH2.

A comparative analysis of the temperature dependence of the absorption spectra of the LH2 complexes from different species of photosynthetic bacteria, i.e., Rhodobacter sphaeroides, Rhodoblastus acidophilus, and Phaeospirillum molischianum, was performed in the temperature range from 4 to 300 K. Qualitatively, the temperature dependence is similar for all of the species studied. The spectral bandwidths of both B800 and B850 bands increases with temperature while the band positions shift in opposite directions: the B800 band shifts slightly to the red while the B850 band to the blue. These results were analyzed using the modified Redfield theory based on the exciton model. The main conclusion drawn from the analysis was that the spectral density function (SDF) is the main factor underlying the strength of the temperature dependence of the bandwidths for the B800 and B850 electronic transitions, while the bandwidths themselves are defined by the corresponding inhomogeneous distribution function (IDF). Slight variation of the slope of the temperature dependence of the bandwidths between species can be attributed to the changes of the values of the reorganization energies and characteristic frequencies determining the SDF. To explain the shift of the B850 band position with temperature, which is unusual for the conventional exciton model, a temperature dependence of the IDF must be postulated. This dependence can be achieved within the framework of the modified (dichotomous) exciton model. The slope of the temperature dependence of the B850 bandwidth is then defined by the value of the reorganization energy and by the difference between the transition energies of the dichotomous states of the pigment molecules. The equilibration factor between these dichotomous states mainly determines the temperature dependence of the peak shift.

Solvation effect of bacteriochlorophyll excitons in light-harvesting complex LH2.

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.

Temperature broadening of LH2 absorption in glycerol solution.

In order to determine the relationship between the pigment-protein and the pigment-pigment interactions, the measurements of absorption spectra of the peripheral light-harvesting complex LH2 from the purple bacteria Rhodobacter sphaeroides solvated in glycerol/buffer solution were carried out in a wide temperature range, from 4 to 250 K. The SDFs used for simulating the temperature dependence of B800 and B850 bands were determined in a parametric form. To fit experimental spectra the overall exciton-phonon coupling had to be assumed to be weak for B850 (lambda/2V approximately 0.3, where lambda is the reorganization energy and V is the nearest-neighbor dipole-dipole coupling for bacteriochlorophylls). At physiological temperatures the intermediate nuclear bath dynamics compares with the magnitude of energy gap fluctuations. Slower dynamics with kappa approximately 0.39, where kappa is the ratio of the nuclear relaxation rate and the line width parameter, determines the spectral shape of B850 whilst faster modulations characterize B800 (kappa approximately 2.39). The static disorder for the B800 band is relatively high with the characteristic value of the inhomogeneous bandwidth Gamma(inh) approximately 120 cm-1, while for the B850 band this value is almost equal to the dipole-dipole coupling strength (Gamma(inh) approximately 360 cm-1). It has been found that the LH2 absorption spectrum is likely to be influenced by the temperature dependence of the dielectric constant of the solution in the high temperature range, when the glycerol/buffer solution is in the liquid state.

Exciton delocalization probed by excitation annihilation in the light-harvesting antenna LH2.

Singlet-singlet annihilation is used to study exciton delocalization in the light harvesting antenna complex LH2 (B800-B850) from the photosynthetic purple bacterium Rhodobacter sphaeroides. The characteristic femtosecond decay constants of the high intensity isotropic and the low intensity anisotropy kinetics of the B850 ring are related to the hopping time tau(h) and the coherence length N(coh) of the exciton. Our analysis yields N(coh) = 2.8+/-0.4 and tau(h) = 0.27+/-0.05 ps. This approach can be seen as an extension to the concept of the spectroscopic ruler.

Kinetic modeling of exciton migration in photosynthetic systems. 3. Application of genetic algorithms to simulations of excitation dynamics in three-dimensional photosystem I core antenna/reaction center complexes.

A procedure is described to generate and optimize the lattice models for spectrally inhomogeneous photosynthetic antenna/reaction center (RC) particles. It is based on the genetic algorithm search for the pigment spectral type distributions on the lattice by making use of steady-state and time-resolved spectroscopic input data. Upon a proper fitness definition, a family of excitation energy transfer models can be tested for their compatibility with the availability experimental data. For the case of the photosystem I core antenna (99 chlorophyll + primary electron donor pigment (P700)), three spectrally inhomogeneous three-dimensional lattice models, differing in their excitation transfer conditions, were tested. The relevant fit parameters were the pigment distribution on the lattice, the average lattice spacing of the main pool pigments, the distance of P700 and of long wavelength-absorbing (LWA) pigments to their nearest-neighbor main pool pigments, and the rate constant of charge separation from P700. For cyanobacterial PS I antenna/RC particles containing a substantial amount of LWA pigments, it is shown that the currently available experimental fluorescence data are consistent both with more migration-limited, and with more trap-limited excitation energy transfer models. A final decision between these different models requires more detailed experimental data. From all search runs about 30 different relative arrangements of P700 and LWA pigments were found. Several general features of all these different models can be noticed: 1) The reddest LWA pigment never appears next to P700. 2) The LWA pigments in most cases are spread on the surface of the lattice not far away from P700, with a pronounced tendency toward clustering of the LWA pigments. 3) The rate constant kP700 of charge separation is substantially higher than 1.2 ps-1, i.e., it exceeds the corresponding rate constant of purple bacterial RCs by at least a factor of four. 4) The excitation transfer within the main antenna pool is very rapid (less than 1 ps equilibration time), and only the equilibration with the LWA pigments is slow (about 10-12 ps). The conclusions from this extended study on three-dimensional lattices are in general agreement with the tendencies and limitations reported previously for a simpler two-dimensional array. Once more detailed experimental data are available, the procedure can be used to determine the relevant rate-limiting processes in the excitation transfer in such spectrally inhomogeneous antenna systems.

Nonlinear annihilation of excitations in photosynthetic systems.

The theory of the singlet-singlet annihilation in quasi-homogeneous photosynthetic antenna systems is developed further. In the new model, the following important contributions are taken into account: 1) the finite excitation pulse duration, 2) the occupation of higher excited states during the annihilation, 3) excitation correlation effects, and 4) the effect of local heating. The main emphasis is concentrated on the analysis of pump-probe kinetic measurements demonstrating the first two above possible contributions. The difference with the results obtained from low-intensity fluorescence kinetic measurements is highlighted. The experimental data with picosecond time resolution obtained for the photosynthetic bacterium Rhodospirillum rubrum at room temperature are discussed on the basis of this theory.

Kinetic modeling of exciton migration in photosynthetic systems. 2. Simulations of excitation dynamics in two-dimensional photosystem I core antenna/reaction center complexes.

Kinetic modeling of the exciton migration in the cyanobacterial photosystem I core complex from Synechococcus sp. was performed by an exact solution of the Pauli master equation for exciton motion. A square two-dimensional 10 x 10 pigment lattice and a Förster dipole-dipole coupling between chromophores was assumed. We calculated decay-associated spectra and lifetimes and compared them to the corresponding experimental data from picosecond fluorescence and transient absorption obtained by global analysis. Seven spectral chlorophyll(Chl) forms, identical in shape but shifted in their absorption maximums, were used to describe the non-homogeneous broadening of the PS I-100 particle absorption spectrum. The optimized Chl lattice arrangement best reproducing the experimental decay-associated spectra as well as the steady-state fluorescence spectrum indicated the long-wavelength-absorbing Chls forming a cluster in the corner of the lattice with the reaction center (RC) placed apart at a distance of two lattice constants. The variable parameters, i.e., the charge separation rate in the RC and the lattice constant a, were found to be optimal at kRC = 2.3 ps-1 and a = 1.14 nm, respectively. The surprising conclusions of the simulations is that Chls with absorption maxima as long a 724 nm have to be taken into account to describe the time-resolved spectra of this PS I particle properly. The dependencies of the exciton decay in the model PS I particle on the excitation wavelength and on the temperature are discussed. We also show that the excited state decay of similar PS I particles that lack the long-wavelength absorbing Chls is nearly mono-exponential. Various critical factors that limit the general reliability of the conclusions of such simulations are discussed in detail.