Modeling of oligomeric-state dependent spectral heterogeneity in the B875 light-harvesting complex of Rhodobacter sphaeroides by numerical simulation

A series of detergent-isolated light-harvesting 1 (LH1, B875) complexes from Rhodobacter sphaeroides, estimated to range in size from (RâBChl2)4 to (RâBChl2)13, was used to study the combined effects of spectral disorder and excitonic interactions on oligomeric-state dependent optical properties. Numerical simulations of absorption and fluorescence emission, excitation, and polarization spectra, based on the structure of the related LH2 complex, were compared to spectra measured experimentally at 77 K (Westerhuis and Niederman, in preparation). The aggregation-state dependence of the polarization spectra was found to be particularly sensitive to the choice of parameters, and vibronic components were included to obtain satisfactory simulations. Good agreement with most experimental features, including the oligomeric-state dependence of the absorption and emission maxima, was obtained only when the inter- and intradimer coupling strengths for adjacent BChls were similar (200-260 cm-1), and the width for the inhomogeneous distribution function (300-400 cm-1) was comparable. The relevance of these findings to existing controversies on the physical origin of spectral heterogeneity observed for the LH1 complex is discussed

Structural information on the light-harvesting complex II of green plants that can be depichered from polarized absorption characteristics.

The atomic model of light-harvesting complex II of green plants (LHCII) reveals a densely packed arrangement of 12 chlorophylls and two carotenoids. At the current resolution of 3.4 Angstrom chlorophylls can only be modeled as ''naked'' tetrapyrrole rings. Consequently, definitive assignments of the identities of the chlorophylls (chlorophyll a or chlorophyll b) and the directions of the transition dipole moments are obstructed. These uncertainties lead to a large number of possible configurations, and a detailed understanding of the structure-function relationship is obscured. It is demonstrated that a large reduction in the number of possible configurations and a considerable amount of additional structural information can be obtained by deciphering global features of the polarized absorption spectra within the context of exciton calculations. It is shown that only a limited number of configurations are able to explain the global features of the linear and circular dichroism spectra of LHCII. Assuming that the preliminary assignment of the identities of the 12 chlorophylls by Kuhlbrandt and co-workers is correct, it is possible to deduce the most likely orientations for most of the chlorophylls. The information presented in this study on the most likely orientations will be important for a detailed understanding of the relation between the structure and spectroscopy

Mixing of exciton and charge-transfer states in Photosystem II reaction centers: Modeling of stark spectra with modified redfield theory

AbstractWe propose an exciton model for the Photosystem II reaction center (RC) based on a quantitative simultaneous fit of the absorption, linear dichroism, circular dichroism, steady-state fluorescence, triplet-minus-singlet, and Stark spectra together with the spectra of pheophytin-modified RCs, and so-called RC5 complexes that lack one of the peripheral chlorophylls. In this model, the excited state manifold includes a primary charge-transfer (CT) state that is supposed to be strongly mixed with the pure exciton states. We generalize the exciton theory of Stark spectra by 1), taking into account the coupling to a CT state (whose static dipole cannot be treated as a small parameter in contrast to usual excited states); and 2), expressing the line shape functions in terms of the modified Redfield approach (the same as used for modeling of the linear responses). This allows a consistent modeling of the whole set of experimental data using a unified physical picture. We show that the fluorescence and Stark spectra are extremely sensitive to the assignment of the primary CT state, its energy, and coupling to the excited states. The best fit of the data is obtained supposing that the initial charge separation occurs within the special-pair PD1PD2. Additionally, the scheme with primary electron transfer from the accessory chlorophyll to pheophytin gave a reasonable quantitative fit. We show that the effectiveness of these two pathways is strongly dependent on the realization of the energetic disorder. Supposing a mixed scheme of primary charge separation with a disorder-controlled competition of the two channels, we can explain the coexistence of fast sub-ps and slow ps components of the Phe-anion formation as revealed by different ultrafast spectroscopic techniques

Superradiance and exciton delocalization in bacterial photosynthetic light-harvesting systems.

LH-2 complexes of Rhodobacter sphaeroides and on the isolated B820 subunit of Rhodospirillum rubrum. From these measurements the superradiance is calculated, which is related to the delocalization of excitations in these complexes. In the B820 preparation we find a radiative rate that is 30 % higher than that of monomeric bacteriochlorophyll, in agreement with a dimer model of this subunit. At room temperature both LH-1 and LH-2 are superradiant relative to monomeric Bchl-a with enhancement factors of 3.8 and 2.8, respectively. In LH-2 the radiative rate does not change significantly upon lowering the temperature to 4 K. LH-1 however exhibits a strong temperature dependence, giving rise to a 2.4 times higher radiative rate at 4 K relative to room temperature. From modeling of the superradiance using a Hamiltonian based on the LH-2 structure and including site inhomogeneity, we conclude that the ratio of inhomogeneity over the coupling betwee

Spectral broadening of interacting pigments: Polarized absorption by photosynthetic proteins.

Excitonic interaction between pigment molecules is largely responsible for the static and dynamic spectroscopic properties of photosynthetic pigment-proteins. This paper provides a new description of its effect on polarized absorption spectroscopy, in particular on circular dichroism (CD). We investigate excitonic spectra of finite width and use "spectral moments" to compare 1) inhomogeneously broadened excitonic spectra, 2) spectra that are (homogeneously broadened by vibrations or electron-phonon interaction, and 3) spectra that are simulated by applying convolution after the interaction has been evaluated. Two cases are distinguished. If the excitonic splitting is smaller than the width of the interacting absorption bands, the broadening of the excitonic spectrum can be approximated by a convolution approach, although a correction is necessary for CD spectra. If the excitonic splitting exceeds the bandwidth, the well-known exchange narrowing occurs. We demonstrate that this is accompanied by redistribution of dipole strength and spectral shifts. The magnitude of a CD spectrum is conveniently expressed by its first spectral moment. As will be shown, this is independent of spectral broadening as well as dispersive shifts induced by pigment-protein interactions. Consequently, it provides a simple tool to relate the experimental CD spectrum of a pigment complex to the excitonic interactions from which it originates. To illustrate the potential of the presented framework, the spectroscopy of the LH2 pigment-protein complex from purple bacteria is analyzed and compared for dimer-like and ring-like structures. Furthermore, it is demonstrated that the variability of the CD of chlorosomes from green bacteria can be explained by small changes in the structure of their cylindrical bacteriochlorophyll c subunits

Local and nonlocal contributions to the linear spectroscopy of light-harvesting antenna systems

In this paper the circular dichroism and absorption spectra of the LH2 complex of Rhodopseudomonas acidophila, for which the atomic structure is known, are analyzed. We show that an analysis based on the distribution of the excitations in real space, and their correlations, to unravel the relation between the atomic structure of the light-harvesting complex and its excitonic properties, is particularly successful. Starting from molecular expressions for the linear susceptibility, we demonstrate that linear spectra can be viewed as originating from the product of coherence correlation functions and geometric structure factors. Effects of homogeneous and inhomogeneous broadening can be incorporated in a natural way and lead to a definition of exciton length as the distance over which coherence correlation functions decay. 1

Ultrafast polarized fluorescence measurements on monomeric and self-associated melittin

The anisotropic and magic-angle fluorescence decay of the single tryptophan (Trp) residue of melittin, a bee venom peptide, was investigated by time-resolved fluorescence anisotropy using a streak camera setup. The peptide was dissolved either in distilled water or in Hepes/NaOH buffer containing low (10 mM) or high (2 M) concentrations of NaCl, the latter resulting in tetramerized melittin. For melittin in distilled water and low NaCl concentration, two anisotropy decay times were found in the order of similar to50 and similar to800 picoseconds, reflecting, local and overall peptide dynamics, respectively, and for tetramerized melittin, two anisotropy decay times of similar to200 and similar to5500 picoseconds were found. Decay-associated spectra of the isotropic fluorescence decay show three time components in the range of similar to20 picoseconds, similar to500 picoseconds, and similar to3500 picoseconds, respectively. The relative amplitudes of the latter two change upon the self-association of melittin. This change can be explained by the existence of different rotamers of Trp in melittin, of which one is more favored in the melittin tetramer than in the melittin monomer

Hydrodynamic studies of a DNA-protein complex Elongation of single stranded nucleic acids upon complexation with the gene 32 protein of phage T4 deduced from electric field-induced birefringence experiments

AbstractShort DNA and RNA fragments complexed with the helix destabilizing protein of bacteriophage T4, GP32, have been studied in solution by electric birefringence and circular dichroism. The birefringence of the complexes is positive and the magnitude indicates that the DNA and RNA fragments become linear and rigid upon protein binding. The field free decay is biphasic. On the basis of a rigid rod approximation the slow relaxation time leads to a base-base distance along the helix axis in the complex from 4.3 to 5.6 Å, an elongation of at least 50% compared to single-stranded DNA

Polarized fluorescence measurements on ordered photosynthetic antenna complexes Chlorosomes of Chloroflexus aurantiacus and B800-B850 antenna complexes of Rhodobacter sphaeroides

We have used a new and relatively easy approach to study the pigment-organization in chlorosomes from the photosynthetic bacterium Chloroflexus aurantiacus and in B800–850 antenna complexes of the photosynthetic purple bacterium Rhodobacter sphaeroides. These particles were embedded in compressed and uncompressed gels and the polarized fluorescence was determined in a 90° setup. Assuming both a rotational symmetric distribution of the particles in the gel and of the transition dipole moments in the particles, the order parameters <P2> and <P4>, describing the orientation of the symmetry axis of the particles with respect to the direction of gel expansion can be determined. Moreover, the direction parameters, describing the orientation of the absorption and emission dipole moments with respect to the symmetry axis of the particles can be obtained.The value of <P2> is essential for quantitative interpretation of linear dichroism measurements and usually it is estimated from theoretical approaches, which may lead to incorrect results. For the rod-like chlorosomes the value of <P2> appears to be the same as predicted by the theoretical approach of Ganago, A. O., M. V. Fok, I. A. Abdourakhmanov, A. A. Solov'ev, and Yu. E. Erokhin (1980. Mol. Biol. [Mosc.]. 14:381–389). The agreement with linear dichroism results, analyzed with this theoretical approach shows that the transition dipole moments are indeed in good approximation distributed in a rotationally symmetric way around the long axis of the chlorosomes. Moreover, it appears those BChl c molecules, which fluoresce, are oriented in the same way with respect to the symmetry axis as the rest of these pigments, with the dipole moments close to parallel to the long axis.The B800–850 complexes appear to orient like discs, whereas the transition dipoles of the BChl a 800- and 850-nm bands are oriented almost perpendicular to the symmetry axis. These findings are in agreement with the minimal model for these complexes proposed by Kramer, H. J. M., R. van Grondelle, C. N. Hunter, W. H. J. Westerhuis, and J. Amesz (1984. Biochim. Biophys. Acta. 156–165).The amount of orientation of the particles appears to vary for different gels and it is lower than predicted by the theory of Ganago et al., showing that application of their approach for these particles leads to incorrect interpretations.The approach that is used in this study allows determination of orientations of those dipole moments, which transfer their excitation energy to the fluorescing species, in contrast to linear dichroism measurements, where the orientations of all absorbing dipole moments are studied. For the polarized fluorescence measurements, the amount of orientation of the particles is determined experimentally, whereas for linear dichroism this amount has to be estimated from theoretical models. The value of <P2> that can be determined from the fluorescence measurements can, however, also be used for a quantitative interpretation of the linear dichroism results