The crystal structure at 4.8 angstrom resolution of the reaction center-light harvesting 1 (RC-LH1) core complex from Rhodopseudomonas palustris shows the reaction center surrounded by an oval LH1 complex that consists of 15 pairs of transmembrane helical alpha- and beta-apoproteins and their coordinated bacteriochlorophylls. Complete closure of the RC by the LH1 is prevented by a single transmembrane helix, out of register with the array of inner LH1 alpha-apoproteins. This break, located next to the binding site in the reaction center for the secondary electron acceptor ubiquinone (UQB), may provide a portal through which UQB can transfer electrons to cytochrome b/c1.
Emerging nonlinear optical spectroscopies enable deeper insight into the intricate world of interactions and dynamics of complex molecular systems. 2D electronic spectroscopy appears to be especially well suited for studying multichromophoric complexes such as light-harvesting complexes of photosynthetic organisms as it allows direct observation of couplings between the pigments and charts dynamics of energy flow on a 2D frequency map. Here, we demonstrate that a single 2D experiment combined with self-consistent theoretical modeling can determine spectroscopic parameters dictating excitation energy dynamics in the bacterial B800-B820 light-harvesting complex, which contains 27 bacteriochlorophyll molecules. Ultrafast sub-50-fs dynamics dominated by coherent intraband processes and population transfer dynamics on a picosecond time scale were measured and modeled with one consistent set of parameters. Theoretical 2D spectra were calculated by using a Frenkel exciton model and modified Fö rster͞ Redfield theory for the calculation of dynamics. They match the main features of experimental spectra at all population times well, implying that the energy level structure and transition dipole strengths are modeled correctly in addition to the energy transfer dynamics of the system. photosynthetic complexes ͉ excitons ͉ multichromophoric systems ͉ ultrafast spectroscopy T wo-dimensional optical experiments constitute a promising addition to the field of ultrafast spectroscopy. The success of 2D IR spectroscopy in adapting techniques from multidimensional NMR is steadily expanding into the visible range, where coherent couplings between electronic transitions, frequencydependent excitation transfer processes, and chromophoreenvironment interactions in complex molecular systems can be investigated with femtosecond time resolution. As demonstrated first by Jonas and coworkers (1-3) for visible-range laser excitation pulses, Fourier analysis of the signal electric field in a phase-controlled, four-wave mixing experiment yields 2D frequency maps representing the full (within the laser pulse spectral window) third-order optical response of the system. 2D electronic experiments performed thus far are based on heterodyne detection of a three-pulse photon echo signal, which is separated from other nonlinear signals by phase matching in a noncollinear beam geometry (1, 2, 4-6), or fluorescence detection after phase cycling using pulse-shaping techniques, as performed by Tian et al. (7). Brixner et al. (4,5) developed a particularly robust experimental setup combining inherent phase stability, phase matching, and heterodyne detection by spectral interferometry, and they later demonstrated that the method was well suited to the study of multichromophoric pigment-protein complexes (8).Multiple third-order nonlinear signals interfere to give the overall three-pulse photon echo signal, and thus quantitative analysis is essential for disentangling contributions to 2D spectra and identifying the source of spectral features. Simulation o...
Steady-state and ultrafast time-resolved optical spectroscopic investigations have been carried out at 293 and 10 K on LH2 pigment-protein complexes isolated from three different strains of photosynthetic bacteria: Rhodobacter (Rb.) sphaeroides G1C, Rb. sphaeroides 2.4.1 (anaerobically and aerobically grown), and Rps. acidophila 10050. The LH2 complexes obtained from these strains contain the carotenoids, neurosporene, spheroidene, spheroidenone, and rhodopin glucoside, respectively. These molecules have a systematically increasing number of π-electron conjugated carbon-carbon double bonds. Steady-state absorption and fluorescence excitation experiments have revealed that the total efficiency of energy transfer from the carotenoids to bacteriochlorophyll is independent of temperature and nearly constant at ~90% for the LH2 complexes containing neurosporene, spheroidene, spheroidenone, but drops to ~53% for the complex containing rhodopin glucoside. Ultrafast transient absorption spectra in the near-infrared (NIR) region of the purified carotenoids in solution have revealed the energies of the S1 (21Ag−) → S2 (11Bu+) excited-state transitions which, when subtracted from the energies of the S0 (11Ag−) → S2 (11Bu+) transitions determined by steady-state absorption measurements, give precise values for the positions of the S1 (21Ag−) states of the carotenoids. Global fitting of the ultrafast spectral and temporal data sets have revealed the dynamics of the pathways of de-excitation of the carotenoid excited states. The pathways include energy transfer to bacteriochlorophyll, population of the so-called S* state of the carotenoids, and formation of carotenoid radical cations (Car•+). The investigation has found that excitation energy transfer to bacteriochlorophyll is partitioned through the S1 (11Ag−), S2 (11Bu+), and S* states of the different carotenoids to varying degrees. This is understood through a consideration of the energies of the states and the spectral profiles of the molecules. A significant finding is that, due to the low S1 (21Ag−) energy of rhodopin glucoside, energy transfer from this state to the bacteriochlorophylls is significantly less probable compared to the other complexes. This work resolves a long-standing question regarding the cause of the precipitous drop in energy transfer efficiency when the extent of π-electron conjugation of the carotenoid is extended from ten to eleven conjugated carbon–carbon double bonds in LH2 complexes from purple photosynthetic bacteria.
The spectroscopic properties of light-harvesting (LH) antennae in photosyntehtic organisms represent a fingerprint that is unique for each specific pigment-protein complex. Because of that, spectroscopic observations are generally combined with structural data from X-ray crystallography to obtain an indirect representation of the excitonic properties of the system. Here, an alternative strategy is presented which goes beyond this empirical approach and introduces an ab initio computational description of both structural and electronic properties and their dependence on the temperature. The strategy is applied to the peripheral light-harvesting antenna complex (LH2) present in purple bacteria. By comparing this model with the one based on the crystal structure, a detailed, molecular level explanation of the absorption and circular dichroism (CD) spectra and their temperature dependence is achieved. The agreement obtained with the experiments at both low and room temperature lays the groundwork for an atomistic understanding of the excitation dynamics in the LH2 system.
The LH2 antenna complexes of purple bacteria occur, depending on light conditions, in various different spectroscopic forms, with a similar structure but different absorption spectra. The differences are related to point changes in the primary amino acid sequence, but the molecular-level relationship between these changes and the resulting spectrum is still not well understood. We undertook a systematic quantum chemical analysis of all the main factors that contribute to the exciton structure, looking at how the environment modulates site energies and couplings in the B800-850 and B800-820 spectroscopic forms of LH2. A multiscale approach combining quantum chemistry and an atomistic classical embedding has been used where mutual polarization effects between the two parts are taken into account. We find that the loss of hydrogen bonds following amino acid changes can only explain a part of the observed blue-shift in the B850 band. The coupling of excitonic states to charge-transfer states, which is different in the two forms, contributes with a similar amount to the overall blue-shift.
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