Effects of Aggregation on the Excitation Dynamics of LH2 from Thermochromatium tepidum in Aqueous Phase and in Chromatophores

Dependence of the hydration status of bacterial light-harvesting complex 2 on polyol cosolvents

Tch. tepidumLH2 hydration correlates with water activity in water–polyol binary solvents as sensitively probed by near infrared electronic spectra and characteristic triplet carotenoid–bacteriochlorophyll interaction bands.

Light Absorption and Energy Transfer in the Antenna Complexes of Photosynthetic Organisms

The process of photosynthesis is initiated by the capture of sunlight by a network of light-absorbing molecules (chromophores), which are also responsible for the subsequent funneling of the excitation energy to the reaction centers. Through evolution, genetic drift, and speciation, photosynthetic organisms have discovered many solutions for light harvesting. In this review, we describe the underlying photophysical principles by which this energy is absorbed, as well as the mechanisms of electronic excitation energy transfer (EET). First, optical properties of the individual pigment chromophores present in light-harvesting antenna complexes are introduced, and then we examine the collective behavior of pigment-pigment and pigment-protein interactions. The description of energy transfer, in particular multichromophoric antenna structures, is shown to vary depending on the spatial and energetic landscape, which dictates the relative coupling strength between constituent pigment molecules. In the latter half of the article, we focus on the light-harvesting complexes of purple bacteria as a model to illustrate the present understanding of the synergetic effects leading to EET optimization of light-harvesting antenna systems while exploring the structure and function of the integral chromophores. We end this review with a brief overview of the energy-transfer dynamics and pathways in the light-harvesting antennas of various photosynthetic organisms.

Challenges facing an understanding of the nature of low-energy excited states in photosynthesis

While the majority of the photochemical states and pathways related to the biological capture of solar energy are now well understood and provide paradigms for artificial device design, additional low-energy states have been discovered in many systems with obscure origins and significance. However, as low-energy states are naively expected to be critical to function, these observations pose important challenges. A review of known properties of low energy states covering eight photochemical systems, and options for their interpretation, are presented. A concerted experimental and theoretical research strategy is suggested and outlined, this being aimed at providing a fully comprehensive understanding.

Combined topographic, spectroscopic, and model analyses of inhomogeneous energetic coupling of linear light harvesting complex II aggregates in native photosynthetic membranes

Light harvesting by LH1 and LH2 antenna proteins in the photosynthetic membranes of purple bacteria has been extensively studied in recent years for the fundamental understanding of the energy transfer dynamics and mechanism. Here we report the inhomogeneous structural organization of the LH2 complexes in photosynthetic membranes, giving evidence for the existence of energetically coupled linear LH2 aggregates in the native photosynthetic membranes of purple bacteria. Focusing on systematic model analyses, we combined AFM imaging and spectroscopic analysis with energetic coupling model analysis to characterize the inhomogeneous linear aggregation of LH2. Our AFM imaging results reveal that the LH2 complexes form linear aggregates with the monomer number varying from one to eight and each monomer tilted along the aggregated structure in photosynthetic membranes. The spectroscopic results support the attribution of aggregated LH2 complexes in the photosynthetic membranes, and the model calculation values for the absorption, emission and lifetime are consistent with the experimentally determined spectroscopic values, further proving a molecular-level understanding of the energetic coupling and energy transfer among the LH2 complexes in the photosynthetic membranes.

Quantum chemical description of absorption properties and excited-state processes in photosynthetic systems

The theoretical description of the initial steps in photosynthesis has gained increasing importance over the past few years. This is caused by more and more structural data becoming available for light-harvesting complexes and reaction centers which form the basis for atomistic calculations and by the progress made in the development of first-principles methods for excited electronic states of large molecules. In this Review, we discuss the advantages and pitfalls of theoretical methods applicable to photosynthetic pigments. Besides methodological aspects of excited-state electronic-structure methods, studies on chlorophyll-type and carotenoid-like molecules are discussed. We also address the concepts of exciton coupling and excitation-energy transfer (EET) and compare the different theoretical methods for the calculation of EET coupling constants. Applications to photosynthetic light-harvesting complexes and reaction centers based on such models are also analyzed.

Ultrafast time-resolved spectroscopy of the light-harvesting complex 2 (LH2) from the photosynthetic bacterium Thermochromatium tepidum

The light-harvesting complex 2 from the thermophilic purple bacterium Thermochromatium tepidum was purified and studied by steady-state absorption and fluorescence, sub-nanosecond-time-resolved fluorescence and femtosecond time-resolved transient absorption spectroscopy. The measurements were performed at room temperature and at 10 K. The combination of both ultrafast and steady-state optical spectroscopy methods at ambient and cryogenic temperatures allowed the detailed study of carotenoid (Car)-to-bacteriochlorophyll (BChl) as well BChl-to-BChl excitation energy transfer in the complex. The studies show that the dominant Cars rhodopin (N=11) and spirilloxanthin (N=13) do not play a significant role as supportive energy donors for BChl a. This is related with their photophysical properties regulated by long π-electron conjugation. On the other hand, such properties favor some of the Cars, particularly spirilloxanthin (N=13) to play the role of the direct quencher of the excited singlet state of BChl.