Carotenoids are crucial for photosynthesis, playing key roles in light harvesting and photoprotection. In this study, spheroidene and bacteriochlorophyll a (Bchl a) were reconstituted into the chromatophores of the carotenoidless mutant Rhodobacter sphaeroides R26.1, resulting in the preparation of high-quality LH2 complexes. Global and target analyses of transient absorption data revealed that incorporating B800 Bchl a significantly enhances excitation energy transfer (EET) efficiency from carotenoids to Bchl a. EET predominantly occurs from the carotenoid S2 state, with additional pathways from the S1 state observed in native LH2. Unique relaxation dynamics were identified, including the generation of the carotenoid S* state in reconstituted LH2 with both spheroidene and B800 Bchl a and the formation of the carotenoid T1 state in reconstituted LH2. These findings underscore the critical influence of pigment composition and spatial organization on energy transfer mechanisms. They provide valuable insights into the molecular interplay that governs excitation energy transfer in photosynthetic light-harvesting systems.

• Keywords
• B800 bacteriochlorophyll a, carotenoid, light-harvesting, photoprotection, purple photosynthetic bacteria, reconstitution,
• MeSH
• Bacterial Proteins * chemistry   metabolism   MeSH
• Bacteriochlorophyll A * chemistry   metabolism   MeSH
• Photosynthesis MeSH
• Carotenoids * metabolism   chemistry   MeSH
• Energy Transfer * MeSH
• Rhodobacter sphaeroides * metabolism   chemistry   MeSH
• Light-Harvesting Protein Complexes * metabolism   chemistry   MeSH
• Publication type
• Journal Article MeSH
• Names of Substances
• Bacterial Proteins * MeSH
• Bacteriochlorophyll A * MeSH
• Carotenoids * MeSH
• spheroidene MeSH Browser
• Light-Harvesting Protein Complexes * MeSH

The peripheral light-harvesting antenna complex (LH2) of purple photosynthetic bacteria is an ideal testing ground for models of structure-function relationships due to its well-determined molecular structure and ultrafast energy deactivation. It has been the target for numerous studies in both theory and ultrafast spectroscopy; nevertheless, certain aspects of the convoluted relaxation network of LH2 lack a satisfactory explanation by conventional theories. For example, the initial carotenoid-to-bacteriochlorophyll energy transfer step necessary on visible light excitation was long considered to follow the Förster mechanism, even though transfer times as short as 40 femtoseconds (fs) have been observed. Such transfer times are hard to accommodate by Förster theory, as the moderate coupling strengths found in LH2 suggest much slower transfer within this framework. In this study, we investigate LH2 from Phaeospirillum (Ph.) molischianum in two types of transient absorption experiments-with narrowband pump and white-light probe resulting in 100 fs time resolution, and with degenerate broadband 10 fs pump and probe pulses. With regard to the split Qx band in this system, we show that vibronically mediated transfer explains both the ultrafast carotenoid-to-B850 transfer, and the almost complete lack of transfer to B800. These results are beyond Förster theory, which predicts an almost equal partition between the two channels.

• Keywords
• Excitation energy transfer, Excitons, LH2, Photosynthesis, Ultrafast spectroscopy,
• MeSH
• Bacteriochlorophylls metabolism   MeSH
• Time Factors MeSH
• Fourier Analysis MeSH
• Carotenoids metabolism   MeSH
• Lasers MeSH
• Energy Transfer * MeSH
• Proteobacteria metabolism   MeSH
• Spectrophotometry, Ultraviolet MeSH
• Light-Harvesting Protein Complexes metabolism   MeSH
• Publication type
• Journal Article MeSH
• Names of Substances
• Bacteriochlorophylls MeSH
• Carotenoids MeSH
• Light-Harvesting Protein Complexes MeSH

Carotenoids are naturally occurring pigments that absorb light in the spectral region in which the sun irradiates maximally. These molecules transfer this energy to chlorophylls, initiating the primary photochemical events of photosynthesis. Carotenoids also regulate the flow of energy within the photosynthetic apparatus and protect it from photoinduced damage caused by excess light absorption. To carry out these functions in nature, carotenoids are bound in discrete pigment-protein complexes in the proximity of chlorophylls. A few three-dimensional structures of these carotenoid complexes have been determined by X-ray crystallography. Thus, the stage is set for attempting to correlate the structural information with the spectroscopic properties of carotenoids to understand the molecular mechanism(s) of their function in photosynthetic systems. In this Account, we summarize current spectroscopic data describing the excited state energies and ultrafast dynamics of purified carotenoids in solution and bound in light-harvesting complexes from purple bacteria, marine algae, and green plants. Many of these complexes can be modified using mutagenesis or pigment exchange which facilitates the elucidation of correlations between structure and function. We describe the structural and electronic factors controlling the function of carotenoids as energy donors. We also discuss unresolved issues related to the nature of spectroscopically dark excited states, which could play a role in light harvesting. To illustrate the interplay between structural determinations and spectroscopic investigations that exemplifies work in the field, we describe the spectroscopic properties of four light-harvesting complexes whose structures have been determined to atomic resolution. The first, the LH2 complex from the purple bacterium Rhodopseudomonas acidophila, contains the carotenoid rhodopin glucoside. The second is the LHCII trimeric complex from higher plants which uses the carotenoids lutein, neoxanthin, and violaxanthin to transfer energy to chlorophyll. The third, the peridinin-chlorophyll-protein (PCP) from the dinoflagellate Amphidinium carterae, is the only known complex in which the bound carotenoid (peridinin) pigments outnumber the chlorophylls. The last is xanthorhodopsin from the eubacterium Salinibacter ruber. This complex contains the carotenoid salinixanthin, which transfers energy to a retinal chromophore. The carotenoids in these pigment-protein complexes transfer energy with high efficiency by optimizing both the distance and orientation of the carotenoid donor and chlorophyll acceptor molecules. Importantly, the versatility and robustness of carotenoids in these light-harvesting pigment-protein complexes have led to their incorporation in the design and synthesis of nanoscale antenna systems. In these bioinspired systems, researchers are seeking to improve the light capture and use of energy from the solar emission spectrum.

• MeSH
• Chlorophyll chemistry   metabolism   MeSH
• Dinoflagellida chemistry   metabolism   MeSH
• Eukaryota metabolism   MeSH
• Photosynthesis * physiology   MeSH
• Glucosides MeSH
• Glycosides MeSH
• Carotenoids * chemistry   metabolism   MeSH
• Lutein chemistry   metabolism   MeSH
• Energy Transfer MeSH
• Rhodopseudomonas metabolism   MeSH
• Light MeSH
• Light-Harvesting Protein Complexes * chemistry   metabolism   MeSH
• Thylakoids metabolism   MeSH
• Xanthophylls metabolism   MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Research Support, N.I.H., Extramural MeSH
• Research Support, U.S. Gov't, Non-P.H.S. MeSH
• Names of Substances
• Chlorophyll MeSH
• Glucosides MeSH
• Glycosides MeSH
• Carotenoids * MeSH
• Lutein MeSH
• neoxanthin MeSH Browser
• peridinin MeSH Browser
• rhodopin glucoside MeSH Browser
• salinixanthin MeSH Browser
• Light-Harvesting Protein Complexes * MeSH
• violaxanthin MeSH Browser
• Xanthophylls MeSH