The current interpretation of excitation energy transfer (EET) processes in natural photosynthesis generally relies on Kasha's rule, suggesting that internal conversion (IC) processes usually outpace any EET between higher excited states. It is, however, known from research on artificial systems that Kasha's rule does not apply to many dyes, especially when found in assembled clusters analogous to photosynthetic chlorophyll (Chl)-protein complexes. In this contribution, a semiempirical Förster-type model is applied to otherwise well-investigated pigments of natural photosynthesis (Chls a, b, c1 and various carotenoids). Strong potential for anti-Kasha processes is identified in all investigated pigments, based on their high Coulomb coupling elements, similar to compounds with already known anti-Kasha properties. The pigments are further found to form strongly delocalized excitons, especially between the higher excited states usually responsible for anti-Kasha pathways. Test calculations with different pigment compositions for various natural light harvesting complexes (LHCII, CP24, CP26, CP29, FCP) demonstrate how the higher band EET network and absorbance could be affected by the presence of accessory pigments: Chl a-only networks should perform anti-Kasha EET, but this is suppressed by the presence of accessory pigments via several mechanisms (exciton disruption, spectral competition, energy sinks and fast, non-Chl a IC). The apparent "special" behavior of photosynthetic systems is thus resolved as the result of pigment mixtures.

• MeSH
• Pigments, Biological * chemistry   MeSH
• Chlorophyll chemistry   MeSH
• Photosynthesis * MeSH
• Carotenoids chemistry   MeSH
• Energy Transfer MeSH
• Light-Harvesting Protein Complexes chemistry   metabolism   MeSH
• Publication type
• Journal Article MeSH
• Names of Substances
• Pigments, Biological * MeSH
• Chlorophyll MeSH
• Carotenoids MeSH
• Light-Harvesting Protein Complexes MeSH

The functions of both (bacterio) chlorophylls and carotenoids in light-harvesting complexes have been extensively studied during the past decade, yet, the involvement of BChl a high-energy Soret band in the cascade of light-harvesting processes still remains a relatively unexplored topic. Here, we present transient absorption data recorded after excitation of the Soret band in the LH2 complex from Rhodoblastus acidophilus. Comparison of obtained data to those recorded after excitation of rhodopin glucoside and B800 BChl a suggests that no Soret-to-Car energy transfer pathway is active in LH2 complex. Furthermore, a spectrally rich pattern observed in the spectral region of rhodopin glucoside ground state bleaching (420-550 nm) has been assigned to an electrochromic shift. The results of global fitting analysis demonstrate two more features. A 6 ps component obtained exclusively after excitation of the Soret band has been assigned to the response of rhodopin glucoside to excess energy dissipation in LH2. Another time component, ~ 450 ps, appearing independently of the excitation wavelength was assigned to BChl a-to-Car triplet-triplet transfer. Presented data demonstrate several new features of LH2 complex and its behavior following the excitation of the Soret band.

• Keywords
• Antenna complex, Carotenoids, Electrochromic shift, Energy transfer, Excess energy, LH2,
• MeSH
• Bacteriochlorophylls metabolism   MeSH
• Beijerinckiaceae MeSH
• Glucosides MeSH
• Carotenoids * metabolism   MeSH
• Light-Harvesting Protein Complexes * metabolism   MeSH
• Publication type
• Journal Article MeSH
• Names of Substances
• Bacteriochlorophylls MeSH
• Glucosides MeSH
• Carotenoids * MeSH
• rhodopin glucoside MeSH Browser
• Light-Harvesting Protein Complexes * MeSH

Phototrophic Gemmatimonadetes evolved the ability to use solar energy following horizontal transfer of photosynthesis-related genes from an ancient phototrophic proteobacterium. The electron cryo-microscopy structure of the Gemmatimonas phototrophica photosystem at 2.4 Å reveals a unique, double-ring complex. Two unique membrane-extrinsic polypeptides, RC-S and RC-U, hold the central type 2 reaction center (RC) within an inner 16-subunit light-harvesting 1 (LH1) ring, which is encircled by an outer 24-subunit antenna ring (LHh) that adds light-gathering capacity. Femtosecond kinetics reveal the flow of energy within the RC-dLH complex, from the outer LHh ring to LH1 and then to the RC. This structural and functional study shows that G. phototrophica has independently evolved its own compact, robust, and highly effective architecture for harvesting and trapping solar energy.

• Publication type
• Journal Article MeSH

RC-LH1-PufX complexes from a genetically modified strain of Rhodobacter sphaeroides that accumulates carotenoids with very long conjugation were studied by ultrafast transient absorption spectroscopy. The complexes predominantly bind the carotenoid diketospirilloxanthin, constituting about 75% of the total carotenoids, which has 13 conjugated C=C bonds, and the conjugation is further extended to two terminal keto groups. Excitation of diketospirilloxanthin in the RC-LH1-PufX complex demonstrates fully functional energy transfer from diketospirilloxanthin to BChl a in the LH1 antenna. As for other purple bacterial LH complexes having carotenoids with long conjugation, the main energy transfer route is via the S2-Qx pathway. However, in contrast to LH2 complexes binding diketospirilloxanthin, in RC-LH1-PufX we observe an additional, minor energy transfer pathway associated with the S1 state of diketospirilloxanthin. By comparing the spectral properties of the S1 state of diketospirilloxanthin in solution, in LH2, and in RC-LH1-PufX, we propose that the carotenoid-binding site in RC-LH1-PufX activates the ICT state of diketospirilloxanthin, resulting in the opening of a minor S1/ICT-mediated energy transfer channel.

• Keywords
• Carotenoids, Energy transfer, Intramolecular charge transfer state, Light-harvesting, Purple bacteria, Ultrafast spectroscopy,
• MeSH
• Bacteriochlorophylls metabolism   MeSH
• Spectrometry, Fluorescence MeSH
• Carotenoids metabolism   MeSH
• Kinetics MeSH
• Signal Processing, Computer-Assisted MeSH
• Energy Transfer * MeSH
• Rhodobacter sphaeroides metabolism   MeSH
• Light-Harvesting Protein Complexes metabolism   MeSH
• Chromatography, High Pressure Liquid MeSH
• Xanthophylls metabolism   MeSH
• Publication type
• Journal Article MeSH
• Names of Substances
• Bacteriochlorophylls MeSH
• Carotenoids MeSH
• spirilloxanthin MeSH Browser
• Light-Harvesting Protein Complexes MeSH
• Xanthophylls MeSH

The major light-harvesting complex of Amphidinium (A.) carterae, chlorophyll-a-chlorophyll-c 2-peridinin-protein complex (acpPC), was studied using ultrafast pump-probe spectroscopy at low temperature (60 K). An efficient peridinin-chlorophyll-a energy transfer was observed. The stimulated emission signal monitored in the near-infrared spectral region was stronger when redder part of peridinin pool was excited, indicating that these peridinins have the S1/ICT (intramolecular charge-transfer) state with significant charge-transfer character. This may lead to enhanced energy transfer efficiency from "red" peridinins to chlorophyll-a. Contrary to the water-soluble antenna of A. carterae, peridinin-chlorophyll-a protein, the energy transfer rates in acpPC were slower under low-temperature conditions. This fact underscores the influence of the protein environment on the excited-state dynamics of pigments and/or the specificity of organization of the two pigment-protein complexes.

• MeSH
• Spectroscopy, Near-Infrared * MeSH
• Time Factors MeSH
• Chlorophyll A MeSH
• Chlorophyll metabolism   MeSH
• Dinoflagellida metabolism   MeSH
• Electrons MeSH
• Carotenoids metabolism   MeSH
• Kinetics MeSH
• Cold Temperature * MeSH
• Energy Transfer MeSH
• Light-Harvesting Protein Complexes metabolism   MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Names of Substances
• Chlorophyll A MeSH
• Chlorophyll MeSH
• Carotenoids MeSH
• peridinin MeSH Browser
• Light-Harvesting Protein Complexes MeSH

Carotenoids are known to offer protection against the potentially damaging combination of light and oxygen encountered by purple phototrophic bacteria, but the efficiency of such protection depends on the type of carotenoid. Rhodobacter sphaeroides synthesizes spheroidene as the main carotenoid under anaerobic conditions whereas, in the presence of oxygen, the enzyme spheroidene monooxygenase catalyses the incorporation of a keto group forming spheroidenone. We performed ultrafast transient absorption spectroscopy on membranes containing reaction center-light-harvesting 1-PufX (RC-LH1-PufX) complexes and showed that when oxygen is present the incorporation of the keto group into spheroidene, forming spheroidenone, reconfigures the energy transfer pathway in the LH1, but not the LH2, antenna. The spheroidene/spheroidenone transition acts as a molecular switch that is suggested to twist spheroidenone into an s-trans configuration increasing its conjugation length and lowering the energy of the lowest triplet state so it can act as an effective quencher of singlet oxygen. The other consequence of converting carotenoids in RC-LH1-PufX complexes is that S(2)/S(1)/triplet pathways for spheroidene is replaced with a new pathway for spheroidenone involving an activated intramolecular charge-transfer (ICT) state. This strategy for RC-LH1-PufX-spheroidenone complexes maintains the light-harvesting cross-section of the antenna by opening an active, ultrafast S(1)/ICT channel for energy transfer to LH1 Bchls while optimizing the triplet energy for singlet oxygen quenching. We propose that spheroidene/spheroidenone switching represents a simple and effective photoprotective mechanism of likely importance for phototrophic bacteria that encounter light and oxygen.

• MeSH
• Bacterial Proteins chemistry   metabolism   MeSH
• Bacteriochlorophylls chemistry   metabolism   MeSH
• Cell Membrane metabolism   MeSH
• Carotenoids chemistry   metabolism   MeSH
• Oxygen metabolism   MeSH
• Molecular Structure MeSH
• Energy Transfer radiation effects   MeSH
• Proteobacteria chemistry   metabolism   MeSH
• Rhodobacter sphaeroides chemistry   metabolism   MeSH
• Spectrophotometry MeSH
• Light MeSH
• Light-Harvesting Protein Complexes chemistry   metabolism   MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Research Support, U.S. Gov't, Non-P.H.S. MeSH
• Names of Substances
• Bacterial Proteins MeSH
• Bacteriochlorophylls MeSH
• Carotenoids MeSH
• Oxygen MeSH
• PufX protein, Rhodobacter MeSH Browser
• spheroidene MeSH Browser
• spheroidenone MeSH Browser
• Light-Harvesting Protein Complexes MeSH

Chlorosomes, the light-harvesting antennae of green photosynthetic bacteria, are based on large aggregates of bacteriochlorophyll molecules. Aggregates with similar properties to those in chlorosomes can also be prepared in vitro. Several agents were shown to induce aggregation of bacteriochlorophyll c in aqueous environments, including certain lipids, carotenes, and quinones. A key distinguishing feature of bacteriochlorophyll c aggregates, both in vitro and in chlorosomes, is a large (>60 nm) red shift of their Q(y) absorption band compared with that of the monomers. In this study, we investigate the self-assembly of bacteriochlorophyll c with the xanthophyll astaxanthin, which leads to the formation of a new type of complexes. Our results indicate that, due to its specific structure, astaxanthin molecules competes with bacteriochlorophylls for the bonds involved in the aggregation, thus preventing the formation of any significant red shift compared with pure bacteriochlorophyll c in aqueous buffer. A strong interaction between both the types of pigments in the developed assemblies, is manifested by a rather efficient (~40%) excitation energy transfer from astaxanthin to bacteriochlorophyll c, as revealed by fluorescence excitation spectroscopy. Results of transient absorption spectroscopy show that the energy transfer is very fast (<500 fs) and proceeds through the S(2) state of astaxanthin.

• MeSH
• Bacterial Proteins chemistry   isolation & purification   metabolism   MeSH
• Bacteriochlorophylls chemistry   isolation & purification   metabolism   MeSH
• Chlorobium chemistry   MeSH
• Photosynthesis MeSH
• Energy Transfer * MeSH
• Spectrum Analysis MeSH
• Light MeSH
• Light-Harvesting Protein Complexes chemistry   metabolism   MeSH
• Xanthophylls chemistry   MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Names of Substances
• astaxanthine MeSH Browser
• bacteriochlorophyll c MeSH Browser
• Bacterial Proteins MeSH
• Bacteriochlorophylls MeSH
• Light-Harvesting Protein Complexes MeSH
• Xanthophylls MeSH

We present a comparative study of xanthorhodopsin, a proton pump with the carotenoid salinixanthin serving as an antenna, and the closely related bacteriorhodopsin. Upon excitation of retinal, xanthorhodopsin exhibits a wavy transient absorption pattern in the region between 470 and 540 nm. We interpret this signal as due to electrochromic effect of the transient electric field of excited retinal on salinixanthin. The spectral shift decreases during the retinal dynamics through the ultrafast part of the photocycle. Differences in dynamics of bacteriorhodopsin and xanthorhodopsin are discussed.

• Publication type
• Journal Article MeSH