title: New insights into the photochemistry of carotenoid spheroidenone in light-harvesting complex 2 from the purple bacterium Rhodobacter sphaeroides.
abstract: Light-harvesting complex 2 (LH2) from the semi-aerobically grown purple phototrophic bacterium Rhodobacter sphaeroides was studied using optical (static and time-resolved) and resonance Raman spectroscopies. This antenna complex comprises bacteriochlorophyll (BChl) a and the carotenoid spheroidenone, a ketolated derivative of spheroidene. The results indicate that the spheroidenone-LH2 complex contains two spectral forms of the carotenoid: (1) a minor, "blue" form with an S2(11Bu+) spectral origin band at 522 nm, shifted from the position in organic media simply by the high polarizability of the binding site, and (2) the major, "red" form with the origin band at 562 nm that is associated with a pool of pigments that more strongly interact with protein residues, most likely via hydrogen bonding. Application of targeted modeling of excited-state decay pathways after carotenoid excitation suggests that the high (92%) carotenoid-to-BChl energy transfer efficiency in this LH2 system, relative to LH2 complexes binding carotenoids with comparable double-bond conjugation lengths, derives mainly from resonance energy transfer from spheroidenone S2(11Bu+) state to BChl a via the Qxstate of the latter, accounting for 60% of the total transfer. The elevated S2(11Bu+) → Qxtransfer efficiency is apparently associated with substantially decreased energy gap (increased spectral overlap) between the virtual S2(11Bu+) → S0(11Ag-) carotenoid emission and Qxabsorption of BChl a. This reduced energetic gap is the ultimate consequence of strong carotenoid-protein interactions, including the inferred hydrogen bonding.
authors.affiliation: Department of Chemistry, Washington University in St. Louis, St. Louis, MO, 63130, USA. niedzwiedzki@wustl.edu.|Photosynthetic Antenna Research Center, Washington University in St. Louis, Campus Box 1138, St. Louis, MO, 63130, USA. niedzwiedzki@wustl.edu.
authors.forename: Dariusz M
authors.initials: DM
authors.lastname: Niedzwiedzki
authors.affiliation: Department of Chemistry, Washington University in St. Louis, St. Louis, MO, 63130, USA.
authors.forename: Preston L
authors.initials: PL
authors.lastname: Dilbeck
authors.affiliation: Department of Chemistry, University of California Riverside, Riverside, CA, 92521, USA.
authors.forename: Qun
authors.initials: Q
authors.lastname: Tang
authors.affiliation: Department of Molecular Biology and Biotechnology, University of Sheffield, Sheffield, S10 2TN, UK.
authors.forename: Elizabeth C
authors.initials: EC
authors.lastname: Martin
authors.affiliation: Department of Chemistry, University of California Riverside, Riverside, CA, 92521, USA.
authors.forename: David F
authors.initials: DF
authors.lastname: Bocian
authors.affiliation: Department of Molecular Biology and Biotechnology, University of Sheffield, Sheffield, S10 2TN, UK.
authors.forename: C Neil
authors.initials: CN
authors.lastname: Hunter
authors.affiliation: Department of Chemistry, Washington University in St. Louis, St. Louis, MO, 63130, USA.
authors.forename: Dewey
authors.initials: D
authors.lastname: Holten
mesh_terms: D002338:Carotenoids / Q000378:metabolism*; D004735:Energy Transfer; D045342:Light-Harvesting Protein Complexes / Q000378:metabolism*; D010777:Photochemistry; D012242:Rhodobacter sphaeroides / Q000378:metabolism*; D013050:Spectrometry, Fluorescence; D013059:Spectrum Analysis, Raman
chemical_list: D045342:Light-Harvesting Protein Complexes; C104880:spheroidenone; D002338:Carotenoids
keywords: Carotenoids; Energy transfer; Light-harvesting complex; Purple bacteria; Ultrafast spectroscopy