Energy Transfer from Carotenoids to Bacteriochlorophylls
The photosynthetic apparatus contains light-harvesting (LH) pigment-protein complexes that capture light energy from the sun and transfer it efficiently to the reaction center. In photosynthetic bacteria, carotenoids supplement the non-optimal LH capacity of bacteriochlorophyll (BChl) in the 400–500 nm region of the visible spectrum. Thus, carotenoid-to-BChl energy transfer provides an essential process for enhancing the ability of these systems to capture light energy and convert it into useful work. Carotenoids have at least two states involved in energy transfer to BChl. These are the S2 state into which absorption from the ground state, S0, is strongly allowed, and a low-lying, S1 state into which absorption is forbidden by symmetry. These two states represent the primary energy donors for carotenoid-to-BChl energy transfer. The S2 state transfers energy with an efficiency between 30 and 70%, the value of which is only slightly dependent on the structure of the carotenoid. The S1-mediated energy transfer pathway depends strongly on the π-electron conjugation length of the carotenoid. This route is essentially closed for carotenoids with eleven or more conjugated carbon-carbon double bonds because in these cases the S1 energy of the carotenoid lies too low to enable transfer to BChl. Besides the main S2 and S1 pathways, the past few years of investigations have raised the prospect of other carotenoid excited states participating in energy transfer. The possibilities include vibrationally hot S1 states, a state denoted S* thought to be formed by a branched deactivation pathway from S2, and a state with symmetry representation 1B u − predicted on the basis of theoretical computations to lie between S1 and S2. This chapter reviews the evidence for these states and discusses their possible involvement as energy donors in the process of light-harvesting in photosynthetic bacteria.
KeywordsChlorophyll Carbonyl Photosynthesis Gall Carotenoid
- BChl a
lithium dodecyl sulfate
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- Desamero RZB, Chynwat V, van der Hoef I, Jansen FJ, Lugtenburg J, Gosztola D, Wasielewski MR, Cua A, Bocian DF and Frank HA (1998) Mechanism of energy transfer from carotenoids to bacteriochlorophyll: Light-harvesting by carotenoids having different extents of π-electron conjugation incorporated into the B850 antenna complex from the carotenoidless bacterium Rhodobacter sphaeroides R-26.1. J Phys Chem B 102: 8151–8162CrossRefGoogle Scholar
- Gradinaru CC, Kennis JTM, Papagiannakis E, van Stokkum IHM, Cogdell RJ, Fleming GR, Niederman RA and van Grondelle R (2001) An unusual pathway of excitation energy deactivation in carotenoids: Singlet-to-triplet conversion on an ultrafast timescale in a photosynthetic antenna. Proc Natl Acad Sci USA 98: 2364–2369PubMedCrossRefGoogle Scholar
- Papagiannakis E, Das SK, Gall A, Stokkum IHM, Robert B, van Grondelle R, Frank HA and Kennis JTM (2003a) Light harvesting by carotenoids incorporated into the B850 light-harvesting complex from Rhodobacter sphaeroides R-26.1: Excited-state relaxation, ultrafast triplet formation, and energy transfer to bacteriochlorophyll. J Phys Chem B 107: 5642–5649CrossRefGoogle Scholar
- Papagiannakis E, van Stokkum IHM, van Grondelle R, Niederman RA, Zigmantas D, Sundström V and Polívka T (2003b) A near-infrared transient absorption study of the excited-state dynamics of the carotenoid spirilloxanthin in solution and in the LH1 complex of Rhodospirillum rubrum. J Phys Chem B 107: 11216–11223CrossRefGoogle Scholar
- Polivka T, Zigmantas D, Herek JL, He Z, Pascher T, Pullerits T, Cogdell RJ, Frank HA and Sundström V (2002) The carotenoid S1 state in LH2 complexes of purple bacteria Rhodobacter sphaeroides and Rhodopseudomonas acidophila: S1 energies, dynamics, and carotenoid radical formation. J Phys Chem B 106: 11016–11025CrossRefGoogle Scholar
- Rondonuwu FS, Watanabe Y, Fujii R and Koyama Y (2003) A first detection of singlet to triplet conversion from the 1Bu - to the 3Ag state and triplet internal conversion from the 3Ag to the 3Bu state in carotenoids: dependence on the conjugation length. Chem Phys Lett 376: 292–301CrossRefGoogle Scholar
- Rondonuwu FS, Yokoyama K, Fujii R, Koyama Y, Cogdell RJ and Watanabe Y (2004) The role of the 1Bu - state in carotenoid-to-bacteriochlorophyll singlet-energy transfer in the LH2 antenna complexes from Rhodobacter sphaeroides G1C, Rhodobacter sphaeroides 2.4.1, Rhodospirillum molischianum and Rhodopseudomonas acidophila. Chem Phys Lett 390: 314–322CrossRefGoogle Scholar
- van Grondelle R (1985) Excitation energy transfer, trapping and annihilation in photosynthetic systems. Biochim Biophys Acta 811: 147–195Google Scholar
- Wohlleben W, Buckup T, Hashimoto H, Cogdell RJ, Herek JL and Motzkus M (2004) Pump-deplete-probe spectroscopy and the puzzle of carotenoid dark states. J Phys Chem 108: 3320–3325Google Scholar