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Excitation Energy Transfer In Photosynthetic Membranes
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Excitation Energy Transfer In Photosynthetic Membranes



Term project for PHYS 598NSM (K. Schulten, Fall 2004).

Term project for PHYS 598NSM (K. Schulten, Fall 2004).



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Excitation Energy Transfer In Photosynthetic Membranes Excitation Energy Transfer In Photosynthetic Membranes Presentation Transcript

  • Excitation Energy Transfer in Light Harvesting Complex II Jiahao Chen December 15, 2004 PHYS 598 NSM
  • Plants as Transducers
    • Light energy  Chemical potential energy
    • The light harvesting process
      • threshold of photochemistry
    M. Kamen, Primary Processes in Photosynthesis , Academic Press: NY, 1963 . This study Light harvesting ~ 0.5 – 100 ps
  • Light Harvesting Complex II
    • Most common photosynthetic protein in plants
    • Energy funneled to reaction center
    • Trimeric in vivo
    • Study monomer properties
    • Components: chlorophyll a (pink), chlorophyll b (grey), protein (cyan).
    LHC-II from spinach. PDB code 1RWT. Liu et. al. , Nature , 428 , 2004 , 287-292.
  • Objectives
    • Quantum and Statistical Physics
      • How long does it take to harvest energy?
    • Chemical Biology
      • What is the efficiency of light harvesting?
  • The Origin and Nature of Excitation
    • Light absorption
    • Photon  exciton
      • electronic excited state
    Ground state hole electron exciton Excited state occupied orbital empty orbital photon
  • How Energy is Transferred
    • Coulomb interaction
      • Coherent/Resonant
      • Incoherent/Hopping
    • Two mechanisms
      • Dexter
      • Förster
    • Range
      • <5 Å
      • 5-12Å
    Different Spin Correlations! hole electron D onor A cceptor Dexter D onor A cceptor Förster
  • F örster Theory
    • Approximations
      • Time-dependent first-order perturbation theory  Fermi’s Golden Rule
      • (Transition) Dipole-dipole interactions only
      • Optically accessible states only
    • Förster formula
  • How to Quantify Efficiency
    • Mean passage time
      • Average time needed to traverse the entire protein
    • Quantum yield
      • Fraction of excitons that make it from start to end
      • Assume dissipation to be the only competing process
  • Computational Procedure Atomic coordinates Distances between centers of mass: R ij Identity: chlorophyll a v. b Transition dipole strength, f Orientation: k factor Förster rate: k ij Quantum yield,  Mean passage time:  results PDB literature parameters
  • Dipole-dipole couplings in LHC-II
  • Energy transfer rates for LHC-II (II) Fastest route Energy ends up sloshing between this pair
  • Results
    • Strongest dipole couplings lead to fastest transition rates
    • Light harvesting efficiency: 98.7%
      • Excitons have half-life of 50 transitions!
      • Excitons can travel long distances before decaying
    • Mean passage time: 13.52 ps
      • Average transition time: 0.97 ps
  • Conclusions
    • Numerical evidence support model of antenna complexes funneling energy to a reaction center
      • Pair that excitation prefers to move in one direction
        • Chl b (650 nm)  Chl a (670 nm) transition
      • Pair that excitation prefers to end up at
    • Directional preference in exciton transfer rates
  • Acknowledgements
    • TCBG
      • Klaus Schulten
      • Melih Şener
    • Martínez Group
      • Todd Martínez
      • Hanneli Huddock