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Excitation Energy Transfer In Photosynthetic Membranes


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Excitation Energy Transfer In Photosynthetic Membranes

  1. 1. Excitation Energy Transfer in Light Harvesting Complex II Jiahao Chen December 15, 2004 PHYS 598 NSM
  2. 2. Plants as Transducers <ul><li>Light energy  Chemical potential energy </li></ul><ul><li>The light harvesting process </li></ul><ul><ul><li>threshold of photochemistry </li></ul></ul>M. Kamen, Primary Processes in Photosynthesis , Academic Press: NY, 1963 . This study Light harvesting ~ 0.5 – 100 ps
  3. 3. Light Harvesting Complex II <ul><li>Most common photosynthetic protein in plants </li></ul><ul><li>Energy funneled to reaction center </li></ul><ul><li>Trimeric in vivo </li></ul><ul><li>Study monomer properties </li></ul><ul><li>Components: chlorophyll a (pink), chlorophyll b (grey), protein (cyan). </li></ul>LHC-II from spinach. PDB code 1RWT. Liu et. al. , Nature , 428 , 2004 , 287-292.
  4. 4. Objectives <ul><li>Quantum and Statistical Physics </li></ul><ul><ul><li>How long does it take to harvest energy? </li></ul></ul><ul><li>Chemical Biology </li></ul><ul><ul><li>What is the efficiency of light harvesting? </li></ul></ul>
  5. 5. The Origin and Nature of Excitation <ul><li>Light absorption </li></ul><ul><li>Photon  exciton </li></ul><ul><ul><li>electronic excited state </li></ul></ul>Ground state hole electron exciton Excited state occupied orbital empty orbital photon
  6. 6. How Energy is Transferred <ul><li>Coulomb interaction </li></ul><ul><ul><li>Coherent/Resonant </li></ul></ul><ul><ul><li>Incoherent/Hopping </li></ul></ul><ul><li>Two mechanisms </li></ul><ul><ul><li>Dexter </li></ul></ul><ul><ul><li>Förster </li></ul></ul><ul><li>Range </li></ul><ul><ul><li><5 Å </li></ul></ul><ul><ul><li>5-12Å </li></ul></ul>Different Spin Correlations! hole electron D onor A cceptor Dexter D onor A cceptor Förster
  7. 7. F örster Theory <ul><li>Approximations </li></ul><ul><ul><li>Time-dependent first-order perturbation theory  Fermi’s Golden Rule </li></ul></ul><ul><ul><li>(Transition) Dipole-dipole interactions only </li></ul></ul><ul><ul><li>Optically accessible states only </li></ul></ul><ul><li>Förster formula </li></ul>
  8. 8. How to Quantify Efficiency <ul><li>Mean passage time </li></ul><ul><ul><li>Average time needed to traverse the entire protein </li></ul></ul><ul><li>Quantum yield </li></ul><ul><ul><li>Fraction of excitons that make it from start to end </li></ul></ul><ul><ul><li>Assume dissipation to be the only competing process </li></ul></ul>
  9. 9. 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
  10. 10. Dipole-dipole couplings in LHC-II
  11. 11. Energy transfer rates for LHC-II (II) Fastest route Energy ends up sloshing between this pair
  12. 12. Results <ul><li>Strongest dipole couplings lead to fastest transition rates </li></ul><ul><li>Light harvesting efficiency: 98.7% </li></ul><ul><ul><li>Excitons have half-life of 50 transitions! </li></ul></ul><ul><ul><li>Excitons can travel long distances before decaying </li></ul></ul><ul><li>Mean passage time: 13.52 ps </li></ul><ul><ul><li>Average transition time: 0.97 ps </li></ul></ul>
  13. 13. Conclusions <ul><li>Numerical evidence support model of antenna complexes funneling energy to a reaction center </li></ul><ul><ul><li>Pair that excitation prefers to move in one direction </li></ul></ul><ul><ul><ul><li>Chl b (650 nm)  Chl a (670 nm) transition </li></ul></ul></ul><ul><ul><li>Pair that excitation prefers to end up at </li></ul></ul><ul><li>Directional preference in exciton transfer rates </li></ul>
  14. 14. Acknowledgements <ul><li>TCBG </li></ul><ul><ul><li>Klaus Schulten </li></ul></ul><ul><ul><li>Melih Şener </li></ul></ul><ul><li>Martínez Group </li></ul><ul><ul><li>Todd Martínez </li></ul></ul><ul><ul><li>Hanneli Huddock </li></ul></ul>