Excitation Energy Transfer in Light Harvesting Complex II

Jiahao ChenDecember 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

Förster 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 betweencenters of mass: Rij

Identity:chlorophyll a v. b

Transition dipolestrength, f

Orientation:k factor

Förster rate: kij

Quantum yield, Mean passage time:

results

PDB

literatureparameters

Dipole-dipole couplings in LHC-II

Energy transfer rates for LHC-II (II)

Fastest route

Energy ends upsloshing betweenthis 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