The Mechanisms of Photosynthetic Light Harvesting and NonPhotochemical Quenching

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The Mechanisms of Photosynthetic Light Harvesting
and Non-Photochemical Quenching Rienk van
Grondelle
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Contents

• Introduction to Photosynthesic Light-Harvesting.
• The Disordered Exciton Model.
• The Dynamics of Energy Transfer in LH1/LH2.
• LHCII
• Photosystem II core
• Photosystem I
• What has been optimized?
• The regulation of Photosynthetic Light-Harvesting
• Switching Proteins

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The Photosynthetic Membrane Architecture For
Excitation Energy Transfer
AFM view Bahatyrova, Frese, Siebert, Olsen, van
der Werf, van Grondelle, Niederman, Bullough,
Otto and Hunter, Nature 430, (2004), 1058
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Some light-harvesting systems

• Plant Light-harvesting complex 2

• Purple bacterial LH2

• Plant photosystem I

• Plant photosystem 2

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B850
Carotenoids
B800
Qy
Qx
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Photosynthetic Light-Harvesting
5 ps
1 ps
1 ps
30 ps
5 ps
1 ps
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3PEPS LH2
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Photosynthetic Light-Harvesting
5 ps
1 ps
1 ps
30 ps
5 ps
1 ps
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When the complex is excited with light, an
electron moves from the Chl dimer to a Pheophytin
(looks like Chlorophyll) and then to a quinone
LIGHT
Chl dimer

3 ps
-
Chl
4 nm
Reactie Centrum
1 ps
-
Ph
200 ms
200 ps
-
-
Q
Fe
1 ps 10-12 s
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How does this work for a simple LH2
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Förster Resonance Energy Transfer

Vdipole-dipole coupling


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Excitonic Interactions
V small
large
Excitonically coupled system
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Exciton Interaction in Dimer


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Excitonic Interactions
V small
large
Excitonically coupled system
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Relaxation In An Exciton Manifold
Vlarge
Delocalized excited state
Jkp(w)
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The Redfield Rate
In the limit of very weak coupling of the
excitonic states to vibrations/phonons we find
for a transition between two elements of the
density matrix rkr and rps
Relaxation between one-exciton states (k,r) and
(p,s), where k?r and p?s for coherence transfer
or decay, or kr and ps for population transfer,
is given by ?nmcnkcnpcmrcmsJkpltvnvmgt.
Overlap Excitonic Wavefunctions
Coupling to Phonons/Vibs
Spectral Density
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V
V
Redfield
Redfield
Förster
Förster
V
V
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How does this work for a simple LH2
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Pump-probe experiments on the bacterial LH2
light-harvesting complex
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The exciton structure for a ring ()
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The exciton structure for a ring (2)
All dipole strength in two next levels
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Superradiance of LH2
Disorder equals or exceeds coupling
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The Disorder-Exciton-Redfield model
A structure (distances, orientations)
Excitons
Disorder
Relaxation
Relaxation between one-exciton states (k,r) and
(p,s), where k?r and p?s for coherence transfer
or decay, or kr and ps for population transfer,
is given by the term ?nmcnkcnpcmrcmsJkpltvnvmgt.
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Redfield Model for Energy Transfer in LH2
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Relaxation in the Exciton Manifold of LH2
Relaxation between one-exciton states (k,r) and
(p,s), where k?r and p?s for coherence transfer
or decay, or kr and ps for population transfer,
is given by the term ?nmcnkcnpcmrcmsJkpltvnvmgt.
k-th exciton state
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Exciton Relaxation in RC-LH1 of Rps. viridis
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Vibrational Wavepackets in LH1 of Rps viridis
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Excitation Transfer in LHCII
Chl a
Chl b
Neoxanthin
Violaxanthin
Lutein
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2.72 Å LHCII atomic modelExcitonic Clusters
Stromal side Lumenal side

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Spectroscopy of LHC-II
ODLD
Low Temperature Fluorescence
calculated