Light Harvesting in Photosynthesis Gabriela Schlau-Cohen Fleming Group.
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Transcript of Light Harvesting in Photosynthesis Gabriela Schlau-Cohen Fleming Group.
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Light Harvesting in Photosynthesis
Gabriela Schlau-CohenFleming Group
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Earth is formed
Life emerges
Photosynthesis
Cyanobacteria
Oxygen-rich atmosphere
Plants
Dinosaurs
Hominids, 3pm
Homo Sapiens, 11:38 pm
US 11:59:58 pm
Age of Earth 4.55 billion yrs
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Prevalence of Photosynthesis
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Photosynthesis 101
Efficient: ~90% of energy absorbed is used to initiate photochemistry
Fast:Turnover rate of photosynthesis : ~ 100-300/sec
Regulated:~75% of absorbed energy can be dissipated with a 1sec response time
2 26CO 6H O 6 12 6 2C H O 6Olight
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Architecture of Photosynthesisis Optimized to:
Cover the solar spectrumProtect against photochemical damage
Separate energy and electron transfer
Regulate the efficiency of lightharvesting and repair damage (PSII)
Transmit excitation to thereaction center with near unit efficiency
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Protein Complexes in the Plant
Cross-section of the leaf
Thylakoid membrane
Chloroplast
Pigment Protein Complexes
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D2
D1
D1
D2
CP43
CP47
CP47
CP43
CP26
LHCII
CP29
CP26
LHCII
CP29
CP24
CP24
LHCII
LHCIIPeripheral antennas
Peripheral antennas
Core Complex
Core Complex
Overview of photosynthetic supercomplex in plants
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Light Harvesting Complex II (LHCII)
• >50% of plant chlorophylls in LHCII
• Functions as antenna complex
• Absorbs light energy and funnels to reaction center
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How does Light Harvesting Really Work?
Can we find a way to image Functionality?
• Are the excited states localized?
• Are the states optimized in both spatial and energetic landscapes?
• Does the excitation hop?
• Does Quantum Mechanics matter?
• Do only optically allowed transitions matter?
• How is photo-induced damage minimized?
• How do we learn about interactions, pathways and mechanisms when all the molecules are chemically identical?
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Spectral Properties of LHCII
•Delocalized excited states allow for energetic jumps
•Chl b localized near Chl a to allow for downhill energy transfer
Carotenoids
Chl a
Chl b
Chl b Vibronic band
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Two-Dimensional Spectroscopy
• Excitation at one wavelength influences emission at other wavelengths
• Diagonal peaks are linear absorption
• Cross peaks are coupling and
energy transfer
Excited StateAbsorption
Inhomogeneous Linewidth
HomogeneousLinewidth
CrossPeak
ωτ (“absorption”)
ωt
(“em
iss
ion
”)
Dimer Model (Theory)
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Dynamics in 2D Spectroscopy
ωτ (cm-1)
ωt (c
m-1)
Dimer Model (Theory)
T=0 fs T=500 fs T=50 ps
ωτ (cm-1) ωτ (cm-1)
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Experimental Conditions• Sample Details
– Wild type LHCII– Extracted from Arabidopsis Thaliana
• Sample Conditions– 65:35 v/v glycerol to LHCII solution– LHCII Solution:
• 10 mM Hepes/HCl pH 7.6• OD670=0.18
– Temperature: 77K– 200 um Silanized quartz cell (SigmaCote)
• Laser Conditions– 650 nm spectral center, 80 nm FWHM– 23 fs pulse (measured by auto-correlation)– 99.7 μW incident on sample
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LHCII 2D Spectra
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LHCII 2D Spectra
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LHCII 2D Spectra
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LHCII 2D Spectra
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LHCII 2D Spectra
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LHCII 2D Spectra
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LHCII 2D Spectra
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LHCII 2D Spectra
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LHCII Spectral Dynamics
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Energy Transfer in LHCII
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Electronic Coupling
1 2Dimer
E
g1
e1
g2
e2
1
2
J
E
J
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Localization of Dynamics
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T = 80 fs
T = 80 fsElectronic Coherence
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Conclusions & Future Directions
• Coupling creates delocalized excited states over multiple chromophores
• Delocalization means spatial overlap of energetically separate chromophores which allows large energetic jumps
• Chl-b arranged near Chl-a for interband energy transfer
• What specific mechanisms give rise to these energy transfer processes?
• What role does quantum coherence play and how does it change at room temperature?
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AcknowledgementsGraham Fleming
Tessa CalhounElizabeth ReadGreg EngelNaomi Ginsberg
References:Rick Trebino, Georgia Tech
http://www.physics.gatech.edu/gcuo/UltrafastOptics/index.html
Blankenship, R.E. Molecular Mechanisms of Photosynthesis, 2004.