1. Ultrafast energy transfer of artificial dyes conjugated to photosynthetic antenna complex: LH2
Miyasaka Lab.
Yoneda Yusuke

2. Contents I. Introduction II. Experimental method III. My work
What is photosynthesis?
The limit of natural photosynthesis.
II. Experimental method
Femtosecond transient absorption (TA) spectroscopy.
III. My work
Ultrafast energy transfer reaction of LH2-Alexa.
IV. Summary

3. What is photosynthesis?
Photosynthesis reaction :
Consume CO2 and H2O in the presence of sunlight, and produce O2 and carbohydrate.
CO2
Sunlight
light
6CO2 + 6H2O → (CH2O)6 + 6O2 O2
Sugar
Cross section of a leaf
Structure of chloroplast
Chloroplast:葉緑体

4. What is photosynthesis?
Light Harvest
Energy transfer e-
Charge Separation
CALVIN
CYCLE
CO2
ATP
ADP
NADPH
NADP+
Sugar h+
ATP
NADPH
Dark Reaction

5. Characteristics of natural photosynthesis
Light harvesting process
1. Antenna complex harvests sun light.
2. Excitation energy is transferred via antenna complex.
3. Excitation energy is transferred to reaction center.
→　Ultrafast reaction (almost 100 % efficiency)!
LH2
LH1 RC
~5 ps
3~5 ps
~35 ps
~700 fs
B800
B850
B875
Energy transfer hν
Excited states
LH: light-harvesting complex
RC: reaction center
Ground states
B800
B850
B850
B875 RC
Energy transfer: エネルギー移動

6. The limit of natural photosynthesis
The structure of light-harvesting complex 2 (LH2)
Sunlight and absorption spectrum of LH2
β-chain
B800
B850
Car
Car
B800
Top view
B850
α-chain
Side view
There is a gap at nm!
Primary photoacceptor in the photosynthetic antenna system of purple bacteria

7. LH2-Alexa conjugate
Alexa Fluor 647
Synthesis process
LH2
To elucidate the process of energy transfer directly, femtosecond transient absorption measurement was carried out!
LH2-Alexa conjugate:LH2-Alexa結合体

8. Transient absorption spectroscopy
(Time resolved electronic spectrum) t
Probe
Pump
Sample
Detector
𝜟𝑨= 𝑨 ′ −𝑨 ΔA ΔA A’ A
:Transient absorbance
:Absorbance in the presence of
transient species
:Ground state absorbance A’ A
20 fs time resolution!
Transient absorption: 過渡吸収

9. Ultrafast energy transfer reaction of LH2-Alexa
Animation
Alexa
B850
The excited state signals of Alexa decay in picosecond time scale and simultaneously taken over by the excited state signals of B850.
The time constants of energy transfer were 2.9 ps and 17 ps.
Excited at 650 nm Excitation power 10.5 nJ
Pulse duration 17 fs　Solvent: TL buffer pH8.0

10. Excitation wavelength dependence
Excited at 530 nm (Car)
Excited at 775 nm (B800)
B800
B850
B850
Car
B800

11. Energy transfer diagram of LH2-Alexa
140 fs S2 Qx
2.9 or 17 ps
In addition to Car, B800 and B850, Alexa can harvest solar energy.
･The time constants of energy transfer
were 2.9 or 17 ps.
･Alexa isn’t involved in the energy transfer from Car.
137 fs
50 fs
50 fs S1
700 fs Qy Qy S0
Car
Alexa
B800
B850
Biophysical Journal, 2006, 90, 2486
J. Chem. Phys, 2013, 139,
Biophysical methods, 1997, 7, 738

12. Excitation energy dependence
Alexa
Linear excitation energy dependence.
455 nm
(Alexa)
845 nm
(B850)
B850
Saturate tendency.
→　Due to exciton-exciton annihilation!
Laser Power

13. The image of Exciton-Exction annihilation
Exciton-Exciton annihilation
The image of Exciton-Exction annihilation
Alexa
B800
B850
４．Promptly annihilation
３．Two or more B850 excitons were generated
２．Energy transfer from Alexa to B850
１．Two or more Alexas were excited
→　The amount of excited B850 was reduced!

14. Summary
・We observed ultrafast energy transfer in LH2-Alexa system with the time constants of 2.9 ps, 17 ps.
→　Energy transfer efficiency > 99%！
・The multiexponential nature indicates that Alexa moieties are not positioned uniformly.
→　If the orientation and position could be controlled, ideal energy transfer would complete within a few picoseconds!
・Exciton-exciton annihilation only takes place after the energy is transferred to the B850.
→　The ring structure of LH2 is conserved！