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

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

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 http://www2.estrellamountain.edu/faculty/farabee/BIOBK/BioBookPS.html

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

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

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 620-750 nm! Primary photoacceptor in the photosynthetic antenna system of purple bacteria

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

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

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

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

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, 034311 Biophysical methods, 1997, 7, 738

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

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!

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！