Ultrafast energy transfer of artificial dyes conjugated to photosynthetic antenna complex: LH2 Miyasaka Lab. Yoneda Yusuke Contents I. Introduction 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? H2O CO2 Sunlight Photosynthesis reaction : Consume CO2 and H2O in the presence of sunlight, and produce O2 and carbohydrate. light 6CO2 + 6H2O → (CH2O)6 + 6O2 O2 Sugar Cross section of a leaf http://www2.estrellamountain.edu/faculty/farabee/BIOBK/BioBookPS.html Structure of chloroplast Chloroplast:葉緑体 What is photosynthesis? Light Harvest Energy transfer Charge Separation h+ CO2 ATP ADP ADP ATP CALVIN CYCLE ATP NADPH Dark Reaction NADPH NADP+ Sugar e- 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 B800 ~700 fs B850 LH2 LH1 ~5 ps RC ~35 ps Energy transfer 3~5 ps B875 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 Top view B800 B850 α-chain Side view Primary photoacceptor in the photosynthetic antenna system of purple bacteria There is a gap at 620-750 nm! LH2-Alexa conjugate Synthesis process LH2 Alexa Fluor 647 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 Sample Detector Pump 𝜟𝑨 = 𝑨′ − 𝑨 ΔA :Transient absorbance A’ :Absorbance in the presence of transient species A :Ground state absorbance ΔA 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) Car B800 B850 Excited at 775 nm (B800) B800 B850 Energy transfer diagram of LH2-Alexa 140 fs S2 137 fs 2.9 or 17 ps Qx 50 fs S1 50 fs 700 fs Qy Qy S0 Car Alexa B800 B850 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. Biophysical Journal, 2006, 90, 2486 J. Chem. Phys, 2013, 139, 034311 BIOPHYSICAL METHODS, 1997, 7, 738 Excitation energy dependence Excitation energy dependence Alexa Linear excitation energy dependence. 455 nm (Alexa) 845 nm (B850) Laser Power B850 Saturate tendency. → Due to exciton-exciton annihilation! Exciton-Exciton annihilation The image of Exciton-Exction annihilation Alexa B800 B850 ３．Two B850 excitons were generated １．Two or more Alexas were ４．Promptly annihilation ２．Energy transfer from Alexaexcited to B850 → 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！
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