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The Initial Photochemical Process of Retinal in Bacter

iorhodopsin

MIYASAKA Lab.Tetsuro KATAYAMA

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Contents

(1)“Probing the ultrafast charge translocation of photoexcited retinal in bacteriorhodopsin”

(1) S.Schenkl, F. van Mourik, G. vander Zwan, S. Haacke, M. Chergui. Science, 2005, 309, 917

Photoisomerization ；光異性化反応

Introduction 　　　・ Bacteriorhodopsin ・ Photoisomerization Experimental ・ Pump-probe spectroscopy Result and Discussion

・ Probing Tryptophan residues Summary

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Organisms which can convert light energy into chemical energy

Halobacterium halobium

Purple membrane consists of bacteriorhodopsin

Halobacteria in the Pink Salt Lakes http://www.striderusa.com/salt.html

http://www.unigiessen.de/~gf1265/GROUPS/KLUG/extrem_halophile.html

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Bacteriorhodopsin resembles photosynthesis and rhodopsin

Bacterio-rhodopsin

Photosynthesis Rhodopsin

Function

Light to energy

Light to energy Light to signal

Initial process

Isomerization of retinal

Charge separation and electron transfer

Isomerization of retinal

Existingplace

Plasma membrane

Chloroplast Retina

Plasma membrane ; 細胞膜Chloroplast ; 葉緑体Retina ; 網膜

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Mechanism of bacteriorhodopsin’s photoisomerization is different from that of rhodopsin

F. Gai et al. , Science, 1998, 279, 1886

Schiff base

Rhodopsin

D.W. McCamant et al. , Science, 2005, 310, 1006

Bacteriorhodopsin

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Cycle of isomerization of retinal

F. Garczarek et al. , Nature , 2006, 439, 109

～ 1μs

～ 100μs

>ms

Process of isomerization to K

BR

J

K

>ms

～ 500fs

～ 3ps

S. O. Smith et al. , Proc. Natl. Acad. Sci. USA , 1984, 81, 2055

7H. Luecke et al. , Science, 1999, 286, 255

Cytosol

Extracellular space

Extracellular space ; Cytosol ; 細胞胞質ゾル細胞外側

Bacteriorhodopsin is a part of solar energy transducer

(1)Deprotonation of the Schiff base, protonation of Asp85

(2)proton release to the extracellular surface (3)reprotonation of the Schiff base, deprotonation of Asp96

(4)reprotonation of Asp96

(5)deprotonation of Asp85 reprotonation of the proton release site.

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Time-dependence of the transient absorption can observed by pump-probe spectroscopy

Probe

Pump

Detector

Delay TimeSampleτ

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What kinds of signals are observed ?

0 t

Photoinduced Absorption

En

erg

y

Equilibrium Nuclear Position

Excitedstate

Groundstate

Equilibrium Nuclear Position ： 平衡核間位置 Photoinduced Absorption 　： 光誘起吸収 Photo-bleaching ： ブリーチングStimulated Emission 　：　誘導放出

0 t

A

⊿

Photo-bleachingStimulated Emission

0 t

A

⊿

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Absorption spectrum of bacteriorhodopsin consists of Trp and retinal absorbance

560nm Retinal absorbance

265 ～ 280nmTryptophan absorbance

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Transient absorption spectra of retinal

F. Gai et al. , Science, 1998, 279, 1886

At 0.316ps

At 31.6ps

Absorbance spectrum

Photoisomerizationwas over

After 31.6psExcited-state absorption

and stimulated emission

disappearedBleach signal

and Photoproduct

absorptionremained

460nm

560nm

630nm

910nm

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Initial process of photoisomerization The first event H→I occurs within 50-100fs

The second eventoccurs within 100-200fs.

The third event occurswithin 200-600fs

The fourth event occurswithin 700fs-3 ps

T. Kobayash.et al., Nature, 2001, 414, 531

J

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Trp residues close to retinal Transition dipole of retinal and Trp86 are parallel

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What do components of signal indicate ?

Decay component was fitted to previous temporal transient absorption (420fs=relaxation to J,3.5ps= relaxation to K)A constant ,weak bleach signal is observed at the longest delay times

-1 0 1 2 3 10 Delay (ps)

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What is rise component ?

Cross-correlation(FWHM=85fs)

Temporal derivativebleach transient(FWHM=150fs)

Rise time was observed

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Exiciton-Coupling model

La,b La,b

S1

Sn

X1, X2, X3 ; Linear coupling of La,b － Sn XX1, XX2 ; Linear coupling of La,b, － S1

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Calculation of the effect of retinal dipole moment change

Bleach signal increase

Dipole moment of retinal increase

A* shifts redA doesn’t change

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Conclusion

Charge 　translocationoccurs

The isomerization takes place after charge translocation.

K

J

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Summary

Time scale of charge translocation and its interplay with the initial twisting and subsequent isomerization were confirmed.

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Landau-zener equation