Fluorescence Resonance Energy Transfer (FRET)
description
Transcript of Fluorescence Resonance Energy Transfer (FRET)
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Fluorescence Resonance Energy Transfer (FRET)
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FRET
Resonance energy transfer can occur when the donor and acceptor molecules are less than 100 A of one another
Energy transfer is non-radiative which means the donor is not emitting a photon which is absorbed by the acceptor
Fluorescence RET (FRET) can be used to spectrally shift the fluorescence emission of a molecular combination.
Resonance Energy Transfer
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FRET The mechanism of FRET involves a donor
fluorophore in an excited electronic state, which may transfer its excitation energy to a nearby
acceptor chromophore
non-radiative fashion through long-range dipole-dipole interactions
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FRET The absorption spectrum of the acceptor must
overlap fluorescence emission spectrum of the donor
Donorfluorescnece
Flu
ores
cnec
e In
tens
ity
Wavelength
Acceptorabsorption
J(λ)
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FRET
Energy Donor excitation state
Emission Acceptor excitation state
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학교 제도 : 교육 제도 중 학교에 관한 제도사회적으로 가장 먼저 공인된 제도 , 형식적 교육 제도1)서구 사회의 학교 제도 - schola : 한가 , 여가를 뜻함 , 오늘날의 학교
school
- 고대 그리스 사회에서 지배계급의 지위와 신분을 유
지하기 위해 소수의 귀족계급을 위해 조직되어 교육 실시
- 중세 유럽사회의 학교는 소수의 성직자나 지도자 양
성 을 위한 교회부속의 사원 학교가 대부분
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FRET
488nm light
excitation
excitation
630nm
light
FITC FITC
520nm
light
TRITC TRITC
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FRET
Distance dependent interaction between the electronic excited states of two molecules
*not sensitive to the surrounding solvent shell of a fluorophore
*Donor-Acceptor 의 Energy transfer 는 거리에 의해 효율이 결정
(~10nm)
Spectral properties of involved chromophore
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FRET
Calculation
Efficiency of Energy Transfer = E = kT/(kT + kf + k’) kT = rate of transfer of excitation energy kf = rate of fluorescence k’ = sum of the rates of all other deexcitation
processes
E = R60/ R60+ R6
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FRET Förster Equation Ro= Forster radius
= Distance at which energy transfer is 50% efficient
= 9.78 x 103(n-4*fd*k2*J)1/6 Å
fd- fluorescence quantum yield of the donor in the absence of acceptor n- the refractive index of the solution k2- the dipole angular orientation of each molecule
j- the spectral overlap integral of the donor and acceptor
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Typical values of R0
Donor Acceptor Ro(Ǻ)Fluorescein Tetramethlrho
damine55
IAEDANS Fluorescein 46
EDANS Dabcyl 33
Fluorescein Fluoresscein 44
BODIPY FL BODIPY FL 57
Fluorescein Qsy7&Qsy9 dyes
61
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FRET
Critical Distance for Common RET Donor-Acceptor Pairs
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FRET
Förster Equation
46
2
417
~
~~~108.8
dF
Rn
kW DA
Dr
DAFörster
Equation
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FRET
Schematic diagram of FRET phenomena
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FRET SUMMARY
Emission of the donor must overlap absorbance of the acceptor
Detect proximity of two fluorophores upon binding
Energy transfer detected at 10-80Ǻ
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FRET
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FRET
Inter-molecular FRET Intra-molecular FRET
Biological application using FRET (ex: cameleon)
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FRET
Biological application using FRET
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OutlineOutline
1. What is fluorescence??
2. Fluorescent molecules
3. Equipment for single-molecule fluorescence experiments
4. Some applications & examples
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fluorescence from moleculesfluorescence from moleculesphysical fundamentsphysical fundaments
photon
photon
molecule in ground state
molecule in excited state
light can induce transitions between electronic states in a molecule
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S0
S1
T0
transition involving emission/absorption of photon
radiationless transition
abso
rptio
n
+hν
fluor
esce
nce
-hν
inte
rnal
co
nver
sion
inte
rsys
tem
cr
ossi
ngin
tern
al
conv
ersi
on
fluorescencefluorescencethe Jablonski diagramthe Jablonski diagram
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fluorescencefluorescenceproperties that can be measuredproperties that can be measured
• spectra (environmental effects)
• fluorescence life times
• polarization (orientation and dynamics)
• excitation transfer (distances ->
dynamics)
• location of fluorescence
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fluorescencefluorescencerequirements for a good fluorophorerequirements for a good fluorophore
• good spectral properties
• strong absorber of light (large extinction coefficient)
• high fluorescence quantum yield
• low quantum yield for loss processes (triplets)
• low quantum yield of photodestruction
• small molecule / easily attachable to biomolecule to be studied
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1.7 Fluorescence quantum yield
knrkr
nrrfluo kk 1
1nrr
rfluo kk
k
S0
S1
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fluorescencefluorescencechromophores: intrinsic or synthetic??chromophores: intrinsic or synthetic??
• common intrinsic fluorophores like tryptophan, NAD(P)H are not good enough
• chlorophylls & flavins work
in most cases extrinsic fluorophores have to be added:
• genetically encoded (green fluorescence protein)
• chemical attachment of synthetic dyes
R
NH
O(H3C)2N N+(CH3)2
O
OCH3
R
NH
NN
N
O
O
CH3
CH3
R
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fluorescencefluorescencea typical synthetic chromophore: a typical synthetic chromophore: tetramethylrhodaminetetramethylrhodamine
• extinction coefficient: ~100,000 Molar-1 cm-1 • fluorescence quantum yield: ~50%• triplet quantum yield <1%• available in reactive forms (to attach to amines,
thiols) and attached to many proteins and other compounds (lipids, ligands to proteins)
400 450 500 550 600 650 700
AbsorptionEmission
Abs
orp
tion
/ Em
issi
on (
a.u.
)
wavelength (nm)
550
550
580
580
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extinction coefficient (): ~100 000 M-1 cm-1
the fluorescence of a single TMR can be measured easilythe fluorescence of a single TMR can be measured easily
absorption cross section () = · 2303 / N0: ~4·10-16 cm2
excitation power: ~100 W/cm2
excitation photon flux = power / photon energy: ~2.5 · 1020 photons·s -1·cm-2
photon energy = h·c/
#excitations·molecule-1·s-1 #exc = flux· ~105 photons·s -1·cm-2
= area of an opaque object with the same that blocks thelight as good as the molecule
dI/I = (·C·NAv/1000)·dL
dI/I = ·2.303·dL
#emitted photons·molecule-1·s-1 #em = #exc·QY ~105 photons·s -1·cm-2
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single-molecule fluorescence microscopysingle-molecule fluorescence microscopy
• excitation source: laser
Lasers cw (ion), pulsed (Nd-YAG, Ti-sapphire, diodes
• detector: - CCD camera, PMT- eyes; PMT, APD, CCD
PhotoMultiplier Tube, Avalanche PhotoDiode,
Charge Coupling Device (signal is usually weak) + electronics
• optics to separate fluorescence from excitation light: filters / dichroic mirrorsmonochromators, spectrographs; filters: colored glass, notch holographic, multidielectric
• optical system with high collection efficiency: high NA objective
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rotation of F1-ATPaserotation of F1-ATPase
Adachi, K., R. Yasuda, H. Noji, H. Itoh, Y. Harada, M. Yoshida, and K. Kinosita, Jr. 2000. Proc. Natl. Acad. Sci. U.S.A. 97:7243-7247
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folding / unfolding of RNAfolding / unfolding of RNA((TetrahymenaTetrahymena ribozymes) ribozymes)
X. Zhuang, L. Bartley, H. Babcock, R. Russell, T. Ha, D. Herschlag, and S. Chu Science 2000 June 16; 288: 2048-2051.
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FLUORESCENCE MEASUREMENTS
• Information given by each property of fluorescence photons:
- spectrum
- delay after excitation (lifetime)
- polarization
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Spectra
Laser exc fluo
Spectrograph
DetectorSample
exc fluo
Fluo. intensity
Excitation spectrumFluorescence spectrum
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Solvent effects
Non-polar solvent
Polar solvent
Energy
Static molecular dipole moment
S0
S1
S1
S0
S1
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Fluorescence Lifetime
Pulsed laser
Sample
Detector
Filter
time
Laser pulses
photons delay
delay, t
number
fluote
/
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Polarization
polarized depolarized
Rigid Fluid
Polarization memory during the fluorescencelifetime : fluo. anisotropy
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Fluorescence Resonance Energy Transfer (FRET)
DAAD RRR
V
ˆˆ314
13
0
Dipole-dipole interaction(near-field)
Donor Acceptor
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Transfer Efficiency
• Fraction of excitations transferred to acceptor
• R0 = Förster radius, maximum 10 nm for large overlap
6
01
1
RRkk
kE
fDDA
DA
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Förster Resonance Energy Transfer
R>10 nm
R<10 nm
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FRET studies of interaction and dynamics(molecular ruler)
Association of two biomolecules
Dynamics ofa biomolecule
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Other specific labeling and imaging
• Possibility to specifically label certain biomolecules, sequences, etc. with fluorophores
• Staining and imaging with various colors• Detection of minute amounts (DNA assays)• Fluorescence lifetime imaging (FLIM)• Fluorescence recovery after photobleaching
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multicolor2-photonmicroscopy
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specific labeling with various colors
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Fluorescence Correlation Spectroscopy
t
I(t)
I(t+
)()()2( tItIg
Keeps track of the fluctuations of the fluorescence intensity.
log
g(2)
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Single molecule spectroscopy
• Single molecule tracking• dynamics of single enzyme• sp-FRET• orientation fluctuations• lifetime measurement