optical tweezers f scatt f grad 1. velocity autocorrelation function from the langevin model kinetic...
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Optical Tweezers
Fscatt
Fgrad
1
Velocity autocorrelation function from the Langevin model
0
/
0
0 dtem
kTdttvv mt
kinetic propertyproperty of equilibrium fluctuations
,300
Ddtt
vv
For 3-dimensional model
Green-Kubo relationship
D
2
kT
Fluctuation – Dissipation Theorem
x-axis
F(x)
Periodic asymmetric potential(Randomly fluctuating)
A simple Brownian ratchet
Random diffusion of protein(Gaussian probability distribution)
Probability for protein to move across the potential barrier to the right (+x) is higher than to move to the left
0 a-b
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Manipulating single molecules
• Attach molecule to magnetic particle and use magnetic fieldMagnetic tweezers
• Attach molecule to dielectric particle and use laser lightOptical tweezers
mirror
AFM tip
photodiode position detector
cantilever
laser
imaging surface
sample
laser beam
focus of optical trap
trapF externalF
optical trap force balances the external force
magnetic bead
external magnets
DNA
surface
Atomic Force Microscope Optical Tweezers Magnetic Tweezers
objective
F
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The physics behind optical tweezers
The change in momentum can be calculated by the difference of momentum flux between entering and leaving a dielectric object
BES
0
1
is the Poynting vector for an electromagnetic waveS
Momentum flux of photons is given by
dAScndtPdd
)/()/(
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Optical Trapping - a>> Conditions for Mie scattering when the particleradius a is larger than the wavelength of the light .
We can use a ray optics argument andlook at the transfer of momentum
a
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Optical TweezersLateral gradient force in non-uniform light
High intensity low intensity
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Axial gradient forces towards focus of laser light
Force due to reflection
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Scattering force and gradient force are separable
nm = refractive index trapping mediumnp= refractive index particlem = np/nm (in the Fscatt, Fgrad equation)
Optical Trapping - a<<
Condition for Rayleigh scattering when the particleradius a is smaller than the wavelength of the light .
Fgrad > Fscatt requires tight focusing
a
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The scalesCan trap 0.1 to 10’s m
1m is…..…the same as 1/100th diameter of a hair.
In water, you can move a particle at about 20-30m per sec.
Sensitivity ~ 1 – 100pN
Require 10mW per trap.
Can rotate at 100’s of Hz.
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Optical Trap Dynamics
Equation of motion of particle in a potential well
restoring force
Brownian motion
Newtonian force
drag force
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Particle in fluid
Solution is of exponential decay
Damping provided by water
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Particle in ideal trap
Spring constant or trap stiffness)
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Solution is a simple harmonic motion
t
Trapped particle in fluid
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Solution is of damped simple harmonic motion
t
The whole picture
Time averaged effect is 0
Stochastic events introduce fluctuations in the particle’s position
Add in the effect of Brownian motion
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Trap dynamics
Look at the movement of the particle in x and y
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Collecting data
How can we collect this data?
Moving 100s nm at a few kHz!!!
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Quadrant Photodiode
Quadrant Photodiode
Intensity distribution signals Dx and Dy
Quadrant Photodiode
linear response for small displacements
Quadrant Photodiode
Quadrant photodiode collects the laser light transmitted through the condenser lens.
Small changes in the transmitted and scattered light are measured.
Advantages
• Large bandwidth 100s kHz• Very fast compared to f0
•High light level as collecting laser light
Disadvantages
• Complex arrangement• Single particle
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sample
microscoop lens
dichroic mirror
position sensitive detector
Camera
lamp
laser
microscoop lens
dichroic mirror
Trap strength or stiffness
Fourier transform to get the power spectrum
Lorenzian
Calibration using the Power spectrum22
Real data
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Calibration (viscous drag force calibration)
Vibrate container with liquid with known amplitude xo and frequency
A
)sin( txx o
)cos( txv o
avvFvis 6
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Calibration (viscous drag force calibration)
AA
Double frequency
Signal is )cos( tAS Get A from fitting A against for different ’s.
A
ax
S
F o6calibration constant
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Bio-applications
The size of particles that can be trapped is ~0.1m to 10’s m
Approximately the same size asmany biological specimen.
e.g. Blood cells, stem cells, DNA molecules
Either trapped directly, or beads used as handles to reduce optical damage.
Ashkin et al. Nature. 330, 768 (1987)
Block et al. Nature. 338, 514 (1989)
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Measuring force/motion
Molloy et al. Biophys J. 68, S298 (1995)
biologicalobject
trapped bead
quadrant detector
imaging lens
• Image trapped bead (handle) onto quadrant detector
• Measure movement of shadow– nm accuracy!– kHz response
• Adjust trap to maintain position gives measurement of force– pN accuracy!
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RNA Polymerase
http://www.stanford.edu/group/blocklab/RNAP.html
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RNA-Polymerase
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e.g. Stretching/twisting of DNA
Perkins et al. Science. 264, 822 (1994)
Wang et al. Science. 282, 902 (1998)
• Attach handles to ends of DNA molecule
• Pull, let go and observe what happens!– understanding of
protein folding
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DNA mechanics
Unzipping a DNA double strand
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