Lecture 8: 17/10

Question 1

How can we measure single myosin forces and movement? How can we measure pN forces in general?

Answer 1

Atomic force microscopes, optical tweezers, magnetic tweezers

Question 2

What are properties of magnetic tweezers?

Answer 2

Can readily rotate, exerts forces on all magnetic particles, very short distances

Question 3

What are properties of AFM?

Answer 3

Precisely defined stress & strain, needs mechanical contact

Question 4

What are properties of flow stretch?

Answer 4

Precisely defined stress & strain, needs flow exposures

Question 5

What are the properties of optical tweezers?

Answer 5

Precisely defined stress & strain, no contact heating can be the issue

Question 6

How is the ray optics regime defined?

Answer 6

Particle&raquo_space; Wavelength of light
Refractive index of particle > surrounding fluid

The diffraction of light gives the particle a kick/transfer of momentum.

Question 7

What happens if particle is not in center of beam intensity?

Answer 7

Asymmetry in intensity, creates asymmetry in force

Question 8

What is Fata Morgana?

Answer 8

Optical illusion causing objects to appear to float when warmer air is on top of cooler air

Question 9

Index of refraction

Answer 9

n = c/v
A measure of how much slower light locally is

Question 10

What happens when light passes into a material of higher index of refraction?

Answer 10

The speed decreases, momentum is conserved

Question 11

What is the equation for the energy of a photon?

Answer 11

E = (h * c) / lambda

Question 12

What is the equation for momentum transfer between different refractive indices?

Answer 12

delta(p) = delta(n) * (h/lambda)

p2 - p1 = (n2 - n1) * (h/lambda)

Question 13

Calculate the total momentum change per second of a 100mW 800 nm going from water (n=1.3) to glass (n=1.5)?

Answer 13

n1 = 1.3, n2 = 1.5, h = plank’s, lambda = 800 nm, P = 100mW

delta(p) of one photon = (n2 - n1) * (h/lambda)
E of one photon = hc / lambda
Photons / Time = Power / (E of one photon)
Total Momentum per time = (delta(p) of one photon) * (photons/time)

Question 14

Explain what leads us to 3D single beam optical tweezers?

Answer 14

We must create not only a radial gradient, but an azimuthal (z) gradient.

When z gradient > scattering force -> the bead is trapped?

Question 15

What is the formula for Stoke’s Law? How is it used for optical traps?

Answer 15

F = 6pirviscosityvelocity

It is used to determine at what velocity the particle pops out of the laser trap; This allows us to create a force vs velocity plot to calibrate the system.

When the flow causes the bead to pop out of the trap, you have exceeded the trap force

Continuously apply force until the bead pops out, plug this force in to calculate for bead velocity at this point.

Question 16

What are the types of optical trap applications?

Answer 16

• Measuring strength or force exerted by motors
• Measuring unfolding or stretching forces
• Measuring relative forces

Question 17

How does ATP concentration impact the single displacement vs time of particles (i.e., how does it influence myosin power stroke)? how does it impact the force?

Answer 17

more ATP = faster cycling
more ATP = greater power (force magnitude is same, applied over shorter duration)

more ATP does not influence step size
Duration increases as ATP concentration decreases
Duration is determined by the peak width

Question 18

What is the average myosin step size obtained?

Answer 18

11nm
(smaller than 40nm, not influenced by ATP concentration)

Question 19

How does an optical stretcher work?

Answer 19

Two divergent lasers shining in the middle, the particles get stretched by the lasers, which can be used to characterize how deformable different cells are (% percentage of cells vs. optical deformability)

Question 20

How does an optical rotator work?

Answer 20

If you rotate the laser, you can rotate the sample/particle.
Makes use of non-uniform cell structures.

Question 21

What is bio optical neuron guidance (BONG)?

Answer 21

Using a femtosecond laser to steer neuron extension and growth structure. Holding on to substructure of growth cone, uses laser to fake a chemoattraction. (?)

Question 22

What are the similarities and differences between magnetic and optical traps?

Answer 22

Similarities:
- Both effective at applying known forces to small particles

Differences:
- Optical traps are more precise in virtually all aspects due to the laser component
- Multi-pole magnetic traps are powerful but complex
- Magnetic traps are useful for long slow measurements or where there is too much scattering/heating

Question 23

What moduli do you expect for the cytoplasm vs cortex?

Answer 23

Cytoplasm - less stiff (Pa)
Cortex - more stiff (kPa)

Question 24

What information does the trap & bead displacement vs time tell us?

Answer 24

The difference in amplitude between the optical trap position and trapped bead position tells us the resistance to displacement (higher amplitude means more resistance).

In phase: elastic solid
90 degrees out of phase: viscous fluid

Question 25

What is the role of vimentin in the cytoplasm?

Answer 25

Vimentin stiffens the elastic cytoplasm, and contributes to ~50% of the modulus

Question 26

How does optical magnetic twisting cytometry (OMTC) work?

Answer 26

Critical tool to examine beads that are partially embedded
Rotates/twists the bead in position

Bead tracking -> Oscillatory Displacement vs Time -> Fit data to sinusoid & extra amplitude -> calculate apparent (complex) modulus from amplitude

Question 27

What is the role of vimentin in the cortex?

Answer 27

Does not impact the moduli of the cell cortex, because the cortex is dominated by actin and is 1000x stiffer than cytoplasm

Question 28

Provide a brief description of magnetic tweezers

Answer 28

• Calculating the applied force a priori is imprecise in this case
• The best approach is to calibrate the current in the magnet vs bead velocity in solution of known viscosity and use Stokes law to determine force

Question 29

Which tools would you use for the following: (add list)

Answer 29

1. Optical Tweezers
2. Magnetic Twisting Cytometry
3. Magnetic Tweezers
4. Optical Tweezers
5. Magnetic Twisting Cytometry, + more
6. Optical Tweezers
7. Optical Stretcher
8. Optical Tweezers

Fast time scale -> Optical
Slow time scale -> Magnetic