Optical Tweezers + DNA Stretching

Lecture Notes stored at: http://rsc.anu.edu.au/~sevick/ Then select the “Lecture” button.

David Carberry, email: carberry@rsc.anu.edu.au

What are Optical Tweezers?

Simply put: a device that uses light to control and manipulate objects. With the proper detection equipment they can be capable of measuring very small forces (as low as 10 fN, or 10-14 N)

Applications for Optical Tweezers

• Cell sorting

• In vitro fertilisation

• Laser atom cooling

• Orbital Angular Momentum expts

• Thermodynamics Research

• Stretching DNA

• +many more

Refraction: Snell’s Law

Photons/hcE

/hp h = 6.626 * 10-34 J.sc = 3.0 * 108 m.s = wavelength

sinsin 21 nn

Very small individual momentum contributions, but becomes significant when used in large “concentrations”.

Thermal Energy

TkB2

1

Laser Profiles

• Different laser profiles depending on the type of application, characterised by the term TEMxy.• Most common is a Gaussian profile, TEM00.• Other profiles offer different functions, eg optical vortices.

All matter has a thermal energy of:

per degree of freedom.kB = Boltzmann constant, 1.3806503 * 10-23 J.K-1

Combining these ideas

This last image indicates how the Optical Tweezers work.

Optical Traps as Potential Wells

Experiments have shown that the forces acting on a particle when it is within the focal plane operate in a similar manner to a simple spring, i.e.:

Integrating this yields the potential energy. This means we know the potential energy profile of the optical trap.

kxF

Stretching Polymers Using Optical Tweezers

Why Stretch DNA?

• If we know the forces applied when stretching a chain, and we change the system, then we can determine the forces and energetics due to that change.

F(DNA + protein) - F(DNA) F(protein)

F(DNA + salt) - F(DNA) F(salt)

• The force profile of the protein leads to a better understanding of how the protein operates and the conditions it requires manipulate it.

• The effects of other solvents and interactions can be tested.

How to Stretch DNA

1. Prepare DNA ends to have two different labels

2. Prepare particles with two different surface chemistries

3. Mix DNA with particle type 1 (say green)

4. Capture particle type 2 in a micropipette (ie yellow)

5. Move green particle close to yellow particle using optical tweezers and HOPE IT WORKS!

Progress so farCan hold particles in micropipette easily. But forming a link is very difficult.

Preliminary ResultsWe do have a few results, an example is shown here. But the difficulties mean we need to work further on improving the experiment.

In summary

Optical Tweezers use the refraction of light to trap and manipulate microscopic particles.

By measuring the position of the particle we measure the force exerted on the particle.

Using these particles we can investigate many different properties, DNA stretching being just one.

DNA stretching is difficult to achieve, but then again we are manipulating one molecule at a time.

Lecture Notes stored at: http://rsc.anu.edu.au/~sevick/ Then select the “Lecture” button.