Magnetic Tweezers

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Magnetic Tweezers

• 20 November 2007
• David J. Niedzwiecki
• djniedzw_at_phy.syr.edu

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What are Magnetic Tweezers?

• Create gradient in the Magnetic Field
• Manipulate this gradient on small scale
• Work with paramagnetic beads to exert minute
  forces

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Paramagnets

• Curries Law
• M is the resulting magnetization
• B is the magnetic flux density of the applied
  field, measured in tesla
• T is absolute temperature
• C is the Curie constant

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First type of Magnetic Tweezers

• Translational Tweezers
• Coiled Electromagnet
• Have paramagnetic core
• Gradient around tip
• Attracts beads

• Biophysical Journal (2005), Vol 88, 2137-2144.

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Second Type of MT

• Rotational Tweezers
• Usually two permanent magnets
• Able rotate magnetic gradient
• Turns beads

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3D manipulation setup

• Set of multiple translational magnets
• Gradients can be more complex
• Able to move in 3-Dimensions

Biophysical Journal (2005), Vol 88, 2137-2144.
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3D manipulation in Cells
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Some Applications of MT

• Microprobes in Living Cells
• Topoisomerases
• Chromatin fiber assembly
• DNA manipulation (Super-Coiling)

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Super-coiling of DNA

• Secure DNA to surface and bead
• Rotational MT turns bead a set number of times
• Use equitation function to find force applied

• Cell Biol. (2003) 81 151-159.

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Force on DNA molecule
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Force vs. Extension for Supercoiled DNA
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Results below 0.4pN

• Extension vs symmetric for positive and
  negative supercoiling
• Formation of plectonemes
• Reduction in tether extension

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Intermediate Forces (1.2pN)

• Negative Coiling
• Extension not effected
• Local denaturation in AT rich regions
• Stretched DNA can hybridize with single-stranded
  DNA probes

• Positive Coiling
• Supercoils still form

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Forces gt 3pN

• No plectoneme formation
• Negative supercoiling continues denaturation
• Positive supercoiling forms pDNA (Pauling DNA)

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Summery

• MT create small gradients in the B field
• Are able to manipulate small beads with high
  precision (pN range)
• Can be used to study systems at the molecular
  level (DNA supercoiling)

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References

• A.H.B. de Vries, B.E. Krenn, R. van Driel and
  J.S. Kanger, Micro Magnetic Tweezers for
  Nanomanipulation Inside Live Cells, Biophysical
  Journal (2005), Vol 88, 2137-2144.
• J. Zaltanova and S. Leuba, Magnetic tweezers a
  sensitive tool to study DNA and chromatin at the
  single-molecule level, Biochem. Cell Biol. (2003)
  81 151-159.
• Referances therein.