This document describes magnetic tweezers, which are instruments that use magnetic fields to manipulate biomolecules. Magnetic tweezers typically exert piconewton forces on superparamagnetic beads attached to molecules like DNA or proteins to study their mechanical properties. They consist of magnets above a microscope that tracks bead position. Magnetic tweezers allow studying properties of single molecules and complexes by twisting or stretching molecules between a surface and bead. They have high resolution but low throughput. Recent advances aim to improve fields and resolution for diverse single molecule applications.

1. MAGNETIC TWEEZER
Nano Photonics
Centre for Nanoscience and Nanotechnology
Jamia Millia Islamia
University
New Delhi
CHANDRAMANI UPADHYAY
M.TECH(Nanotechnology)
Centre for Nanoscience and Nanotechnology
JMI University
New - Delhi

2. Outline
• INTRODUCTION
• CONTRUCTION
• PRINCIPLE
• EXPERIMENT SETUP
• WORKING PRINCIPLE
• PRONS
• CRONS
• APPLICATION
• FUTURE ASPECTS
• REFERNECES

3. INTRODUCTION
What Is Magnetic Tweezer ?
 Magnetic tweezers (MT) are scientific instruments for the manipulation of biomolecules. This apparatus exerts forces and
torques to individual molecules or groups of molecules. It can be used to measure the tensile strength or the force
generated by molecules.
 Most commonly magnetic tweezers are used to study mechanic properties of biological macromolecules like DNA or
proteins in single-molecule experiments. Other applications are the rheology of soft matter, and studies of force-regulated
processes in living cells. Forces are typically on the order of pico- to nanonewtons. Due to their simple architecture,
magnetic tweezers are a popular biophysical technique.

4. CONSTRUCTION
A magnetic tweezers apparatus consists of magnetic micro-particles, which can be manipulated with the help of an external
magnetic field. The position of the magnetic particles is then determined by a microscopic objective with a camera.
Configuration of the MAGNETIC TWEEZER

5. Principle and Experiment Setup
Basic magnetic tweezers consist of a pair of permanent magnets placed above the sample holder of an inverted
microscope outfitted with a CCD camera.
Experiment preparation's
Typically, a DNA/RNA/Protein molecule is first anchored to a surface with one end and with the other to a probe,
through which force is applied.
The probe is usually a trapped micron sized super paramagnetic bead, the displacement of which allows measurement
of the force. For anchoring the molecule , the substrate and the probe are prepared in a specific way.
The preparation of the molecule(ds DNA),the surface and
the probe before the anchoring.

6. General Considerations:
Molecule attachment –specific end-binding, support infinite loads, inert surface.
Measurement concerns –ability to measure accurate position of probe.

7. WORKING PRICIPLE
 Superparamagnetic beads– size of the order of μm, thermal ordering of magnetic domains on
external magnetic field B.
 Permanent magnet–Strongest Nedymium magnets, collocated with a 1mm gap, 3D is not possible
in this setup, constant force experiments.
 Magnetic interactions –Measure displacement of sensor tethered to a fixed surface by polymer
using equivalence to an inverted pendulum.

8. Manipulation of single molecule with MT
DNA condensation and de-condensation involves the molecular conformation of supercoil–twisted and compacted
state.
A magnetic field is used to torsionally constrain the DNA molecules enabling us to investigate the structure and
topology of supercoiled DNA.

9. Prons or Advantage's
 Magnetic tweezer rheology :-
Magnetic tweezers can be used to measure mechanical properties such as rheology, the study of matter flow and elasticity, in
whole cells. The phagocytosis method previously described is useful for capturing a magnetic bead inside a cell. Measuring the
movement of the beads inside the cell in response to manipulation from the external magnetic field yields information on the
physical environment inside the cell and internal media rheology: viscosity of the cytoplasm, rigidity of internal structure, and ease
of particle flow.
 Single-molecule experiments :-
Magnetic tweezers as a single-molecule method is decidedly the most common use in recent years. Through the single-molecule
method, molecular tweezers provide a close look into the physical and mechanical properties of biological macromolecules.
Similar to other single-molecule methods, such as optical tweezers, this method provides a way to isolate and manipulate an
individual molecule free from the influences of surrounding molecules.
 Single-complex studies :-
Magnetic tweezers go beyond the capabilities of other single-molecule methods, however, in that interactions between and within
complexes can also be observed. This has allowed recent advances in understanding more about DNA-binding proteins, receptor-
ligand interactions, and restriction enzyme cleavage. A more recent application of magnetic tweezers is seen in single-complex
studies.

10. Cons or Disadvantages
An important drawback of magnetic tweezers is the low temporal and spatial resolution due to
the data acquisition via video-microscopy.
However, with the addition of a high-speed camera, the temporal and spatial resolution has been
demonstrated to reach the Angstrom-level.

11. Application of MT
 Bead Tracking
The heart of magnetic tweezers is a computer program that tracks the bead in 3D space and reports its position in real time. In a
typical experiment, the bead is observed through a static oil immersion objective and its image projected onto a digital camera .
To compensate for instrumental drift, one might choose to track simultaneously two beads, one of which is tethered to DNA and
the other is stuck to the surface and serves as a reference. In this set up, detecting the horizontal location of the beads is quite
straightforward.
 Attachment of DNA to the Surface
Attachment of DNA to both the magnetic bead and the surface of the flow cell is essential for single molecule studies. This is
achieved by combining together an unmodified DNA with derivatized DNA handles. Such handles can be prepared in a variety
of ways. Perhaps the easiest way involves PCR using modified dNTPs.
 DNA Stretching
The persistence length is the primary parameter that defines extension of polymers by applied forces. The persistence length of
double stranded DNA measured using single molecule stretching agrees well with numerous previous studies.

12. Future Aspects
 Magnetic tweezers are powerful instruments well suited for diverse single molecule applications.
They are lauded for their ability to twist macromolecules in addition to stretching. However, they
have a number of other unique advantages. In particular, they provide a technical solution that
does not use intense irradiation of the sample, which inevitably leads to accelerated degradation.
 Furthermore, the tweezers generate a nearly homogeneous force field over large distances. As a
result, no adjustments are needed during the course of experiment to compensate for enzyme
translocation. Additionally, the flow chamber can be readily modified for multiplex applications
so that many reactions can be followed in parallel. Finally, magnetic tweezers can handle multiple
DNAs as easily as they handle single molecules.
 Recent advances in hybrid single molecule techniques, which combine magnetic tweezers with
microfluidic systems and fluorescence microscopy, opened new possibilities of experimental
design. These advances also helped to solve one of the original weaknesses of the method, its
difficulties in generating strong magnetic fields. The use of micro manufactured magnetic poles
allows a significant reduction in the separation between the bead and the pole, which helps
produce stronger field gradients and stiffer magnetic traps. Thanks to such developments, sub-
nanometer resolution has become possible.

13. Reference's
 Neuman KC, Nagy A. Single-molecule force spectroscopy: optical tweezers, magnetic tweezers
and atomic force microscopy. Nat Methods (2008) 5:491–505. doi: 10.1038/nmeth.
 Kruithof M, Chien FT, Routh A, Logie C, Rhodes D, van Noort J. Single-molecule force
spectroscopy reveals a highly compliant helical folding for the 30-nm chromatin fiber. Nat
Struct Mol Biol. (2009) 16:534–40. doi: 10.1038/nsmb.
 Tanase, Monica; Biais, Nicolas; Sheetz, Michael (2007). "Chapter 20: Magnetic Tweezers in
Cell Biology". In Wang, Yu-li; Discher, Dennis E. (eds.). Cell Mechanics. Methods in Cell
Biology. 83. Elsevier Inc. pp. 473–493.
 Lipfert, Jan; Hao, Xiaomin; Dekker, Nynke H. (June 2009). "Quantitative Modeling and
Optimization of Magnetic Tweezers". Biophysical Journal. 96 (12): 5040–
5049. Bibcode:2009BpJ....96.5040L. doi:10.1016/j.bpj.2009.03.055.