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Nucleic Acids Research Advance Access published online on September 3, 2009
Nucleic Acids Research, doi:10.1093/nar/gkp725
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© The Author(s) 2009. Published by Oxford University Press.
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/2.5/uk/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Methods Online

A high-resolution magnetic tweezer for single-molecule measurements

Kipom Kim1 and Omar A. Saleh1,2,*

1Materials Department and 2Biomolecular Science and Engineering Program, University of California, Santa Barbara, CA 93106, USA

*To whom correspondence should be addressed. Tel: +1 805 893 8814; Fax: +1 805 893 8486; Email: saleh{at}engineering.ucsb.edu

Received May 1, 2009. Revised July 26, 2009. Accepted August 17, 2009.

Magnetic tweezers (MT) are single-molecule manipulation instruments that utilize a magnetic field to apply force to a biomolecule-tethered magnetic bead while using optical bead tracking to measure the biomolecule’s extension. While relatively simple to set up, prior MT implementations have lacked the resolution necessary to observe sub-nanometer biomolecular configuration changes. Here, we demonstrate a reflection-interference technique for bead tracking, and show that it has much better resolution than traditional diffraction-based systems. We enhance the resolution by fabricating optical coatings on all reflecting surfaces that optimize the intensity and contrast of the interference image, and we implement feedback control of the focal position to remove drift. To test the system, we measure the length change of a DNA hairpin as it undergoes a folding/unfolding transition.


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