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Optical Force Transducers and Polymer Elasticity

Practical optical tweezers can apply forces up to a few hundred piconewtons. This range is ideally suited for measuring the elastic properties of polymers, including biopolymers. Chu and coworkers attached colloidal spheres to strands of DNA, trapped the spheres in optical tweezers and stretched out the DNA in fluid flows [17]. The strand's elongation provided direct evidence that DNA is not a free draining polymer as sometimes has been assumed.

Several groups have attached one or more colloidal spheres to the ends of polymers and measured the polymers' responses to controlled forces applied and monitored by optical tweezers. A sphere's displacement from its equilibrium position in a well-calibrated tweezer provides a measure of the applied force. Typically, this displacement is measured by gauging the changing pattern of laser light scattered by the particle. These measurements have led, for example, to improved estimates for the bending stiffness and stretching modulus of DNA [18, 19, 20, 21, 22]. Overstretching DNA [21] furthermore leads to uncoiling of the helical structure and controlled separation of the strands in appropriately prepared samples.

Operated in a feedback loop to maintain constant tension, optical tweezer force transducers have been used to measure the forces exerted on DNA by RNA transcriptase [23, 24]. The remarkably high forces developed in this process suggest how the enzyme manages to uncoil DNA during normal transcription.



David G. Grier
Mon Feb 17 21:35:47 CST 1997