Physics Colloquium
Thursday, February 24th, 2005, 4:00 P.M.

Arthur LaPorta

Cornell University

Torque to Biological Molecules with Optical Tweezers

The use of optical tweezers to move microscopic objects and measure linear forces at the piconewton scale has opened up a world of exciting experiments in single-molecule biophysics. This technique allows us to watch as an individual gene is transcribed from DNA to RNA by an enzyme called RNA polymerase.  I will describe an experiment  which determined how an drug called Microcin kills bacteria by interfering with their ability to transcribe genes. I will go on to describe an extension of this technique that makes it possible to trap particles in a specific angular orientation and measure the torque acting on the particle.  The instrument takes advantage of the tendency of materials with anisotropic optical properties to align with the polarization state of an optical field. Using this technique we are able to rotate the particles by varying the polarization state of the trapping beam, and we are able to determine the angular deviation and torque acting on the particles by detecting amount of angular momentum transfered to the trapping beam.  Experiments with quasi-spherical quartz particles (approximately 500 nm diameter) indicate that rotation rates of 30 Hz and torques of several hundred pN-nm can be achieved with reasonable trap intensities.  The detection scheme is able to measure torques of a few pN-nm with bandwidth 1 kHz, and is sufficiently sensitive to observe the rotational Brownian motion of the trapped particle.  By combining our ability to measure instantaneous torque as a function of time and to make essentially instantaneous adjustments to the polarization angle, we have implemented a feedback scheme that can actively stabilize the applied torque under a varying load.  Finally, I will discuss our preliminary results using the system to study DNA elasticity under rotational constraint.