title: Practical axial optical trapping.
abstract: We describe a new method for calibrating optical trapping measurements in which tension is applied in the direction of the laser beam to a molecule tethered between a surface and an optically trapped bead. Specifically, we present a generally-applicable procedure for converting from the measured scattering intensity and the measured stage displacement to applied tension and bead-coverslip separation, using measurements of the light intensity scattered from an untethered, trapped bead. Our calibration accounts for a number of effects, including aberrations and the interference of forward-reflected bead-scattered light with the trapping beam. To demonstrate the accuracy of our method, we show measurements of the DNA force-versus-extension relation using a range of laser intensities, and show that these measurements match the expected extensible wormlike-chain (WLC) behavior. Finally, we also demonstrate a force-clamp, in which the tension in a tether is held fixed while the extension varies as a result of molecular events.
authors.affiliation: Integrated Graduate Program in Physical and Engineering Biology, Yale University, New Haven, Connecticut 06511, USA.
authors.forename: A H
authors.initials: AH
authors.lastname: Mack
authors.forename: D J
authors.initials: DJ
authors.lastname: Schlingman
authors.forename: L
authors.initials: L
authors.lastname: Regan
authors.forename: S G J
authors.initials: SG
authors.lastname: Mochrie
mesh_terms: D001696:Biomechanical Phenomena; D002138:Calibration; D004247:DNA / Q000737:chemistry / Q000378:metabolism; D007834:Lasers; D052898:Optical Tweezers; D012542:Scattering, Radiation; D012815:Signal Processing, Computer-Assisted
chemical_list: D004247:DNA