Force Spectroscopy with Dual-Trap Optical Tweezers: Molecular Stiffness Measurements and Coupled Fluctuations Analysis

TL;DR

This paper investigates the effects of trap and tether misalignment in dual-trap optical tweezers, providing theoretical and experimental methods to analyze, calibrate, and measure molecular stiffness and DNA persistence length.

Contribution

It introduces criteria to identify and quantify misalignments, and presents a novel force measurement setup and calibration method for dual-trap optical tweezers.

Findings

Misalignment effects can be distinguished and quantified.

A new calibration method for trap and tether stiffness is proposed.

Measured DNA persistence length decreases with contour length.

Abstract

Dual trap optical tweezers are often used in high-resolution measurements in single-molecule biophysics. Such measurements can be hindered by the presence of extraneous noise sources, the most prominent of which is the coupling of fluctuations along different spatial directions, which may affect any optical tweezers setup. In this paper we analyze, both from the theoretical and the experimental points of view, the most common source for these couplings in Dual Trap Optical Tweezers setups: the misalignment of traps and tether. We give criteria to distinguish different kinds of misalignment, to estimate their quantitative relevance and to include them in the data analysis. The experimental data is obtained in a novel dual trap optical tweezers setup which directly measures forces. In the case in which misalignment is negligible we provide a method to measure the stiffness of traps and tether based on variance analysis. This method can be seen as a calibration technique valid beyond the linear trap region. Our analysis is then employed to measure the persistence length of ds-DNA tethers of three different lengths spanning two orders of magnitude. The effective persistence length of such tethers is shown to decrease with the contour length, in accordance with previous studies.