Counter-propagating dual-trap optical tweezers based on linear momentum conservation

TL;DR

This paper introduces a dual-trap optical tweezers system that measures forces via linear momentum conservation, enabling precise single-molecule experiments with low drift and robust calibration, including manipulation of very short molecular tethers.

Contribution

The paper presents a novel counter-propagating dual-trap optical tweezers setup that directly measures forces through momentum conservation, improving calibration robustness and enabling manipulation of very short molecular tethers.

Findings

Force measurement based on momentum conservation is robust and calibration-independent.

The setup can perform fluctuation-based molecular stiffness characterization.

It allows manipulation of very short molecular tethers, near contact limits.

Abstract

We present a dual-trap optical tweezers setup which directly measures forces using linear momentum conservation. The setup uses a counter-propagating geometry, which allows momentum measurement on each beam separately. The experimental advantages of this setup include low drift due to all-optical manipulation, and a robust calibration (independent of the features of the trapped object or buffer medium) due to the force measurement method. Although this design does not attain the high-resolution of some co-propagating setups, we show that it can be used to perform different single molecule measurements: fluctuation-based molecular stiffness characterization at different forces and hopping experiments on molecular hairpins. Remarkably, in our setup it is possible to manipulate very short tethers (such as molecular hairpins with short handles) down to the limit where beads are almost in contact. The setup is used to illustrate a novel method for measuring the stiffness of optical traps and tethers on the basis of equilibrium force fluctuations, i.e. without the need of measuring the force vs molecular extension curve. This method is of general interest for dual trap optical tweezers setups and can be extended to setups which do not directly measure forces.