High-resolution dual-trap optical tweezers with differential detection: alignment of instrument components

Simple way to correct the drift in surface-coupled optical tweezers using the laser reflection pattern

The surface-coupled optical tweezers are widely used to resolve small units of motion in biology. However, such motions could readily be interfered by the drift between the trap and surface. We present a simple and low-cost method to correct the drift both actively and passively based on video tracking the distance between the laser reflection pattern and the reference bead. As a result, we achieved sub-nanometer resolution and stability for the stuck bead over a broad range of averaging time (0.002-100 s) as demonstrated by the Allan deviation analysis. The sub-nanometer resolution was further manifested with step measurement. Finally, in double-stranded DNA and DNA hairpin stretching experiments, an extension resolution of 1-2 nm with the stability over 120 s has been demonstrated under a constant force. This work thus provides an easy way to bring the benefit of nanometer resolution and long-term stability to the surface-coupled optical tweezers.

From folding to function: complex macromolecular reactions unraveled one-by-one with optical tweezers

Single-molecule manipulation with optical tweezers has uncovered macromolecular behaviour hidden to other experimental techniques. Recent instrumental improvements have made it possible to expand the range of systems accessible to optical tweezers. Beyond focusing on the folding and structural changes of isolated single molecules, optical tweezers studies have evolved into unraveling the basic principles of complex molecular processes such as co-translational folding on the ribosome, kinase activation dynamics, ligand-receptor binding, chaperone-assisted protein folding, and even dynamics of intrinsically disordered proteins (IDPs). In this mini-review, we illustrate the methodological principles of optical tweezers before highlighting recent advances in studying complex protein conformational dynamics - from protein synthesis to physiological function - as well as emerging future issues that are beginning to be addressed with novel approaches.

Design of a high-quality optical conjugate structure in optical tweezers

We propose an approach to realize a high-quality optical conjugate of a piezo-driven mirror (PM) in optical tweezers. Misalignments between the optical beam and the steering center of the PM are analyzed mathematically. The decentrations in different directions cause different changes, either a position change of the conjugate plane or a spot variation of the beam during PM steering. On the other hand, these misalignment-introduced problems provide the information to check the assembling errors. Thus a wanted conjugate plane of the PM can be effectively and precisely achieved according to the detection signals. This approach is also available to deal with multifactor coupling error. At the end, the procedure for error analysis is given by testing homebuilt optical tweezers.

Force spectroscopy with dual-trap optical tweezers: molecular stiffness measurements and coupled fluctuations analysis

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 article, 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, to our knowledge, novel dual-trap optical-tweezers setup that 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 dsDNA 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.

High-resolution dual-trap optical tweezers with differential detection: an introduction

Optical traps or "optical tweezers" have become an indispensable tool in understanding fundamental biological processes. The ability to manipulate and probe individual molecules or molecular complexes has led to a new, more refined understanding of the mechanical properties of the fundamental building blocks of the cell, and of the mechanism by which molecular machines function. The field has seen a steady stream of technological advances that have greatly refined the technique. One major effort has been in developing methods to resolve motions at the angstrom level--the fundamental length scale for many biological processes. This drive has only recently come to fruition with the advent of high-resolution optical trapping techniques that can now detect movements on the scale of a single base pair of DNA, 3.4 A. Here we briefly review the basic concepts and components of optical traps and the single-molecule experiments in which they are used.

High-resolution dual-trap optical tweezers with differential detection: instrument design

Optical traps or "optical tweezers" have become an indispensable tool in understanding fundamental biological processes. In this article, we present the design of a high-resolution optical tweezers, which is designed to minimize sensitivity to environmental noise. When the tether is attached to a stage, resolution is typically limited by drift and fluctuations in the relative position of the stage and trap-forming objective, cued by environmental noise. In a dual-trap design, however, the biological system is decoupled entirely from the stage by utilizing two optical traps to hold both ends of the tether. By holding each end of the molecule in an identical fashion, such dual-trap optical tweezers are less sensitive to drift and fluctuations of mechanical components. In our instrument, we form two traps from two orthogonally polarized beams generated by a single laser. As a result, many optical components and much of the optical path traveled are common to both trapping beams, thus minimizing differential movements of one optical trap relative to the other. Additionally, this economical use of optics creates an instrument that is simple and compact, reducing the number of elements susceptible to environmental fluctuations.

High-resolution dual-trap optical tweezers with differential detection: managing environmental noise

Optical traps or "optical tweezers" have become an indispensable tool in understanding fundamental biological processes. The ability to manipulate and probe individual molecules or molecular complexes has led to a new, more refined understanding of the mechanical properties of the fundamental building blocks of the cell, and of the mechanism by which molecular machines function. The minimization and management of noise are the keys to building a high-resolution optical tweezers. The high degree of stability needed to observe angstrom motions of a biological system places rather stringent requirements on the environment in which the instrument operates. Specifically, environmental noise is a concern when it leads to differential movements of the optical trap relative to the second attachment point of the tether. In this article, we discuss what entails a quiet environment--what sources of noise are relevant and how they affect the instrument--and provide guidelines for choosing an appropriate location for a high-resolution instrument.

High-resolution dual-trap optical tweezers with differential detection: data collection and instrument calibration

Optical traps or "optical tweezers" have become an indispensable tool in understanding fundamental biological processes. Dual-trap optical tweezers with differential detection have the advantage that the instrument is decoupled from sources of environmental noise, thereby minimizing interference of signal and maximizing stability and spatial resolution. In this article, we describe methods of data detection and calibration for our dual-trap design.

High-resolution dual-trap optical tweezers with differential detection: minimizing the influence of measurement noise

Optical traps or "optical tweezers" have become an indispensable tool in understanding fundamental biological processes. The environment, the instrument components, and the overall design all have important roles in determining the spatial resolution of an optical trap. However, even with careful attention to these details, the spatial resolution of the optical tweezers is still fundamentally limited by the forces that induce Brownian fluctuations of the trapped microspheres. In this article, we focus on the ways in which the design and implementation of an experiment--the choice of bead size, tether length, and even the method in which the motions of the beads are monitored--can have a large impact on the fundamental resolution of the measurement. By investigating how the Brownian fluctuations depend on experimental parameters and the detection method, we develop a set of guidelines for designing a high-resolution experiment, with a prescription for minimizing the effect of Brownian fluctuations, thus maximizing the resolution of the experiment.