Universal Axial Fluctuations in Optical Tweezers

TL;DR

This paper identifies axial bead fluctuations in optical tweezers as an instrumental artifact that can mimic molecular conformational changes, and provides a universal method to distinguish between them for accurate measurements.

Contribution

It introduces a general formula and strategy to separate axial fluctuations from true conformational transitions in optical tweezers experiments.

Findings

Axial fluctuations occur on a timescale similar to folding transitions.

A universal formula describes axial fluctuations across setups.

Disentangling axial and conformational fluctuations improves data interpretation.

Abstract

Optical tweezers allow the measurement of fluctuations at the nano-scale, in particular fluctuations in the end-to-end distance in single molecules. Fluctuation spectra can yield valuable information, but they can easily be contaminated by instrumental effects. We identify axial fluctuations, i.e. fluctuations of the trapped beads in the direction of light propagation, as one of these instrumental effects. Remarkably, axial fluctuations occur on a characteristic timescale similar to that of conformational (folding) transitions, which may lead to misinterpretation of the experimental results. We show that a precise measurement of the effect of force on both axial and conformational fluctuations is crucial to disentangle them. Our results on axial fluctuations are captured by a simple and general formula valid for all optical tweezers setups and provide experimentalists with a general strategy to distinguish axial fluctuations from conformational transitions.