Reconstructing the free energy landscape of a polyprotein by single-molecule experiments

TL;DR

This study uses atomic force microscopy and fluctuation relations to reconstruct the equilibrium free energy landscape of a polyprotein, revealing key unfolding length scales and their relation to kinetic properties.

Contribution

It introduces a method to estimate the equilibrium free energy landscape of a polyprotein from single-molecule pulling experiments using fluctuation relations.

Findings

The free energy landscape has a regular shape with a typical unfolding length of about 20 nm.

The equilibrium unfolding length is much larger than the kinetic unfolding length.

The method provides insights into the relationship between equilibrium and kinetic unfolding properties.

Abstract

The mechanical unfolding of an engineered protein composed of eight domains of Ig27 is investigated by using atomic force microscopy. Exploiting a fluctuation relation, the equilibrium free energy as a function of the molecule elongation is estimated from pulling experiments. Such a free energy exhibits a regular shape that sets a typical unfolding length at zero force of the order of 20 nm. This length scale turns out to be much larger than the kinetic unfolding length that is also estimated by analyzing the typical rupture force of the molecule under dynamic loading.