Force-clamp experiments reveal the free energy profile and diffusion coefficient of the collapse of proteins

We present force-clamp data on the collapse of ubiquitin polyproteins in response to a quench in the force. These nonequilibrium trajectories are analyzed using a general method based on a diffusive assumption of the end-to-end length to reconstruct a downhill free energy profile at 5pN and an energy plateau at 10pN with a slow diffusion coefficient on the order of~100nm^2/s. The shape of the free energy and its linear scaling with the protein length give validity to a physical model for the collapse. However, the length independent diffusion coefficient suggests that internal rather than viscous friction dominates and thermal noise is needed to capture the variability in the measured times to collapse.