Abstract

The conformational energy landscape of a protein out of equilibrium is poorly understood. We use single-molecule force-clamp spectroscopy to measure the kinetics of unfolding of the protein ubiquitin under a constant force. We discover a surprisingly broad distribution of unfolding rates that follows a power law with no characteristic mean. The structural fluctuations that give rise to this distribution reveal the architecture of the protein|[rsquo]|s energy landscape. Following models of glassy dynamics, this complex kinetics implies large fluctuations in the energies of the folded protein, characterized by an exponential distribution with a width of 5–10kBT. Our results predict the existence of a |[lsquo]|glass transition|[rsquo]| force below which the folded conformations interconvert between local minima on multiple timescales. These techniques offer a new tool to further test statistical energy landscape theories|[nbsp]|experimentally.