1. Center for Biophysics and Computational Biology, University of Illinois, Urbana Illinois 61801, USA
2. Departments of Chemistry and Physics, University of Illinois, Urbana Illinois 61801, USA

Correspondence to: Martin Gruebele^1,2 Correspondence and requests for materials should be addressed to M.G. (Email: gruebele@scs.uiuc.edu).

Abstract

Many small proteins seem to fold by a simple process explicable by conventional chemical kinetics and transition-state theory. This assumes an instant equilibrium between reactants and a high-energy activated state¹. In reality, equilibration occurs on timescales dependent on the molecules involved, below which such analyses break down¹. The molecular timescale, normally too short to be seen in experiments, can be of a significant length for proteins. To probe it directly, we studied very rapidly folding mutants of the five-helix bundle protein _6–85, whose activated state is significantly populated during folding. A time-dependent rate coefficient below 2 s signals the onset of the molecular timescale, and hence the ultimate speed limit for folding². A simple model shows that the molecular timescale represents the natural pre-factor for transition state models of folding.