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Single Molecule FRET of Protein Folding Dynamics

We have been using single molecule Förster resonance energy transfer (FRET) for the investigation of protein folding, with a focus on non‐native states of proteins; these are conformationally more heterogeneous than native structures and have thus been more difficult to access experimentally. Single molecule FRET has allowed us to map long‐range distance distributions in the unfolded state of the small cold shock protein CspTm1, even at low concentrations of denaturant, where the unfolded molecules are collapsed, and where the majority of protein molecules are already folded. Using advanced correlation methods, we determined the reconfiguration time of unfolded CspTm2 and found a decrease in its intramolecular diffusion coefficient upon collapse. This increase in the roughness of the free energy surface is accompanied by the formation of some secondary structure, as determined with kinetic synchrotron circular dichroism experiments1. Complete fluorescence intensity correlation functions from picoseconds to seconds are an increasingly important tool to investigate protein dynamics3 and quantify previously inaccessible characteristics of the free energy surfaces for folding. In a next step, the same methods now allow us to investigate the folding and dynamics of proteins in the context of cellular factors, such as molecular chaperones.4