Experimental Evidence for Millisecond–Timescale Structural Evolution Following the Microsecond–Timescale Folding of a Small Protein

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Prior work has shown that small proteins can fold (i.e., convert from unstructured to structured states) within 10 μs. Here we use time-resolved solid state nuclear magnetic resonance (ssNMR) methods to show that full folding of the 35-residue villin headpiece subdomain (HP35) requires a slow annealing process that has not been previously detected. ¹³C ssNMR spectra of frozen HP35 solutions, acquired with a variable time τₑ at 30 °C after rapid cooling from 95 °C and before rapid freezing, show changes on the 3-10 ms timescale, attributable to slow rearrangements of protein sidechains during τₑ.