Detection of a Transient Intermediate in a Rapid Protein Folding Process by Solid-State Nuclear Magnetic Resonance
Kan-Nian Hu, Wai-Ming Yau and Robert Tycko*
We describe the use of solid-state NMR spectroscopy to characterize a partially folded state of the 35-residue helical protein HP35 created by rapid freeze-quenching from a thermally unfolded state on the 10−20 μs time scale. Two-dimensional solid-state 13C NMR spectra of 13C-labeled HP35 in frozen glycerol/water solution exhibit two sets of signals, one corresponding to strongly unfolded protein molecules and the other to an ensemble of molecules having native helical secondary structure but incomplete tertiary structure. The NMR data indicate that secondary structure forms within the freeze-quenching time scale but that full folding involves a slower phase of structural annealing. The 5 μs folding time observed in earlier studies of HP35 by time-resolved optical techniques may not represent the time scale for full folding.