Conformational changes during the nanosecond-to-millisecond unfolding of ubiquitin.

Steady-state and transient conformational changes upon the thermal unfolding of ubiquitin were investigated with nonlinear IR spectroscopy of the amide I vibrations. Equilibrium temperature-dependent 2D IR spectroscopy reveals the unfolding of the beta-sheet of ubiquitin through the loss of cross peaks formed between transitions arising from delocalized vibrations of the beta-sheet. Transient unfolding after a nanosecond temperature jump is monitored with dispersed vibrational echo spectroscopy, a projection of the 2D IR spectrum. Whereas the equilibrium study follows a simple two-state unfolding, the transient experiments observe complex relaxation behavior that differs for various spectral components and spans 6 decades in time. The transient behavior can be separated into fast and slow time scales. From 100 ns to 0.5 ms, the spectral features associated with beta-sheet unfolding relax in a sequential, nonexponential manner, with time constants of 3 micros and 80 micros. By modeling the amide I vibrations of ubiquitin, this observation is explained as unfolding of the less stable strands III-V of the beta-sheet before unfolding of the hairpin that forms part of the hydrophobic core. This downhill unfolding is followed by exponential barrier-crossing kinetics on a 3-ms time scale.