Hydration and Packing along the Folding Pathway of SH3 Domains by Pressure-Dependent NMR^†

The volumetric properties associated with protein folding transitions reflect changes in protein packing and hydration of the states that participate in the folding reaction. Here, NMR spin relaxation techniques are employed to probe the folding−unfolding kinetics of two SH3 domains as a function of pressure so that the changes in partial molar volumes along the folding pathway can be measured. The two domains fold with rates that differ by ∼3 orders of magnitude, so their folding dynamics must be probed using different NMR relaxation experiments. In the case of the drkN SH3 domain that folds via a two-state mechanism on a time scale of seconds, nitrogen magnetization exchange spectroscopy is employed, while for the G48M mutant of the Fyn SH3 domain where the folding occurs on the millisecond time scale (three-step reaction), relaxation dispersion experiments are utilized. The NMR methodology is extremely sensitive to even small changes in equilibrium and rate constants, so reliable estimates of partial molar volumes can be obtained using low pressures (1−120 bar), thus minimizing perturbations to any of the states along the folding reaction coordinate. The volumetric data that were obtained are consistent with a similar folding mechanism for both SH3 domains, involving early chain compaction to states that are at least partially hydrated. This work emphasizes the role of NMR spin relaxation in studying dynamic processes over a wide range of time scales.