Protein dynamics in supercooled water: the search for slow motional modes.

The impact of studying protein dynamics in supercooled water for identifying slow motional modes on the micros time scale is demonstrated. Backbone 15N spin relaxation parameters were measured at -13 degrees C for ubiquitin, which plays a central role for signaling proteolysis, cellular trafficking and kinase activation in eukaryotic organisms. A hitherto undetected motional mode involving Val 70 was found, which may well play an important role for ubiquitin recognition. The measurement of rotating frame 15N relaxation times as a function of the spin-lock field allowed determination of the correlation time of this motional mode, which would not have been feasible above 0 degrees C.