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 μ s time scale is demonstrated. Backbone ¹⁵N spin relaxation parameters were measured at - 13 °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 °C.