Water spin lattice relaxation time T₁ of ²H of frozen ubiquitin solution and the resulting rotation correlation time τ_C.

A) ²H T₁ of non-frozen water measured with a simple saturation recovery and quadrupolar echo pulse sequence. The theoretical minimum assuming a single τ_C and an isotopic motion is indicated with a dashed horizontal line. B) Temperature dependence of the non-frozen water τ_C calculated using Equation 1, the T₁ relaxation times shown in A), and a quadrupolar coupling constant and asymmetry parameter of Cq = 195 kHz and η = 0.1 respectively. Two possible τ_C values correspond to each T₁ in the temperature according to Equation 1. Since τ_C is expected to decrease at lower temperatures the most likely τ_Cs are shown with full circles. The equally likely τ_Cs corresponding to the minimal T₁ are shown in gray.