Figure 3. 2D DARR spectra of U-¹³C-¹⁵N labeled ubiquitin in frozen H₂O solution at 264 K and 213 K.

A) Spectrum recorded at 264 K sample temperature (red). B) Spectrum recorded at 213 K sample temperature (blue). The 1D spectra shown are extracted form the δ₁ slices indicated inside the 2D spectra. The two spectra illustrate the extent of line broadening induced by lowering the temperature in frozen protein solutions.