Protein Linewidth and Solvent Dynamics in Frozen Solution NMR

Solid-state NMR of proteins in frozen aqueous solution is a potentially powerful technique in structural biology, especially if it is combined with dynamic nuclear polarization signal enhancement strategies. One concern regarding NMR studies of frozen solution protein samples at low temperatures is that they may have poor linewidths, thus preventing high-resolution studies. To learn more about how the solvent shell composition and temperature affects the protein linewidth, we recorded ¹H, ²H, and ¹³C spectra of ubiquitin in frozen water and frozen glycerol-water solutions at different temperatures. We found that the ¹³C protein linewidths generally increase with decreasing temperature. This line broadening was found to be inhomogeneous and independent of proton decoupling. In pure water, we observe an abrupt line broadening with the freezing of the bulk solvent, followed by continuous line broadening at lower temperatures. In frozen glycerol-water, we did not observe an abrupt line broadening and the NMR lines were generally narrower than for pure water at the same temperature. ¹H and ²H measurements characterizing the dynamics of water that is in exchange with the protein showed that the ¹³C line broadening is relatively independent from the arrest of isotropic water motions.