Minute Additions of DMSO Affect Protein Dynamics Measurements by NMR Relaxation Experiments through Significant Changes in Solvent Viscosity

Studies of protein−ligand binding often rely on dissolving the ligand in dimethyl sulfoxide (DMSO) to achieve sufficient solubility, and then titrating the ligand solution into the protein solution. As a result, the final protein−ligand solution contains small amounts of DMSO in the buffer. Here we report how the addition of DMSO impacts studies of protein conformational dynamics. We used ¹⁵N NMR relaxation to compare the rotational diffusion correlation time (τ_C) of proteins in aqueous buffer with and without DMSO. We found that τ_C scales with the viscosity of the water−DMSO mixture, which depends sensitively on the amount of DMSO and varies by a factor of 2 across the relevant concentration range. NMR relaxation studies of side chains dynamics are commonly interpreted using τ_C as a fixed parameter, obtained from backbone ¹⁵N relaxation data acquired on a separate sample. Model-free calculations show that errors in τ_C, arising from mismatched DMSO concentration between samples, lead to significant errors in order parameters. Our results highlight the importance of determining τ_C for each sample or carefully matching the DMSO concentrations between samples.