Warnings on the sample preparation
Sample activity should be controlled after every batch purification, in order to get information about the active protein concentration. Ideally it would be nice to have a single protein batch to aliquot in NMR buffer and freeze-thaw. This should prevent batch dependence on the sample concentration. In this case however it is pivotal to check the target activity after unfreezing since in several cases unfolding and aggregation might occur.
If the ligands are more soluble in DMSO, it might be a good idea to add a small percentage of deuterated DMSO in the sample 2-5\%. One should check that the DMSO does not affect the protein stability.