Some impurities in the peptide may be toxic to cells
, if these peptides are used for cell culture studies. Dithiothreitol (DTT) may be tolerated at less than 1M concentration in most cells. For crude peptides that go through the common ether precipitation process, the residual amount of DTT should be relatively small. For peptides that have gone through a purification step, the peptide is generally free of DTT.