I've recently crystallized a protein under a condition containing 1.1 M ammonium tartrate. (I was able to get a few other hits but only these crystals diffract below 2.5 A). Upon refinement, it appears that the electron density in the active site shows a mixture of tartrate and the native ligand. This occurs even after overnight soaks with 50 mM of the ligand ( the Km is about 1 mM). Would you recommend dehydrating crystals with PEG when soaking them with ligand or some different method to minimize the presence of tartrate?