Dear Simon,
DMSO concentrations lower than 5% usually do not alter crystallizability
of a protein. In case you want to avoid this solvent may I suggest you
trying out two methods that worked for me.
1. If you grow crystals in PEGs or similar molecules you might try to
solubilize the compound in a small amount of precipitant solution. This is
helpful if you want just a 1:4 protein:ligand ratio.
2. Sometimes solubility is low even in 5% DMSO (or diluted solutions of
glycerol, alcohols and similar molecules). In these cases setting up drops
in the presence a saturated solution and some precipitate of the compound
may also lead to good co-crystals.
As one molecule passes from the saturated solution to the “bound” state, a
new molecule is solubilized from the precipitate, which gradually
dissolves and passes from the solution to its binding site in the protein.
Indeed, this sounds like a “soaking” experiment and it works well if the
compound is coloured, so that you can see if the crystals actually become
of the same colour. Just remember to wash them thoroughly before
measurements, in order to remove traces of the ligand precipitate that
would result in bad diffraction pattern.
Hope this will help you.
Marco
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