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We are currently trying to obtain ligand-bound complexes for one of our proteins by soaking and/or co-crystallisation. We have had prior success for this protein, but using  a different class of ligands. The new ligand (in DMSO) remains in solution (more or less) when mixed with the reservoir/cryoprotectant, and the diffraction pattern survives the soaking nicely. Annoyingly though,  all we see are density peaks that match the size of DMSO and become more pronounced when increasing [DMSO] (to help solubilisation of the ligand). Soaking times varied between minutes to 16 hours. Kd was measured ( ~ 10 uM) in the solution state.

We have tried pyridine (which keeps the ligand in solution, but kills diffraction in an instant), and DMF (which doesn't keep the ligand in solution when mixing with cryoprotectant). 

I am wondering whether the community has suggestions for alternative organic solvents that have been used to solubilise hydrophobic ligands, and are reasonably gentle to the protein crystal. 

Thank you. 

Klaus

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                    Klaus Fütterer, Ph.D.
                Reader in Structural Biology
                  Undergraduate Admissions

School of Biosciences   P: +44-(0)-121-414 5895
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