Screening Using Polymorphs for the Crystallization of Protein− Ligand Complexes

An efficient crystallization screening method is important in drug design to yield high resolution crystallographic structures of protein−ligand complexes to understand inhibitor selectivity and potency for various members of an enzyme family. The strategy starts with a single condition for each protein−ligand complex, and more trials encompassing all polymorph crystallization conditions are included later, eventually defaulting to a more extensive screening for difficult cases. The polymorph screening approach relies on an intrinsic positive feedback mechanism. New polymorphs are constantly discovered since certain ligands favor variant lattices. The new best diffracting polymorph is selected for single-conditions testing, ensuring that as more forms are discovered, the resolution of the structures obtained improves. Continual optimization of the conditions for all crystal forms yields new solutions that become increasingly effective in protein−ligand crystallization trials. More polymorphs imply more lattices suitable to accommodate ligands of greater diversity. Wider seeding opportunities combined with optimized enzyme-specific crystallization conditions improves the outcome and accelerates the screening process so that a conventional full-range crystallization screening is only rarely needed. Having tested this approach with a large repertoire of 100 ligands and 4 enzymes, we expect the method to perform equally well on similar drug-discovery projects.