Protein–macrocycle polymorphism: crystal form IV of the Ralstonia solanacearum lectin–sulfonato-calix[8]arene complex

Controlled protein assembly and crystallization is necessary as a means of generating diffraction-quality crystals as well as providing a basis for new types of biomaterials. Water-soluble calixarenes are useful mediators of protein crystallization. Recently, it was demonstrated that Ralstonia solanacearum lectin (RSL) co-crystallizes with anionic sulfonato-calix[8]arene (sclx(8)) in three space groups. Two of these co-crystals only grow at pH ≤ 4 where the protein is cationic, and the crystal packing is dominated by the calixarene. This paper describes a fourth RSL–sclx(8) co-crystal, which was discovered while working with a cation-enriched mutant. Crystal form IV grows at high ionic strength in the pH range 5–6. While possessing some features in common with the previous forms, the new structure reveals alternative calixarene binding modes. The occurrence of C (2)-symmetric assemblies, with the calixarene at special positions, appears to be an important result for framework fabrication. Questions arise regarding crystal screening and exhaustive searching for polymorphs.