Exploring humidity effects on polycrystalline human insulin–ligand complexes: preliminary crystallographic insights

This study investigates the effect of relative humidity (RH) on the crystal structures of human insulin (HI) complexes with organic ligands, <i>m</i>-cresol and <i>m</i>-nitro-phenol, using <i>in situ</i>X-ray powder diffraction (XRPD) with a controlled-humidity chamber. Co-crystallization at pH 7.5 produced hexagonal microcrystals (space group <i>R</i>3) for both protein-ligand complexes. The corresponding single-crystal X-ray diffraction structures were solved: HI-<i>m</i>-cresol (PDB entry 9ibb, 1.84 Å) and HI-<i>m</i>-nitro-phenol (PDB entry 9qld, 2.55 Å). Pawley analysis of the <i>in situ</i> XRPD data revealed structural stability up to 70% RH, with no phase transitions observed. At lower humidity levels, reduced diffraction intensities indicated loss of crystallinity, which was fully restored upon rehydration to 95% RH. Notably, each complex exhibited distinct changes in unit-cell parameters during dehydration-rehydration cycles. These results highlight the critical role of controlling environmental factors in structure-based drug design and pharmaceutical manufacturing, and demonstrate how organic ligands can enhance the stability of protein crystals, offering valuable insights for pharmaceutical development.