A Multi-Technique Experimental and Computational Approach To Study the Dehydration Processes in the Crystals of Endomorphin Opioid Peptide Derivative

Abstract

When molecular crystals undergo partial dehydration, determining the crystal contents and precise localization of the remaining water in the crystal lattice becomes challenging, especially when the quality of crystals after dehydration is not suitable for X-ray diffraction studies. In this work, we describe a methodology that allows the determination and refinement of the structures of partially dehydrated crystals employing complementary experimental (advanced solid-state NMR techniques, differential scanning calorimetry, elemental analysis) and computational (gauge-including projector-augmented wave density functional theory) techniques. We present the power of the methodology using an opioid peptide derivative, endomorphin-2-OH (EM2-OH) heptahydrate. The advanced solid state NMR techniques 2D-PASS, inv-HETCOR, and cross polarization variable contact (CP-VC) carried out with very fast magic angle spinning were used as a source of the constraints showing differences and similarities in the structures and local molecular dynamics for crystallized and dehydrated samples. A crystal structure prediction employing the gauge-including projector-augmented wave (GIPAW) method was used for the determination and refinement of dehydrated EM2-OH. After dehydration, three out of the initial seven water molecules remain in the lattice of the EM2-OH crystals, with two water molecules located in the pockets made by the pseudocyclic conformations and one forming a bridge between two independent EM2-OH molecules.