Computational and experimental study of reversible hydration/dehydration processes in molecular crystals of natural products – a case of catechin

Abstract

When employing a complementary approach that joins theoretical (crystal structure prediction, CSP, gauge-including projector-augmented wave density functional theory, GIPAW DFT) and experimental (solid-state nuclear magnetic resonance, SSNMR) techniques, we prove that detectable changes in the water content in a crystal lattice can be monitored as a continuous or quantum process by using border values of the measurable number of water molecules in the crystal, depending on the observation probe.In the case of a 13 C nucleus, which "feels" the presence of water in its nearest neighbourhood, the subtle changes in the structure of the water channel during the dehydration/rehydration process are not reflected until the water loss exceeds a certain number of molecules. Here, the dehydration of (+)-catechin 4.5-hydrate with a loss of ca. 1-1.5 and 2-2.5H 2 O molecules leads to microcrystalline form II (partially dehydrated) or amorphous form III, respectively. The CSP calculations for the partially dehydrated (+)-catechin (form II), as verified by experimental 1 H and 13 C shieldings, resulted in two similar probable crystal structures, which have 2 and 2.5 water molecules in the unit cell. CrystEngCommThis journal is † Electronic supplementary information (ESI) available: 1 H-13 C FSLG HETCOR spectra of forms I and II, 13 C CPMAS NMR spectra of form I and II with different water content, DFT calculations of (+)-catechin CSTs for structures with variable C3-C2-C11-C16 dihedral angle, hydrogen bonding network for each of 26 structures obtained in the prediction of the partially dehydrated catechin crystals and the description of the energy share estimation of each water molecule. See

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