Complex transitions between dihydrate and anhydrate forms of ectoine - unexpected behavior of a highly hygroscopic compatible solute in the solid state

The crystalline dihydrate of the compatible solute ectoine undergoes a loss of water at ambient conditions and transforms into a highly hygroscopic anhydrate form. This unexpected transition is studied combining single-crystal and powder X-ray, single crystal neutron diffraction measurements and quantum mechanical calculations. Overall, our study provides a rationale for the important uptake and loss of water by ectoine in the solid state. Crystallizing the compatible solute ectoine from water yields a metastable dihydrate that readily degrades to a highly hygroscopic anhydrate at ambient conditions; this strange behavior is examined and a rationale is presented.