Understanding the Solid-State Hydration Behavior of a Common Amino Acid: Identification, Structural Characterization, and Hydration/Dehydration Processes of New Hydrate Phases of l‑Lysine

The crystal structure of l-lysine was reported very recently and represented the final member of the set of 20 directly encoded proteinogenic amino acids to have a crystal structure of the enantiomerically pure form determined. Under ambient conditions, l-lysine has a strong propensity to undergo hydration by incorporation of water into the crystal structure, and dehydration under rigorously dry conditions is required to obtain anhydrous l-lysine. In the present paper, we explore the hydration of l-lysine, which has led to the identification and characterization of a hemihydrate phase and a monohydrate phase. As the hydration processes give rise to microcrystalline powder samples of the hydrate phases, powder X-ray diffraction has been exploited for crystal structure determination of both l-lysine hemihydrate and l-lysine monohydrate. The conditions for interconversion between anhydrous l-lysine and the two hydrate phases have been investigated by dynamic vapor sorption measurements, leading to an understanding of the hydration/dehydration behavior in this system.