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.