High-pressure preference for reduced water content in porous zinc aspartate hydrates

The zinc aspartate (ZnAsp₂) complex, a common dietary supplement, preferentially crystallizes as the dihydrate (ZnAsp₂·2H₂O) from aqueous solution. Under normal conditions the dihydrate easily transforms into the sesquihydrate (ZnAsp₂·1.5H₂O). The dihydrate crystal structure is triclinic, space group P1, and the sesquihydrate is monoclinic, space group C2/c. However, their structures are closely related and similarly consist of zinc aspartate ribbons parallel to pores accommodating water molecules. These porous structures can breathe water molecules in and out depending on the temperature and air humidity. High pressure above 50 MPa favours the sesquihydrate, as shown by recrystallizations under pressure and compressibility measured by single-crystal X-ray diffraction up to 4 GPa. This preference is explained by the reduced volume of the sesquihydrate and water compressed separately, compared with the dihydrate. The sesquihydrate undergoes an isostructural phase transition when the voids collapse at 0.8 GPa, whereas no phase transitions occur in the dihydrate, because its pores are supported by increased water content.