Abstract

The antibiotic cephalexin [systematic name: d-7-(2-amino-2-
phenylacetamido)-3-methyl-8-oxo-5-thia-1-azabicyclo[4.2.0]-
oct-2-ene-2-carboxylic acid] forms a range of isomorphic
solvates, with the maximum hydration state of two water
molecules formed only at high relative humidities. The water
content of the structure reported here (C16H17N3O4S�1.9H2O)
falls just short of this con®guration, having three independent
cephalexin molecules, one of which is disordered, and 5.72
observed water molecules in the asymmetric unit. The facile
nature of the cephalexin solvation/desolvation process is
found to be facilitated by a complex channel structure, which
allows free movement of solvent in the crystallographic a and
b directions.