Structure determination of the stable anhydrous phase of alpha-lactose from X-ray powder diffraction.

The stable anhydrous form of alpha-lactose has been obtained by the dehydration of alpha-lactose monohydrate in methanol. An X-ray powder diffraction pattern was recorded at room temperature with a laboratory diffractometer equipped with an INEL curved sensitive detector CPS120. The starting structural model of this form was found by a Monte-Carlo simulated annealing method. The structure was obtained through Rietveld refinements and the minimization of crystalline energy for the localization of the H atoms of the hydroxyl groups. Soft restraints were applied to bond lengths and angles. Networks of O-H...O hydrogen bonds account for the crystalline cohesion. A comparison is made between the hydrogen-bond networks of this form and those of the monohydrate and hygroscopic anhydrous forms of alpha-lactose.