Structural and dynamic properties of crystalline and amorphous phases in raffinose-water mixtures

Purpose. To obtain an improved characterisation of the raffinose-water solid-solid and solid-liquid state diagram, and to study the thermophysical behaviour of the solid amorphous phase. This information is expected to shed light on the potential of raffinose as a pharmaceutical excipient, for stabilising labile preparations at high temperatures. Methods. X-ray diffraction, scanning electron microscopy, polarised-light microscopy, differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were applied to study raffinose pentahydrate and its behaviour during progressive dehydration. Results. Isothermal dehydration of raffinose pentahydrate led to its gradual amorphisation, but also to minor changes in the diffractograms, suggesting the probability of lower stable hydrates. Their existence was confirmed by DSC. Anhydrous raffinose was found to be completely amorphous, and this was supported by the gradual disappearance of birefringence during dehydration. In contrast, electron micrographs, taken during the dehydration process, exhibited no changes in the original ultrastructural crystal morphology. The widths of the glass-to-fluid transitions and the absolute specific heats of crystalline and amorphous phases in the vitreous and fluid states were used to estimate some structural and relaxation characteristics of amorphous raffinose-water mixtures. Conclusions. Raffinose forms the most "fragile" glass of those pharmaceutical excipients for which data are available. In its thermomechanical properties, it is superior to trehalose and should therefore be effective as a long-term stabiliser for dried biopharmaceutical preparations at temperatures up to 65 degrees C.