Solid-State Properties of Tobramycin

Tobramycin I obtained from two different sources was subjected to powder X-ray diffractometry, thermal analyses, and Karl Fischer titrimetry. It was concluded to be tobramycin monohydrate (C₁₈H₃₇N₅O₉ · H₂O). When heated in the differential scanning calorimeter (DSC), the dehydration of I resulted in the formation of metastable anhydrous tobramycin, which melted at 164°C. This was followed by the crystallization of the stable anhydrous tobramycin, which then melted at 217°C. The polymorphic transition was concluded to be monotropic and the calculated free energy difference between the metastable and the stable forms, at 25°C, was 348 cal · mol⁻¹. Both the heating rate in the DSC and the sample size had a significant influence on the enthalpy values of most of the thermal events. These observations were attributed to the presence of trace amounts of moisture in the sample. No detectable decomposition of I occurred when it was heated up to 224°C.