Although freeze dried formulations are commonly characterized using differential scanning calorimetry (DSC), a protein-rich system behaves as a "strong glass", and the glass transition temperature, T(g), cannot be directly determined by DSC. A strong glass means a small heat capacity change at T(g), triangle upC(p), and a very broad glass transition region, or a large triangle upT(g). However, direct experimental evidence for a small triangle upC(p) and a large triangle upT(g) have been lacking. Here, we utilize extrapolation of thermal analysis data in protein:disaccharide mixtures to evaluate T(g), triangle upT(g), and triangle upC(p) for "pure" human growth hormone (hGH) from low to moderate residual water. We find that triangle upT(g) is indeed large and triangle upC(p) is very small. Also, the T(g) for pure hGH decreases from a value of about 136 degrees C when dry to around 25 degrees C at 12% water. This glass transition is not the onset of mobility within the protein molecule but rather signals onset of whole molecule rotation and translation. We also observe complex pre-T(g) thermal events in the DSC data, which are interpreted as consequences of relaxation events, largely due to the disaccharide, and are characteristic of freeze dried systems having a broad distribution of relaxing substates.
(c) 2007 Wiley-Liss, Inc.