Solid State Chemistry of Proteins: I. Glass Transition Behavior in Freeze Dried Disaccharide Formulations of Human Growth Hormone (HGH)

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, Tg, cannot be directly determined by DSC. A strong glass means a small heat capacity change at Tg, ▵Cp, and a very broad glass transition region, or a large ▵Tg. However, direct experimental evidence for a small ▵Cp and a large ▵Tg have been lacking. Here, we utilize extrapolation of thermal analysis data in protein:disaccharide mixtures to evaluate Tg, ▵Tg, and ▵Cp for “pure” human growth hormone (hGH) from low to moderate residual water. We find that ▵Tg is indeed large and ▵Cp is very small. Also, the Tg for pure hGH decreases from a value of about 136°C when dry to around 25°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‐Tg 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 substatesand the American Pharmacists Association J Pharm Sci 96: 2765-2776, 2007