Solid state chemistry of proteins: I. glass transition behavior in freeze dried disaccharide formulations of human growth hormone (hGH)

✍ Scribed by Michael J. Pikal; D.R. Rigsbee; M.L. Roy

• Publisher: John Wiley and Sons
• Year: 2007
• Tongue: English
• Weight: 184 KB
• Volume: 96
• Category: Article
• ISSN: 0022-3549
• DOI: 10.1002/jps.20960

✦ Synopsis

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 , DC p , and a very broad glass transition region, or a large DT g . However, direct experimental evidence for a small DC p and a large DT g have been lacking. Here, we utilize extrapolation of thermal analysis data in protein:disaccharide mixtures to evaluate T g , DT g , and DC p for ''pure'' human growth hormone (hGH) from low to moderate residual water. We find that DT g is indeed large and DC p is very small. Also, the T g for pure hGH decreases from a value of about 1368C when dry to around 258C 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.