Protein glass transition temperature

The proximity of this liquid-liquid transition to the protein-glass transition temperature is suggestive. Clearly, at temperatures below 220 K or so, the dynamics of water and protein are highly coupled. A recent computer simulation study has shown that the stmctural relaxation of protein requires relaxation of the water HB network and translational displacement of interfacial water molecules. It is, therefore, clear that the dynamics of water at the interface can play an important role. This is an interesting problem that deserves further investigation. 

The presence of a solvent, especially water, and/or other additives or impurities, often in nonstoichiometric proportions, may modify the physical properties of a solid, often through impurity defects, through changes in crystal habit (shape) or by lowering the glass transition temperature of an amorphous solid. The effects of water on the solid-state stability of proteins and peptides and the removal of water by lyophilization to produce materials of certain crystallinity are of great practical importance although still imperfectly understood. 

As the temperature is lowered further, the viscosity of the unfrozen solution increases dramatically until molecular mobility effectively ceases. This unfrozen solution will contain the protein, as well as some excipients, and (at most) 50 per cent water. As molecular mobility has effectively stopped, chemical reactivity also all but ceases. The consistency of this solution is that of glass, and the temperature at which this is attained is called the glass transition temperature Tg-. For most protein solutions, Tg- values reside between -40 °C and -60 °C. The primary aim of the initial stages of the freeze-drying process is to decrease the product temperature below that of its Tg- value and as quickly as possible in order to minimize the potential negative effects described above. 

Reduction in the water holding capacity of the corneum can also make the corneocyte proteins brittle and vulnerable to cracking. Keratins in the corneum have a glass transition temperature just below the body temperature28 and this is sensitive to humidity levels. Glass transition temperature is the point below which the material is brittle. As the humidity/water content of the SC decreases, glass transition temperature increases to values above the body temperature thus making the corneocytes brittle at body temperature. 

Temperature-dependent pure dephasing rates of MbCO in three solvents show identical power law behavior at low temperatures. At intermediate temperatures there is a break in the power law arising from the solvent-influenced protein glass transition. Above this point the data in glassy trehalose are exponentially activated. The other solvents, which at elevated temperatures are liquids, have additional solvent viscosity-dependent contributions to the pure dephasing rate. 

Figure 7.11. Glass transition temperature of the 7S and IIS soy globulins as a funetion of moisture content by DSC and RMS (Copyright 1998 Horn Understanding phase transitions and chemical complexing reactions in 7S and IIS soy protein fractions by Morales-Diaz and Kokini (In Phase/State Transitions in Foods, Rao and Hartel (Eds.)). Reproduced by permission of Roudedge/Taylor Francis Group, LLC.)...

The effect of ions on denaturation temperatures is well-studied for dilute solutions. Effects related to the Hoffineister series are apparent. Unfortunately, no systematic studies of ions on protein glass transitions are available, and it cannot be assumed that these will have a neghgible effect on the materials properties during extrusion. 

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