Water protein-glass transition

In Figure 2.8(a) we have plotted the MSD of a water molecule (bound at time t = 0) after f = 10 ps over the whole temperature range. It shows a nonmonotonic temperature dependence. Because the protein motion is coupled to the water d)mamics it is expected that the MSD of the protein will also show similar behavior, which would lead to a protein-glass transition and eventually to the loss of protein functionality. 

Toumier, A.L., Xu, J., and Smith, J.C. Translational hydration water dynamics drives the protein glass transition, Biophys.., 85,1871, 2003. 

Protein-glass transition at 200 K role of water dynamics... 

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. 

The transition temperatures of carbohydrates and proteins are significantly affected by water. It is often reported that an increase in water content results in a substantial decrease in transition temperatures (Slade and Levine 1995). For example, the glass transition of dehydrated food solids decreases as a result of water sorption (i.e., water uptake from its surroundings) and their properties may change from those of the glassy solid to viscous liquids or syrup (e.g., sugar systems) or leathery material (e.g., protein systems) in an isothermal process. 

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