Glass Transition of Hydration Water and Structural Flexibility of Myoglobin
The structure of proteins can be characterized by a few conformational states (R and T structure of hemoglobin). Fluctuations around these states have been termed “conformational substates” (1). The original definition of substates was based on motions which freeze near 200 K leading to static disorder at low temperatures (2,3). A more precise description requires information about the type of motion and the interactions involved. One particular aspect of this problem is the role of hydration water. We studied the O-H stretching band and the specific heat of hydration water in myoglobin films and crystals as a function of temperature and hydration. The ir spectra reveal that the surface water is amorphous and solid at low temperatures. Fig.1 displays the glass transition between the solid and the liquid phase at various water concentrations. No transition is found below 0.24 g H2O/g protein. The spectrum is that of liquid bound water even at low temperatures. Even though all charged and polar sites are hydrated at this concentration, the water still forms isolated patches on the protein surface. The contribution of the temperature independent component decreases with increasing water content. The spectrum of amorphous ice appears between 0.25 and 0.3 g/g. The glass transition thus involves fluctuations of connected water clusters including water around nonpolar sites. Melting starts above 190 K.
KeywordsGlass Transition Heme Iron Hydration Water Static Disorder Hydrogen Bond Length
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- 6.Doster, W., Bachleitner, A., Dunau, R., Hiebl, M., Lüscher, E. (1986) Biophys.J. to be publishedGoogle Scholar