Characterizing the secondary hydration shell on hydrated myoglobin, hemoglobin, and lysozyme powders by its vitrification behavior on cooling and its calorimetric glass->-liquid transition and crystallization behavior on reheating

For hydrated metmyoglobin, methemoglobin, and lysozyme powders, the freezable water fraction of between similar to 0.3-0.4 g water/g protein up to similar to 0.7-0.8 g water/g protein has been fully vitrified by cooling at rates up to similar to 1500 K min(-1) and the influence of cooling rate characterized by x-ray diffractograms. This vitreous but freezable water fraction started to crystallize at similar to 210 K to cubic ice and at similar to 240 K to hexagonal ice. Measurements by differential scanning calorimetry have shown that this vitreous but freezable water fraction undergoes, on reheating at a rate of 30 K min(-1), a glass-->liquid transition with an onset temperature of between similar to 164 and similar to 174 K, with a width of between similar to 9 and similar to 16 degrees and an increase in heat capacity of between similar to 20 and similar to 40 J K-1 (mol of freezable water)(-1) but that the glass transition disappears upon crystallization of the freezable water. These calorimetric features are similar to those of water imbibed in the pores of a synthetic hydrogel but very different from those of glassy bulk water. The difference to glassy bulk water's properties is attributed to hydrophilic interaction and H-bonding of the macromolecules' segments with the freezable water fraction, which thereby becomes dynamically modified. Abrupt increase in minimal or critical cooling rate necessary for complete vitrification is observed at similar to 0.7-0.8 g water/g protein, which is attributed to an abrupt increase of water's mobility, and it is remarkably close to the threshold value of water's mobility on a hydrated protein reported by Kimmich et al. (1990, Biophys. J. 58:1183). The hydration level of similar to 0.7-0.8 g water/g protein is approximately that necessary for completing the secondary hydration shell.