Biopolymers, hydration shell

Protein crystals contain between 25 and 65 vol% water, which is essential for the crystallisation of these biopolymers. A typical value for the water content of protein crystals is 45% according to Matthews et al. l49,150). For this reason it is possible to study the arrangement of water molecules in the hydration-shell by protein-water and water-water interactions near the protein surface, if one can solve the structure of the crystal by X-ray or neutron diffraction to a sufficiently high resolution151 -153). 

Summarizing Chapter 3 we can conclude there are many indications that biopolymers may have different hydrations 1. Primary hydrate 1 -2 H20 per unit 2. bonded water 10-20 H20 3. secondary hydrate shells up to 50 H20 and 4. bulk water. The secondary hydrates may solve ions. 

The importance of the 4-8 x 10-8 cm spacing between hydroxyl oxygen atoms is clearly demonstrated in the aqueous chemistry of polysaccharides (Suggett, 1973). In some instances, the reactivity of a mismatched OH group can differ from one which is matched to this spacing. However, the lack of extensive and specific hydration shells around macromolecules makes the significance of this repeating distance in biopolymers mysterious (Berendsen, 1973). 

The precipitation of the eutectics from solutions upon cooling occurs when the free energy of the solute molecules in the crystalline state becomes less than that of the molecules in solution. Therefore, a change in the tanperature of precipitation of the eutectics indicates a change in the chanical potentials of the components of the solutions in the presence of protein molecules. On concentration of the solution, this may occur in the hydrate shells of the protein molecules. Since for the organic solute molecules close to the macromolecules not only do the molecules of the solvent but also the structural elanents of the protein molecules form the medium, it is most probable that the observed phenomenon of a rise in the temperature of freezing is based on the interaction of the molecules of the organic additive with the biopolymer. 

D. Beglov and B. Roux. Dominant solvations effects from the primary shell of hydration Approximation for molecular dynamics simulations. Biopolymers, 35 171-178, 1994. 

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