X-ray and neutron scattering analyses of hydration shells: a molecular interpretation based on sequence predictions and modelling fits.

Solution scattering is a low resolution diffraction method that provides important structural data on proteins. The ability to model scattering curves by recourse to known crystal structures for proteins under study significantly improves the resolution (and the utility) of the method because of the strict constraints that the crystal structures impose. For these structure determinations, a molecular description of the effect of hydration shells is needed. In calibration studies used for X-ray scattering curve modelling, it has been reproducibly found that a hydration shell is required. In molecular terms, this results from the higher electron density of the hydration shell compared to that of bulk water, which then becomes similar to that of the protein. This is well represented by a level of 0.3 g H(2)O/g glycoprotein and a water molecule volume of 0.0245 nm(3). Procedures for the addition of a hydration shell to a sphere model of a protein are described. For neutron scattering fits, it is not necessary to incorporate a hydration shell, as to a good approximation this is not detectable. In molecular terms, this apparent absence of the neutron hydration shell results from the effect of proton exchange on the scattering densities of bulk water and bound water which causes these to be similar but different from that of the protein.