Neutron small-angle scattering studies of ribonuclease in mixed aqueous solutions and determination of the preferentially bound water.

Neutron small-angle measurements of ribonuclease A in mixed solutions of ethanol-water and glycerol-water have been used to estimate the region near the molecule that only contains water. When glycerol is used as probe, the region corresponds to an interaction parameter xi of 0.23 +/- 0.05 g of water/g of protein. For ethanol xi is -0.07 +/- 0.05, corresponding to the macromolecule being equally accessible to ethanol and water. The observations of the radius of gyration in the mixed solutions are used to show that the volume excluding glycerol is found on the exterior of the protein.