Nuclear magnetic resonance study of water sorbed on serum albumin.

The nuclear magnetic resonance line widths of water sorbed on solid bovine serum albumin have been determined as functions of temperature and water content of the albumin samples. The results indicate that, depending on its amount, the sorbed water may exist in three different states or arrangements. The first of these involves direct binding of water molecules to available polar groups on the protein, with water-water interactions being of relatively little significance. The addition of more water allows interactions among the water molecules and leads to the formation of a hydration shell, the structure of which is markedly different from that of liquid water and is determined by the protein substrate. At any temperature there is a water content past which the influence of the protein becomes of less importance than the interactions of water molecules among themselves, the hydration shell structure is disrupted, and the water structure approaches that of liquid water. The amount of water strongly influenced by the solid protein is apparently less than that for the protein in solution.