Inelastic incoherent neutron scattering studies of water interacting with biological macromolecules.

The interaction between water and biological macromolecules in living organisms is of fundamental importance in a range of processes. We have studied water-DNA and water-proteolipid membrane systems over a range of hydration states using inelastic incoherent neutron scattering. We find a relatively sharp transition for both systems at a water concentration above which bulk solvent can be detected. Below this concentration, bulk water is essentially absent, i.e., all the water in the system is interacting with the biological macromolecules. This water is strongly perturbed as judged by its energy transfer spectrum, with a broader and lower energy transition than bulk water in the 50-75 meV (approximately 400-600 cm(-1)) range. Taking into account the differing geometry of (cylindrical) DNA and (planar) membranes, the number of water shells perturbed by each system was estimated. A conclusion is that in living organisms a large proportion of the cellular water will be in a state quite distinct from bulk water. The data add to the growing evidence that water structure in the vicinity of biological macromolecules is unusual and that the proximal water behaves differently compared to the bulk solvent.