SIMULATION OF WATER AROUND A MODEL PROTEIN HELIX .1. 2-DIMENSIONAL PROJECTIONS OF SOLVENT STRUCTURE

The water structure around a model alpha-helix in solution is investigated by molecular simulation. A marked contrast is seen between the hydration of the alpha and beta carbons on one side of the helix axis and of the carbonyl oxygen on the other side. Three well-defined peaks in the oxygen density, loosely corresponding to three layers of water, are found around the two hydrophobic atoms. In contrast, around the hydrophilic carbonyl oxygen the first two peaks merge into a single large peak. Analysis of the water orientation shows that on average the water dipoles are parallel to the surface of the helix. However, near the carbonyl oxygen they are slightly tipped inward and also have a significant component in the helical direction. The overall dipole orientation is consistent with the charges on the helix. Analysis of the hydrogen distribution reveals an inward pointing hydrogen near the carbonyl oxygen with one clearly preferred position for hydrogen bonding. In contrast, water molecules around the two hydrophobic atoms point their hydrogens outward and adopt the clathrate arrangement found in the hydration of small apolar solutes, Particular emphasis is put on developing a representation for the results in terms of two-dimensional projections, displayed in ''gray-level''. These projections exploit the symmetry of the helix to show the complex, three-dimensional structure of the water distribution better than one-dimensional profiles.