Hydration of amino acid side chains: dependence on secondary structure.

Energy calculations have been used to study the hydration sites around the polar groups of serine, threonine and tyrosine side chains. These hydration sites depend not only on the hybridization of the polar group but also on the local secondary structure, the chi 1 side chain torsion angle and the position of the hydroxyl hydrogen atom. For tyrosine side chains, two solvent sites are found approximately in the plane of the ring. Even for serine and threonine side chains only two minimum energy sites are found in general of which one is in an expected position within hydrogen bonding of the hydroxyl hydrogen atom (unless this is blocked from interaction with solvent molecules by, for example, Oi-4 or Oi-3. The position of the second of these sites depends not only on the position of the hydroxyl oxygen but also on neighbouring main chain atoms to which it can also hydrogen bond. There is good agreement with the solvent distributions obtained from crystallographic data.