Unfolding of an alpha-helix in water.

We describe a 1 ns molecular dynamics simulation of an 18-residue peptide (corresponding to a portion of the H helix of myoglobin) in water. The initial helical conformation progressively frays to a more disordered structure, with the loss of internal secondary structure generally proceeding from the C-terminus toward the N-terminus. Although a variety of mechanisms are involved in the breaking of helical hydrogen bonds, the formation of transient turn structures, with i----i + 3 hydrogen bonds, and bifurcated hydrogen-bond structures intermediate between alpha and turn or 3(10) structures is a common motif. In some cases a single water molecule is inserted into an internal hydrogen bond, but it is also common to have several water molecules involved in transient intermediates. Overall, the results provide new information about the detailed mechanisms by which helices are made and broken in aqueous solution.