Free-Energy Calculations of the Interactions of Helical Poly(L-Proline) with Water*

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Conformational energies, consisting of nonbonded and electrostatic interactions, have been calculated for poly(L-proline) I and II helices, of six-residues length, with a water molecule hydrogen-bonded to each imide group. For polyproline I, the Traub-Shmueli prolyl-ring geometry was used, whereas for polyproline II, calculations were done for the 2 ring geometries of Leung and Marsh and, in some cases, for the several ring geometries of Ramachandran, et al. The chain torsion angles ω(N-C′) and Ψ(Cα-C′) were varied, as were the two angles that specify the orientations of the water molecules. By summation over the latter, it was possible to calculate a free energy at each ω,Ψ that incorporated the conformational entropy of the water molecules. Such ω,Ψ maps reveal that binding of water can cause changes in the equilibrium conformation of polyproline, as well as in the energetics of the chain with respect to ω and Ψ. This has important consequences on the statistics of the chain. Considerations of prolyl-ring self-energies, as well as the conformational entropies of the chains, show that polyproline II is more stable in water than is polyproline I, and determine which ring geometry is favored for polyproline in water.

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