Competition between secondary structures in gas phase polyalanines

¹  Université de Lyon, Université Lyon 1, CNRS, LASIM - 43 Bd du 11 Novembre 1918, F-69622 Villeurbanne cedex, France
²  Laboratoire de Chimie et Physique Quantiques, IRSAMC, Université Paul Sabatier - 118 Route de Narbonne, F-31062 Toulouse cedex, France

Received: 18 April 2007
Accepted: 25 July 2007

Abstract

The temperature-dependent conformations of alanine-rich polypeptides are investigated using generalized ensemble Monte Carlo simulations. Pure polyalanines form α helices at low temperature, but exhibit an intermediate -sheet structure below the coil transition. For the substituted peptide WA₁₃, the simulation predicts the conformation to be more stable than helices already at low temperatures, and the motif is further favored by entropy. Measurements of the electric dipole of this peptide do not provide evidence for helical structures even at room temperature. These experimental observations are thus compatible with our suggestion of conformations, even though random-coil structures cannot be ruled out. Finally, we show how to stabilize α helices by an intense electric field, possibly leading to electrofreezing behavior.