Volume 79, Number 6, September 2007
Article Number 66003
Number of page(s) 6
Section Condensed Matter: Structural, Mechanical and Thermal Properties
Published online 21 August 2007
EPL, 79 (2007) 66003
DOI: 10.1209/0295-5075/79/66003

Competition between secondary structures in gas phase polyalanines

P. Poulain1, F. Calvo1, 2, R. Antoine1, M. Broyer1 and Ph. Dugourd1

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 in final form 25 July 2007; published September 2007
published online 21 August 2007

The temperature-dependent conformations of alanine-rich polypeptides are investigated using generalized ensemble Monte Carlo simulations. Pure polyalanines form $\alpha $ helices at low temperature, but exhibit an intermediate $\beta $-sheet structure below the coil transition. For the substituted peptide WA13, the simulation predicts the $\beta $ conformation to be more stable than helices already at low temperatures, and the $\beta $ 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 $\beta $ conformations, even though random-coil structures cannot be ruled out. Finally, we show how to stabilize $\alpha $ helices by an intense electric field, possibly leading to electrofreezing behavior.

64.60.Cn - Order-disorder transformations; statistical mechanics of model systems.
02.70.Uu - Applications of Monte Carlo methods.
87.14.Ee - Proteins.

© Europhysics Letters Association 2007