Neutron scattering reveals a complex dynamics in polypeptide chains, with two main onsets of anharmonicity whose physical origin and biological role are still debated. In this study the dynamics of strategically selected homomeric polypeptides is investigated with elastic neutron scattering using different energy resolutions and compared with that of a real protein. Our data spotlight the dependence of anharmonic transition temperatures and fluctuation amplitudes on energy resolution, which we quantitatively explain in terms of a two-site model for the protein-hydration water energy landscape. Experimental data strongly suggest that the protein dynamical transition is not a mere resolution effect but is due to a real physical effect. Activation barriers and free energy values obtained for the protein dynamical transition allow us to make a connection with the two-well interaction potential of supercooled-confined water proposed to explain a $\mathrm{\text{low}}\mathrm{\text{−}}\mathrm{\text{density}}\to \mathrm{\text{high}}\mathrm{\text{−}}\mathrm{\text{density}}$ liquid-liquid transition.

• Received 4 May 2012

DOI:https://doi.org/10.1103/PhysRevLett.109.128102