Anharmonic onsets in polypeptides revealed by neutron scattering: Experimental evidences and quantitative description of energy resolution dependence

Neutron scattering measurements on protein powders reveal two deviations from harmonic dynamics at low temperature, whose molecular origin, physical nature and biological relevance are still matter of discussion. In this study we present a new experimental and theoretical approach to evidence the resolution dependence of anharmonic onsets: the use of strategically selected homo-meric polypeptides allows revealing the exact resolution dependence; a two-site energy landscape model, where resolution effects are explicitly taken into account, is able to interpret quantitatively the experimental data in terms of energy landscape parameters. The energetic description provided by this analysis, together with recent experimental evidences obtained on chemically and structurally different peptide systems, allows us to connect the protein/water energy landscape structure with the two-wells water interaction potential proposed to explain the low-density → high-density liquid-liquid transition observed in supercooled water. (author)