Native proteins are surface-molten solids: application of the Lindemann criterion for the solid versus liquid state

Since the internal motions of proteins play an essential role in their biological function, it is important to characterize them in a fundamental way. The Lindemann criterion for the solid state is applied to molecular dynamics simulations and temperature-dependent X-ray diffraction data of proteins. It is found that the interior of native proteins is solid-like, while their surface is liquid-like. When the entire protein becomes solid-like at low temperature ( approximately 220 K), the protein is inactive. Thus, the surface-molten solid nature of proteins in their native state permits the dynamics required for function, while preserving their stability. Comparison with rare gas clusters and polymer models indicates that their thermodynamic phase diagrams have many elements in common with those of proteins.