Towards a complete characterization of the δ-dispersion in dielectric spectroscopy of protein-water systems

In this computational study, we aim at a complete characterization of the delta-process occurring in dielectric spectroscopy of protein-water systems, with a focus on the consistent interpretation in relation to other experimental observables in the field of biomolecular hydration. A large-scale and long-time molecular dynamics simulation of the protein ubiquitin in aqueous solution offers an adequate basis for the investigation of dielectric properties involving long-ranged and slow collective processes, and their comparison to other experiments calculated from the very same trajectories. An expedient definition of components contributing to the dielectric spectrum reveals at least five different delta-subprocesses. While the previously described cross-correlation between water and protein still plays a major role, we find additional dispersions stemming from protein and water self-correlations. In particular, a contribution of the first hydration shell with moderate retardation relative to bulk water can be linked to the single-particle behavior of water molecules, as observed in magnetic relaxation dispersion. The principle character of the complete delta-process, already once anticipated in experimental literature, is now explicitly calculated and analyzed in great detail, as presented in this study.