Water interfacing hydrated proteins carry properties distinct from those of the bulk and is often described as a separate entity, a “biological water.” Combining depolarized light scattering with absorption of THz radiation helps to adress experimentally the question of which dynamical and structural properties of hydration water deserve this distinction. Such experimental observables are analyzed by computer simulations of hydrated proteins to distinguish different aspects of the density and orientational fluctuations of hydration water. The dynamics of the density fluctuations of the hydration shells reflect the coupled dynamics of the protein and water and do not require a special distinction as “biological water.” The orientations of water molecules in the hydration shell carry dramatically different physics and do require a separation into a sub-ensemble. Depending on the property considered, the perturbation of water’s orientational structure induced by the protein propagates 3–5 hydration shells into the bulk at normal temperature. Anomalies in the absorpsion of THz radiation are related to spatially extended cross correlations between the dipole moments of the protein and the dipole moment of the hydration layer.

Depolarized light scattering and terahertz absorption of protein solutions

253rd National Meeting (2017)

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