More than one dynamic crossover in protein hydration water

TL;DR

This study investigates the dynamics of hydration water around proteins, revealing two distinct dynamic crossovers at approximately 252 K and 181 K linked to changes in hydrogen bond fluctuations and reordering, supported by experiments and models.

Contribution

It demonstrates the presence of two dynamic crossovers in protein hydration water and relates them to specific heat maxima through combined experimental and modeling approaches.

Findings

Two dynamic crossovers at ~252 K and ~181 K identified.

Crossovers linked to hydrogen bond fluctuations and reordering.

Model and experimental results show strong agreement.

Abstract

Studies of liquid water in its supercooled region have led to many insights into the structure and behavior of water. While bulk water freezes at its homogeneous nucleation temperature of approximately 235 K, for protein hydration water, the binding of water molecules to the protein avoids crystallization. Here we study the dynamics of the hydrogen bond (HB) network of a percolating layer of water molecules, comparing measurements of a hydrated globular protein with the results of a coarse-grained model that has been shown to successfully reproduce the properties of hydration water. With dielectric spectroscopy we measure the temperature dependence of the relaxation time of protons charge fluctuations. These fluctuations are associated to the dynamics of the HB network of water molecules adsorbed on the protein surface. With Monte Carlo (MC) simulations and mean--field (MF) calculations we study the dynamics and thermodynamics of the model. In both experimental and model analyses we find two dynamic crossovers: (i) one at about 252 K, and (ii) one at about 181 K. The agreement of the experiments with the model allows us to relate the two crossovers to the presence of two specific heat maxima at ambient pressure. The first is due to fluctuations in the HB formation, and the second, at lower temperature, is due to the cooperative reordering of the HB network.