On the origins of apparent fragile-to-strong transition of protein hydration waters

TL;DR

This study uses 2H NMR to investigate protein hydration water dynamics, clarifying that previously reported transition temperatures are not indicative of fragile-to-strong transitions but are related to defect diffusion mechanisms.

Contribution

It demonstrates that the observed crossover temperatures are not fragile-to-strong transitions and links water reorientation to defect diffusion in hydrogen-bond networks.

Findings

X1 transition does not exist.

X2 correlates with thermally activated tetrahedral jumps.

Water motion involves defect diffusion in hydrogen-bond networks.

Abstract

2H NMR is used to study the mechanisms for the reorientation of protein hydration water. In the past, crossovers in temperature-dependent correlation times were reported at T_x1=225K (X1) and T_x2=200K (X2). We show that neither X1 nor X2 are related to a fragile-to-strong transition. Our results rule out an existence of X1. Also, they indicate that water performs thermally activated and distorted tetrahedral jumps at T<T_x2, implying that X2 originates in an onset of this motion, which may be related to a universal defect diffusion in materials with defined hydrogen-bond networks.