The Dynamic Transition of Protein Hydration Water

TL;DR

This study investigates the dynamics of hydration water on biomolecular surfaces, showing that previously observed anomalies near 220 K are due to a calorimetric glass transition at 170 K, not a fragile-to-strong transition.

Contribution

Using advanced neutron backscattering and improved analysis, the paper refutes the existence of a fragile-to-strong transition in hydration water and clarifies the origin of the 220 K anomaly.

Findings

No evidence of fragile-to-strong transition in hydration water.

The 220 K anomaly is linked to a calorimetric glass transition at 170 K.

Relaxation times align with other techniques when data is properly analyzed.

Abstract

Thin layers of water on biomolecular and other nanostructured surfaces can be supercooled to temperatures not accessible with bulk water. Chen et al. [PNAS 103, 9012 (2006)] suggested that anomalies near 220 K observed by quasi-elastic neutron scattering can be explained by a hidden critical point of bulk water. Based on more sensitive measurements of water on perdeuterated phycocyanin, using the new neutron backscattering spectrometer SPHERES, and an improved data analysis, we present results that show no sign of such a fragile-to-strong transition. The inflection of the elastic intensity at 220 K has a dynamic origin that is compatible with a calorimetric glass transition at 170 K. The temperature dependence of the relaxation times is highly sensitive to data evaluation; it can be brought into perfect agreement with the results of other techniques, without any anomaly.