Slow Dynamics of Constrained Water in Complex Geometries

TLDR

Water’s role in many chemical and biological processes remains poorly understood, especially when confined in restricted environments that disrupt its hydrogen‑bond network; recent ultrafast laser spectroscopy studies have begun to probe these constrained water dynamics. This review surveys recent experimental and theoretical studies on the dynamics of confined water. The authors compile and analyze data from ultrafast laser spectroscopy, NOE, and NMRD experiments to elucidate confined water behavior. Confined water exhibits a bimodal response: a fast bulk‑like subpicosecond component and a slow component lasting hundreds to thousands of picoseconds that accounts for 10–30 % of the response, attributed to dynamic exchange between free and bound water.

Abstract

Although water is often hailed as the lubricant of life, a detailed understanding of its role in many chemical and biological processes still eludes us. In many natural systems, water is confined in an environment where its free movement is restricted and its three-dimensional hydrogen-bonded network is disrupted. Very recently, several groups applied ultrafast laser spectroscopy to study the dynamics of the constrained water molecules. It is observed that the dynamic behavior of the confined water molecules is markedly different from that of the ordinary water molecules. The most striking result is the bimodal response of confined water, with one bulk water-like subpicosecond component and a much slower component in a time scale of hundreds or thousands of picoseconds. This slow second component constitutes 10−30% of the total response and is crucial in the understanding of the role of water in complex chemical and biological processes. The origin of the slow component has been a subject of intense recent debate and has recently been attributed to a dynamic exchange between free and bound water. This interpretation seems to be in accord with the conclusion reached independently by intermolecular solute−water NOE and NMRD studies. In this article, a review of the recent experimental and theoretical work in this area is presented.