# Structural properties of water confined by phospholipid membranes

## TL;DR

This study investigates how water's local structure and dynamics are altered near phospholipid membranes, revealing that membrane interfaces influence water structure up to 2.5 nm away, beyond the expected 1 nm scale.

## Contribution

It introduces a sensitive local order metric to analyze water structure near membranes, showing extended influence of the membrane interface on water's properties.

## Key findings

- Water near membranes shows increased intermediate range order (IRO).
- Dynamical slowdown occurs within 1 nm of the membrane.
- Structural effects of the membrane extend up to 2.5 nm from the interface.

## Abstract

Biological membranes are essential for the cell life and hydration water provides the driving force for their assembly and stability. Here we study the structural properties of water in a phospholipid membrane. We characterize local structures inspecting the intermediate range order (IRO) adopting a sensitive local order metric, recently proposed by Martelli et al., which measures and grades the degree of overlap of local environments with structures of perfect ice. Close to the membrane, water acquires high IRO and changes its dynamical properties, e.g., slowing down its translational and rotational degrees of freedom in a region that extends over $\simeq 1$ nm from the membrane interface. Surprisingly, we show that at a distance as far as $\simeq 2.5$ nm from the interface, although the bulk-like dynamics is recovered, water's IRO is still slightly higher than in bulk at the same thermodynamic conditions. Therefore, the water-membrane interface has a structural effect at ambient conditions that propagates further than the often-invoked $1$ nm-length scale, a results that should be taken carefully into account when analyzing experimental data of water confined by membranes and could help us understanding the role of water in biological systems.