Network Topology in Water Nanoconfined between Phospholipid Membranes

TL;DR

This study investigates the structure and dynamics of water confined between phospholipid membranes, revealing enhanced intermediate-range order and complex hydrogen bond network alterations that are crucial for biological processes.

Contribution

It provides new insights into the relationship between water's structure, hydrogen bonding, and confinement near biological membranes, highlighting the existence of bound and unbound water interfaces.

Findings

Water density and dynamics recover at ~1 nm from membranes

Enhanced intermediate-range order extends beyond bulk behavior

Identification of bound-unbound water interface at ~0.8 nm

Abstract

Water provides the driving force for the assembly and stability of many cellular components. Despite its impact on biological functions, a nanoscale understanding of the relationship between its structure and dynamics under soft confinement has remained elusive. As expected, water in contact with biological membranes recovers its bulk density and dynamics at $\sim 1$ nm from phospholipid headgroups but surprisingly enhances its intermediate-range order (IRO) over a distance, at least, twice as large. Here, we explore how the IRO is related to the water's hydrogen bond network (HBN) and its coordination defects. We characterize the increased IRO by an alteration of the HBN up to more than eight coordination shells of hydration water. The HBN analysis emphasizes the existence of a bound-unbound water interface at $\sim 0.8$ nm from the membrane. The unbound water has a distribution of defects intermediate between bound and bulk water, but with density and dynamics similar to bulk, while bound water has reduced thermal energy and much more HBN defects than low-temperature water. This observation could be fundamental for developing nanoscale models of biological interactions and for understanding how alteration of the water structure and topology, for example, due to changes in extracellular ions concentration, could affect diseases and signaling. More generally, it gives us a different perspective to study nanoconfined water.