Dielectric Properties of Water at Lipid Interfaces

Lipid membranes are fundamental to cellular structure and function, with interfacial water playing a pivotal role in maintaining membrane stability and mediating biological processes. However, systematic studies on the dielectric properties of water molecules in the vicinity of lipid headgroups are still lacking. Herein, we reveal significant variations in the local dielectric constant of hydration water adjacent to different lipid headgroups, using molecular dynamics simulations. These differences arise from the varying Coulombic attraction exerted by different lipid headgroups on nearby water molecules. In particular, the headgroup of POPC exerts a stronger Coulombic attraction on surrounding water molecules, which not only reduces their dynamic behavior and dipolar fluctuations but also prolongs their dipole relaxation time, ultimately leading to a lower local dielectric constant. Notably, compared to POPC, POPA exerts a weaker Coulombic attraction on nearby water molecules, leading to enhanced dipolar fluctuations and shorter relaxation times. These effects collectively account for the higher dielectric constant of hydration water near the POPA headgroup. Our findings underscore the critical role of lipid headgroups in governing interfacial water dielectric properties and offer new insights into membrane-associated biophysical mechanisms.