Water Librations in the Hydration Shell of Phospholipids

The hydrophilic phosphate moiety in the headgroup of phospholipids forms strong hydrogen bonds with water molecules in the first hydration layer. Time-domain terahertz spectroscopy in a range from 100 to 1000 cm^-1 reveals the influence of such interactions on rotations of water molecules. We determine librational absorption spectra of water nanopools in phospholipid reverse micelles for a range from w₀ = 2 to 16 waters per phospholipid molecule. A pronounced absorption feature with maximum at 830 cm^-1 is superimposed on a broad absorption band between 300 and 1000 cm^-1. Molecular dynamics simulations of water in the reverse micelles suggest that the feature at 830 cm^-1 arises from water molecules forming one or two strong hydrogen bonds with phosphate groups, while the broad component comes from bulk-like environments. This behavior is markedly different from water interacting with less polar surfaces.