Against the rules: pressure induced transition from high to reduced order

Abstract

Envisioning the next generation of drug delivery nanocontainers requires more in-depth information on the fundamental physical forces at play in bilayer membranes. In order to achieve this, we combine chemical synthesis with physical–chemical analytical methods and probe the relationship between a molecular structure and its biophysical properties. With the aim of increasing the number of hydrogen bond donors compared to natural phospholipids, a phospholipid compound bearing urea moieties has been synthesized. The new molecules form interdigitated bilayers in aqueous dispersions and self-assemble at soft interfaces in thin layers with distinctive structural order. At lower temperatures, endothermic and exothermic transitions are observed during compression. The LC₁ phase is dominated by an intermolecular hydrogen bond network of the urea moieties leading to a very high chain tilt of 52°. During compression and at higher temperatures, presumably this hydrogen bond network is broken allowing a much lower chain tilt of 35°. The extremely different monolayer thicknesses violate the two-dimensional Clausius–Clapeyron equation.