Binding and distribution of water molecules in DPPC bilayers doped with β-sitosteryl sulfate

We have comparatively studied the behavior of water molecules associated with the DPPC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine) bilayers in the presence and absence of β–sitosteryl sulfate (PSO₄) with the help of differential scanning calorimetry (DSC) and SAXS (small-angle x-ray scattering) techniques. The DSC heating endotherms were analyzed to understand the intermolecular interactions between water molecules and the lipid headgroups. The strongly bound, weakly bound, and free water (SB-Water, WB-Water, and FW-Water, respectively) were thus identified in the bilayers and the impact of incorporating PSO₄ was evaluated. The SAXS data provided the supporting evidence for the impact of PSO₄ on the intake of water into the bilayer. Regardless of the presence or absence of PSO₄, the SB-Water existed in the system as the non-freezable fraction. On the other hand, the WB-Water and FW-Water fractions, both of which are freezable, exhibited freezing and melting behaviors that differed from each other significantly. The enthalpies of fusion of the WB-Water, which differed from that of the FW-Water, also varied with the mole fractions of PSO_4. PSO₄ enhanced the fraction of WB -water in the bilayer while at the same time reducing the fractions of SB-Water and FW-Water. The optimum retainability and the ease of release of the available water makes this system efficient for maintaining skin homeostasis if used in cosmetic and pharmaceutical formulations.