The interdependence of thermodynamic, mechanical, and structural properties in saturated phospholipid membranes.

Lipid membranes undergo conformational and structural changes when they transition from the gel phase to the fluid phase. These phase transitions are accompanied by changes in the lipid packing and the membrane mechanical properties, as previously reported in experimental studies and molecular dynamics simulations. Here, we report a simple direct approach for calculating the mechanical properties of single-component lipid membranes using size-dependent calorimetry measurements, performed on lipid vesicles with different mean diameters. Using differential scanning calorimetry (DSC), we find that the melting temperature, associated with the gel-to-fluid phase transition, increases with the vesicle diameter until it approaches an asymptotic value representative of uncurved membranes. More importantly, fits of the transition temperature as a function of vesicle size show a clear dependence on the structural and mechanical properties of the membranes, i.e., the area-per-lipid and the area compressibility modulus. Our method is validated by comparing the obtained parameters against results from small-angle X-ray scattering measurements and Langmuir monolayer compression studies. The findings indicate that the thermodynamic properties of vesicular lipid membranes depend on their nanoscale curvature and corresponding energetic contributions for membrane stretching and bending. When combined with measurements of the bending rigidity modulus by neutron spin-echo spectroscopy, our approach yields the interleaflet coupling constant, a sought-after quantity describing the extent of interactions between the two membrane leaflets. The application of this approach to saturated phospholipid membranes with molecular additives (e.g., cholesterol) provides a platform to assess the resultant effects on interleaflet interactions.