Area per Lipid in DPPC-Cholesterol Bilayers:Analytical Approach

TL;DR

This paper presents an analytical model for calculating the area per molecule in DPPC-Cholesterol bilayers, capturing non-linear effects of cholesterol concentration and predicting related thermodynamic properties.

Contribution

The study introduces a flexible strings model with optimized parameters to accurately describe lipid areas and thermodynamic behavior in cholesterol-containing bilayers.

Findings

Model accurately reproduces area per lipid data from simulations.

Predictions include NMR order parameter and thermal expansion coefficient.

Non-linear dependence of area on cholesterol concentration is explained.

Abstract

Area per molecule in a DPPC-Cholesterol bilayers depends non-linearly on the cholesterol concentration. Using flexible strings model of lipid membranes we calculate area per molecule in DPPC-Cholesterol mixtures in the biologically relevant concentrations range. Few parameters of the model are optimized for a perfect agreement with the area per lipid data available from molecular dynamics simulations. Lateral pressure at the hydrophilic interface, {\gamma}, is taken to be proportional to the cholesterol concentration. Non-linearity arises as a consequence of the non-linear dependence of thermodynamical equilibrium area of molecules on {\gamma}. DPPC lipid is modeled as flexible string of finite thickness and a given bending rigidity, while cholesterol molecule is modeled as rigid rod with finite thickness and infinite rigidity. Using parameters fitted to reproduce area per molecule dependence on cholesterol concentration, we had further calculated our model predictions for the NMR order parameter of DPPC lipid chains and coefficient of thermal area expansion. The microscopic nature of the model allows to consider a broad range of thermodynamic phenomena.