CHARMM36 united atom chain model for lipids and surfactants

Area of Science:

• Computational chemistry
• Biophysics
• Materials science

Background:

• Accurate molecular simulations of lipids and surfactants are crucial for predicting experimental properties.
• Previous united atom (UA) models, like CHARMM27/27r, had limitations, particularly in bilayer simulations requiring imposed surface area ensembles.
• These limitations restricted their application to pure bilayer systems, necessitating a more versatile model.

Purpose of the Study:

• To develop and validate a new UA chain model, C36-UA, based on the CHARMM36 (C36) all-atom lipid parameters.
• To assess the C36-UA model's accuracy in reproducing bulk properties, lipid membrane characteristics, and surfactant self-assembly.
• To provide a computationally efficient alternative to all-atom force fields for large-scale molecular dynamics simulations.

Main Methods:

• Developed a UA chain model (C36-UA) derived from CHARMM36 all-atom lipid parameters.
• Performed molecular dynamics (MD) simulations of alkanes (heptane, pentadecane) to evaluate C36-UA's accuracy for density, heat of vaporization, and self-diffusion.
• Simulated various lipid bilayers (saturated and unsaturated) and DMPC/cholesterol mixtures to test membrane properties and cholesterol interactions.
• Investigated the self-assembly of dodecylphosphocholine (DPC) surfactant using C36-UA to determine aggregation numbers.

Main Results:

• C36-UA accurately reproduced bulk properties of alkanes, including density, heat of vaporization, and self-diffusion constants.
• Simulations of lipid bilayers using C36-UA yielded accurate surface area per lipid, X-ray/neutron form factors, and chain order parameters.
• The C36-UA model performed well in mixed systems with cholesterol and accurately predicted surfactant aggregation numbers (53 ± 11 DPC molecules at 0.45 M).

Conclusions:

• The C36-UA force field provides a computationally efficient and accurate alternative to the all-atom C36 lipid force field.
• This model effectively captures the behavior of lipids and surfactants in various systems, including bilayers and micelles.
• The C36-UA model is well-suited for large-scale molecular dynamics simulations, potentially including systems with proteins, due to its reduced computational cost and maintained accuracy.