Permeation of beta-defensin-3 encapsulated with polyethylene glycol in lung surfactant models at air-water interface

Coarse grained molecular dynamics of the permeation of the peptide human beta-defensin-3 (HBD3) in two different lung surfactant models (BLES and CUROSURF) at surface tension of 20 mN m^-1 was performed in order to simulate the effect of charge distribution on the permeation process. We found that HBD3 permeates in lung surfactant models at surface tension of 20 mN m^-1. However, it sticks to the phospholipid polar heads, possibly having its bactericidal properties inactivated by electrostatic interaction. The HBD3 encapsulated with polyethylene glycol (PEG) molecules does permeate, but it does not interact with the polar heads. The Gibbs free energies of this process in both models were estimated to be -39 and -64 Kcal mol^-1 for HBD3 and PEG-encapsulated HBD3, respectively. The peptide HBD3 was pulled from the polar heads to the water phase overcoming an energy barrier of around 17 and 22 Kcal mol^-1, respectively. The peptide was encapsulated with PEG molecules in order to reduce this barrier, thereby decreasing to roughly 12 and 0 Kcal mol^-1 in BLES and CUROSURF models, respectively. After permeation of the HBD3 encapsulated with PEG, it freely moves in the water phase with little or none interaction with the phospholipid head groups. Lung surfactant with low charge distribution seems to have less or none electrostatic interaction with HBD3, possibly preserving its antimicrobial activity.