Lipid monolayer disruption caused by aggregated carbon nanoparticles

Carbon nanoparticles (CNP) have significant impact on the Pulmonary Surfactant (PS), the first biological barrier in the respiratory system. CNPs -abundant in the environment due to combustion -can translocate into our bodies by crossing the alveolar epithelium barrier, and they can be retained in the lungs due to slow clearance. The physical mechanisms of how CNPs perturb PS remain unclear yet such knowledge is crucial for developing effective strategies against the adverse effects of CNPs. Molecular dynamics (MD) simulations of model PS monolayers in the presence of C-60 fullerenes were performed at time scales of tens of microseconds and varying C-60 content. In contrast to bilayers, fullerenes affected both structural and dynamic properties of the PS monolayer. Surface tension/area isotherms of the monolayer were changed by fullerenes and perturbations of the physical structure of the PS monolayer became major at high fullerene concentrations due to fullerene aggregation. At high compression (area per molecule of 0.48 nm(2)), the monolayer became unstable and collapsed forming a bilayer in the water phase. At low compression, pore formation occurred. Free energy calculations suggest that increasing fullerene concentration leads to decreased preference for the fullerenes to reside inside the monolayer. However, the free energy barrier for transferring fullerene out of the monolayer is rather large at all fullerene concentrations; spontaneous translocation of fullerene out of the monolayer is thermodynamically unfavorable. The results illustrate some of the potentially harmful effects of CNPs on the respiratory system and also the physical mechanism of how CNPs disturb pulmonary surfactant. This may be related to the difficulty of CNP clearance from lung surfactant. The total simulation time was 370 microseconds.