Simulations of skin barrier function: free energies of hydrophobic and hydrophilic transmembrane pores in ceramide bilayers

Abstract

Transmembrane pore formation is central to many biological processes such as ion transport, cell fusion, and viral infection. Furthermore, pore formation in the ceramide bilayers of the stratum corneum may be an important mechanism by which penetration enhancers such as dimethylsulfoxide (DMSO) weaken the barrier function of the skin. We have used the potential of mean constraint force (PMCF) method to calculate the free energy of pore formation in ceramide bilayers in both the innate gel phase and in the DMSO-induced fluidized state. Our simulations show that the fluid phase bilayers form archetypal water-filled hydrophilic pores similar to those observed in phospholipid bilayers. In contrast, the rigid gel-phase bilayers develop hydrophobic pores. At the relatively small pore diameters studied here, the hydrophobic pores are empty rather than filled with bulk water, suggesting that they do not compromise the barrier function of ceramide membranes. A phenomenological analysis suggests that these vapor pores are stable, below a critical radius, because the penalty of creating water-vapor and tail-vapor interfaces is lower than that of directly exposing the strongly hydrophobic tails to water. The PMCF free energy profile of the vapor pore supports this analysis. The simulations indicate that high DMSO concentrations drastically impair the barrier function of the skin by strongly reducing the free energy required for pore opening.

FIGURE 1

Molecular structures of ceramide 2 and dimethylsulfoxide (DMSO).

FIGURE 2

By mechanically constraining the lipid density in the center of a fluidized ceramide 2 bilayer in a 0.6 mol fraction DMSO solvent, a hydrophilic transmembrane pore of radius R is created. Snapshots are shown of a slice of the bilayer through the center of the pore (center of box) and of a top-down view of the bilayer (shown without water). Water is colored cyan, DMSO yellow, and ceramide carbon atoms are colored gray, nitrogen atoms blue, oxygen atoms red, and hydrogen atoms white.

FIGURE 3

Free energy F(R) as a function of pore radius R for the ceramide bilayer with 0.6 mol fraction DMSO. The solid line shows the fit of the data to Eq. 2 for bilayers with a transmembrane pore; the dashed line is a quadratic fit for the intact membranes with a strong local lipid-density reduction.

FIGURE 4

Effect of the constraint on the ceramide bilayer without DMSO, where R is the radius of the pore. Snapshots are shown of a slice of the bilayer through the center of the pore (center of box) and of a top-down view of the bilayer (shown without water). Water is colored cyan and ceramide carbon atoms are colored gray, nitrogen atoms blue, oxygen atoms red, and hydrogen atoms white.

FIGURE 5

Cartoons of pore structures. A hydrophilic pore is formed when the hydrophilic headgroups rearrange to shield the hydrocarbon tails from the water. In a hydrophobic pore, the headgroups do not rearrange and the tails are exposed to the water. In a vapor pore, water does not enter the interior of the pore.

FIGURE 6

Free energy F(R) as a function of pore radius R for the ceramide bilayer in the gel phase. Also shown are the elastic F_elas, water-vapor F_wv, and tail-vapor F_tv contributions to the total free energy F_vapor.

FIGURE 7

Snapshots of a slice through the strips of bilayer in (a) pure water, (b) 0.1 mol fraction DMSO, and (c) 0.6 mol fraction DMSO, showing the bilayer edge.