Dual-resolution molecular dynamics simulation of antimicrobials in biomembranes

Abstract

Triclocarban and triclosan, two potent antibacterial molecules present in many consumer products, have been subject to growing debate on a number of issues, particularly in relation to their possible role in causing microbial resistance. In this computational study, we present molecular-level insights into the interaction between these antimicrobial agents and hydrated phospholipid bilayers (taken as a simple model for the cell membrane). Simulations are conducted by a novel 'dual-resolution' molecular dynamics approach which combines accuracy with efficiency: the antimicrobials, modelled atomistically, are mixed with simplified (coarse-grain) models of lipids and water. A first set of calculations is run to study the antimicrobials' transfer free energies and orientations as a function of depth inside the membrane. Both molecules are predicted to preferentially accumulate in the lipid headgroup-glycerol region; this finding, which reproduces corresponding experimental data, is also discussed in terms of a general relation between solute partitioning and the intramembrane distribution of pressure. A second set of runs involves membranes incorporated with different molar concentrations of antimicrobial molecules (up to one antimicrobial per two lipids). We study the effects induced on fundamental membrane properties, such as the electron density, lateral pressure and electrical potential profiles. In particular, the analysis of the spontaneous curvature indicates that increasing antimicrobial concentrations promote a 'destabilizing' tendency towards non-bilayer phases, as observed experimentally. The antimicrobials' influence on the self-assembly process is also investigated. The significance of our results in the context of current theories of antimicrobial action is discussed.

Figure 1.

Structures of the antimicrobial molecules studied. (a) Triclocarban (TCC); (b) triclosan (TCS).

Figure 2.

Free energies of transfer from water to selected z-distances along the bilayer normal. To facilitate interpretation, different regions across the system have been marked, namely the bulk water region, the lipid headgroup region, the lipid glycerol region and the hydrocarbon tail core. Approximate boundaries between these regions are defined by the vertical dotted lines (red, TCS; green, TCC).

Figure 3.

Antimicrobial order parameters, defined in equation (4.1), plotted as a function of the distance between the antimicrobial centre of mass and the bilayer centre along the z-axis, that is, along the direction normal to the bilayer interfacial plane. To facilitate interpretation, different regions across the system have been marked, namely the bulk water region, the lipid headgroup region, the lipid glycerol region and the hydrocarbon tail core. Approximate boundaries between these regions are defined by the vertical dotted lines (red, TCS; green, TCC).

Figure 4.

Simulation snapshots from dual-resolution z-constraint simulations. The antimicrobials (a) TCC and (b) TCS are both constrained at a distance of 1.3 nm from the bilayer centre. CG colour code: water molecules are transparent blue; lipid headgroups are transparent grey; lipid tails are transparent green. AL solute colour code: carbon is cyan; hydrogen is white; oxygen is red; chlorine is yellow. Images prepared with VMD [93].

Figure 5.

Electron density profiles. (a) Bilayers including TCC. (b) Bilayers including TCS. All bilayers comprise 128 DOPC lipids; the number of antimicrobial molecules in the specific system is indicated by the different colours (colour codes: brown, 64; orange, 27; blue, 16; green, 8; red, 4; black, 0).

Figure 6.

Electron density profiles of the antimicrobial solute molecules only. (a) TCC; (b) TCS (brown, 64; orange, 27; blue, 16; green, 8; red, 4).

Figure 7.

Lateral pressure profiles. (a) Bilayers including TCC. (b) Bilayers including TCS. All bilayers comprise 128 DOPC lipids; the number of antimicrobial molecules in the specific system is indicated by the different colours (brown, 64; orange, 27; blue, 16; green, 8; red, 4; black, 0).

Figure 8.

Monolayer spontaneous curvature as a function of the antimicrobial concentration (blue, TCC; red, TCS).

Figure 9.

Electrostatic potential profiles. (a) Bilayers including TCC. (b) Bilayers including TCS. All bilayers comprise 128 DOPC lipids; the number of antimicrobial molecules in the specific system is indicated by the different colours (brown, 64; orange, 27; blue, 16; green, 8; red, 4; black, 0).

Figure 10.

Snapshots from the self-assembly of a system comprising 27 TCS antimicrobials, 128 DOPC lipids and 5760 water molecules. The AL antimicrobial molecules are coloured orange. The CG lipid headgroups and tails are black and green, respectively; water is blue. Images prepared with VMD [93].