Tristearin bilayers: structure of the aqueous interface and stability in the presence of surfactants

Abstract

We report results of atomistic molecular dynamics simulations of an industrially-relevant, exemplar triacylglycerol (TAG), namely tristearin (TS), under aqueous conditions, at different temperatures and in the presence of an anionic surfactant, sodium dodecylbenzene sulphonate (SDBS). We predict the TS bilayers to be stable and in a gel phase at temperatures of 350 K and below. At 370 K the lipid bilayer was able to melt, but does not feature a stable liquid-crystalline phase bilayer at this elevated temperature. We also predict the structural characteristics of TS bilayers in the presence of SDBS molecules under aqueous conditions, where surfactant molecules are found to spontaneously insert into the TS bilayers. We model TS bilayers containing different amounts of SDBS, with the presence of SDBS imparting only a moderate effect on the structure of the system. Our study represents the first step in applying atomistic molecular dynamics simulations to the investigation of TAG-aqueous interfaces. Our results suggest that the CHARMM36 force-field appears suitable for the simulation of such systems, although the phase behaviour of the system may be shifted to lower temperatures than is the case for the actual system. Our findings provide a foundation for further simulation studies of the TS-aqueous interface. +61 (0)3 5227 1103; Tel: +61 (0)3 5247 9160 † Electronic supplementary information (ESI) available: The area of example bilayers from the Set A and B simulations; the 2D radial distribution function of the TS acyl tails; the order parameter of the acyl tails of the TS molecules in simulations of Set A and B; snapshots showing the gel-liquid crystalline phase transition; snapshots showing the presence of SDBS aggregates; the number of SDBS molecules embedded in the bilayer during the Set C, D, E, F and G simulation runs; density proles of the Set C, D, E, F and G simulation runs; snapshots showing the packing of the SDBS molecules in the bilayers; order parameters of the acyl tails of TS and SDBS molecules; the sulphur/phenyl ring-water radial distribution functions of SDBS. See

Elucidating the mechanisms of nanodiamond-promoted structural disruption of crystallised lipid

“…49 The water model used was that of the modified version of TIP3P compatible with the CHARMM FF. 50,51 Previous work has shown that CHARMM36 FF is suitable for the simulation of the TSaqueous interfaces 41 (although there was some evidence that the FF may over-stabilise the TS). Development details for the nanodiamond FF are provided in the ESI † section 'Nanodiamond FF parameters'.…”

“…The initial structures of the TS bilayers were taken from systems that had previously been equilibrated at 300 or 350 K. 41 Previous work has found that the structure of pure TS bilayers in contact with liquid water, modelled with the CHARMM 36 FF, break down at ⇠ 365.5 367 K. Therefore, our simulations here were performed at both 300 and 350 K. In addition, a number of simulations where the system was heated were performed to determine the gel/liquid-crystalline phase transition temperature. To better characterise the interactions of the different components of the system (TS, ND and surfactant) a range of different systems were considered.…”

“…However, our previous studies indicated that the spontaneous insertion of surfactant from the solution phase into the TS bilayers is associated with very long timescales. 41 While advanced sampling strategies might in future be gainfully applied to explore the timescale of this process, the success of such approaches hinges on an appreciation of the likely initial and final states of this complex pathway. To this end, the Set C and D simulations may capture the initial states of this process, but will not be appropriate for investigating the final stages, i.e.…”

“…There have been relatively few atomistic molecular simulations reported for TAGs, [34][35][36][37][38][39][40][41] with the greater part of these previous studies detailing investigations of the bulk phase properties. [34][35][36][37][38] To the authors' knowledge there has been only one study that reported the use of MD simulations to investigate the TAG-aqueous interface.…”

“…[34][35][36][37][38] To the authors' knowledge there has been only one study that reported the use of MD simulations to investigate the TAG-aqueous interface. 41 In this study, a TS bilayer in contact with liquid water was simulated at a range of temperatures, both in the presence and absence of the anionic surfactant SDBS. In the absence of any surfactant it was found that below ⇡ 365 K the TS bilayers were stable in a solid phase, with the lipid tails ordered laterally in a hexagonal structure.…”