Thermal conductivity and rectification in asymmetric archaeal lipid membranes

Nature employs lipids to construct nanostructured membranes that self-assemble in an aqueous environment to separate the cell interior from the exterior environment. Membrane composition changes among species and according to environmental conditions, which allows organisms to occupy a wide variety of different habitats. Lipid bilayers are phase-change materials that exhibit strong thermotropic and lyotropic phase behavior in an aqueous environment, which may also cause thermal rectification. Among different types of lipids, archaeal lipids are of great interest due to their ability to withstand extreme conditions. In this paper, nonequilibrium molecular dynamics simulations were employed to study the nanostructures and thermal properties of different archaeols and to investigate thermal rectification effects in asymmetric archaeal membranes. In particular, we are interested in understanding the role of bridged phytanyl chains and cyclopentane groups in controlling the phase transition temperature and heat flow across the membrane. Our results indicate that the bridged phytanyl chains decrease the molecular packing of lipids, whereas the existence of cyclopentane rings on the tail groups increases the molecular packing by enhancing the interactions between isoprenoid chains. We found that macrocyclic archaeols have the highest thermal conductivity, whereas macrocyclic archaeols with two cyclopentane rings have the lowest. The effect of the temperature on the variation of thermal conductivity was found to be progressive. Our results further indicate that small thermal rectification effects occur in asymmetric archaeol bilayer membranes at around 25 K temperature gradient. The calculated thermal rectification factor was around 0.09 which is in the range of rectification factor obtained experimentally for nanostructures such as carbon nanotubes (0.07). Such phenomena may be of biological significance and could also be optimized for use in various engineering applications.