Nano-mechanical characterization of asymmetric DLPC/DSPC supported lipid bilayers

Abstract

Asymmetric distribution of lipid molecules in the inner and outer leaflets of the plasma membrane is a common occurrence in the membrane formation. Such asymmetric arrangement is a crucial parameter to manipulate the properties of the cell membrane. It controls signal transduction, endocytosis, exocytosis in the cells. The artificial membrane is often used to study the lateral and transverse arrangement of the lipid molecules in place of the cell membrane. Nano-mechanical characterization of the model membrane helps to understand the mechanical stability of the lipid bilayer. The stability is sensitive to the variations in the lipid composition and their local organization. In this article, we present both topographical and nano-mechanical properties of lipid bilayer characterized by atomic force microscopy (AFM). The results show that the asymmetric lipid bilayer formation is an intrinsic character. We have selected a bi-component fluid-gel phase 1,2-dilauroyl-sn-glycero-3-phosphocholine:1,2-disteroyl-sn-glycero-3-phosphocholine (DLPC: DSPC) system for our studies. We have observed domain formation and phase separation in the bilayer by increasing the composition of the gel phase DSPC. In force spectroscopy studies, we determine the mechanical strength of the bilayer for unique mixtures of DLPC: DSPC by measuring the breakthrough force. These results also show the effect of asymmetry in the lipid bilayer. Besides AFM studies, we have implemented a coarse-grained (CG) molecular dynamics (MD) simulation using the gromacs package at room temperature and 1 bar pressure. The results from the simulation study have been compared with AFM study. It was found that the simulation studies corroborated the findings from AFM such as an increase in the bilayer thickness, change in the phase state, asymmetric and symmetric domain formation in the lipid bilayer.

Keywords: Atomic force microscopy; Coarse-grained simulations; DLPC; DSPC; Lipid asymmetry; Lipid membranes; Martini force field.