Dynamical Transitions and Diffusion Mechanism in DODAB Bilayer

P S Dubey,V K Sharma,V Garcia Sakai,H Srinivasan,S Mitra,R Mukhopadhyay

doi:10.1038/s41598-018-19899-6

Abstract

Dioctadecyldimethylammonium bromide (DODAB), a potential candidate for applications in drug transport or DNA transfection, forms bilayer in aqueous media exhibiting a rich phase behavior. Here, we report the detailed dynamical features of DODAB bilayer in their different phases (coagel, gel and fluid) as studied by neutron scattering techniques. Elastic intensity scans show dynamical transitions at 327 K in the heating and at 311 K and 299 K during cooling cycle. These results are consistent with calorimetric studies, identified as coagel-fluid phase transition during heating, and fluid-gel and gel-coagel phase transitions during cooling. Quasielastic Neutron Scattering (QENS) data analysis showed presence of only localized internal motion in the coagel phase. However, in the gel and fluid phases, two distinct motions appear, namely lateral motion of the DODAB monomers and a faster localized internal motion of the monomers. The lateral motion of the DODAB molecule is described by a continuous diffusion model and is found to be about an order of magnitude slower in the gel phase than in the fluid phase. To gain molecular insights, molecular dynamics simulations of DODAB bilayer have also been carried out and the results are found to be in agreement with the experiment.

Highlights

Lipids and surfactants are amphiphilic molecules, comprised of a hydrophilic head and hydrophobic hydrocarbon chain(s) that self-assemble under favorable conditions in aqueous medium, to form micelles, vesicles, liposomes or even more complex structures
The chain melting temperature (Tm), hydrated DODAB bilayer can be in gel, subgel or coagel phases depending on the concentration and temperature
To investigate the phase behavior of DODAB bilayers, differential scanning calorimetry (DSC) measurements were carried out on 70 mM DODAB vesicles in heating and cooling cycles

Summary

Introduction

Lipids and surfactants are amphiphilic molecules, comprised of a hydrophilic head and hydrophobic hydrocarbon chain(s) that self-assemble under favorable conditions in aqueous medium, to form micelles, vesicles, liposomes or even more complex structures. A variety of techniques have been employed to unravel the complex phase diagram of DODAB bilayers[20,21,22,23,24,25,26], which can be traced out with four major phases – coagel, subgel, gel and fluid/liquid crystalline[24,25,26]. The chain melting temperature (Tm), hydrated DODAB bilayer can be in gel, subgel or coagel phases depending on the concentration and temperature. In all these phases, the lipid chains are ordered, nearly in all-trans configuration, but differ in their packing density, hydration and dynamics. Together with structural and morphological properties of DODAB, the molecular dynamics of lipids in different phases can provide us with a more comprehensive picture of the system’s phase behavior. Given the computing resources at hand nowadays, MD simulations can probe molecular motions in spatial and temporal domains similar to QENS, and give us the opportunity to directly validate a physical model used to describe the QENS data

Methods

Results

Conclusion