Publication Date

April 2016

Advisor(s)

Francis Starr

Major

Physics

Language

English (United States)

Abstract

Lipid monolayers and bilayers are soft condensed matter systems with similar structural properties and phase behaviors that play critical roles in many biological processes. Monolayers are more easily studied experimentally and therefore often are used to infer the properties of bilayers; we examine the degree to which this inference holds. We explicitly compare the organizational structure and dynamical behavior of these systems though molecular dynamics simulations of single-component dipalmitoylphosphatidylcholine (DPPC) lipid monolayers and bilayers. DPPC is the most common lipid component in monolayers and is also prevalent in bilayers. We implement the coarse-grained MARTINI model in simulations performed at zero surface tension and various temperatures near the liquid-liquid phase transition of DPPC; both the monolayer and bilayer exhibit a high-density, low-mobility phase and a low-density, high-mobility phase in this region. Dynamical and structural properties of the low-density phase of the monolayer and bilayer are nearly identical; however, the high-density phase shows significant dynamical differences between these systems. Salient distinctions between the high-density monolayers and bilayers include the diffusion coefficient, which is at least 1-2 orders of magnitude larger in monolayers than in bilayers, and the non-Gaussian parameter, which reveals that the degree of non-Gaussian dynamics in bilayers is approximately 2 orders of magnitude stronger than that of monolayers. The dynamical properties of high-density bilayers, unlike those of monolayers, are consistent with dynamical heterogeneity, a universal phenomenon in many soft-condensed matter systems where particle interactions are strong relative to the total thermal energy. We investigate the structural and organizational origins of these dynamical differences and show that inter-leaflet interactions in the bilayer play an important role in the emergence of heterogeneous dynamics. Consequently, we find limitations in using monolayers as a model system to understand the dynamics of bilayers of DPPC, a finding that may extend to many other compositions.

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