The Molecular Structure of the Liquid Ordered Phase

Edward Lyman (Department of Physics and Astronomy, University of Delaware)

A cholesterol rich fluid phase phase of lipid bilayers called "liquid ordered" has been known for several decades. However, the detailed, molecular scale structure and thermodynamic driving forces responsible for the formation of this phase are still debated. In this talk I will present evidence from molecular dynamics simulations for substructure within the liquid-ordered phase of lipid bilayers. This substructure, identified in a 10 µsec all-atom trajectory of liquid-ordered/liquid-disordered coexistence (Lo/Ld), is composed of saturated hydrocarbon chains packed with local hexagonal order, and separated by interstitial regions enriched in cholesterol and unsaturated chains. Lipid hydrocarbon chain order parameters calculated from the Lo phase are in excellent agreement with 2H NMR measurements; the local hexagonal packing is also consistent with 1H-MAS NMR spectra of the Lo phase, NMR diffusion experiments, and small angle neutron scattering. The balance of cholesterol-rich to local hexagonal order is proposed to control the partitioning of membrane components into the Lo regions, which have been frequently associated with formation of so-called rafts.

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