Cholesterol (Chol) is in cell membranes at concentrations ranging from <5% (mole %) in mitochondrial membranes to upwards of 40% in the plasma membrane of eukaryotic cells. In addition, Chol is a primary regulator of membrane properties and affects the lipid membrane phase behavior, structure, elasticity and viscosity. A detailed phase diagram of 1,2-dimyristoyl-sn-glycerol-3-phosphocholine (DMPC)/Chol mixture based on electron spin resonance (ESR) experiments was proposed in a previous study by Almeida et al. [1]. At temperatures higher than the melting transition temperature (T_m) of DMPC, it undergoes phase transitions from a liquid disordered phase (L_d) to a liquid ordered phase (L_o) with a phase coexistence region (L_d+L_o) in between while increasing Chol mole concentration. The aim of the present study is to understand the structural characteristics, effective bending modulus (κ_eff) and membrane viscosity (η_m) of model DMPC/Chol mixtures in the L_d and L_d+L_ophases.

In this presentation, small-angle neutron scattering (SANS) measurements of the nanostructure and neutron spin echo (NSE) measurements of the nanoscale dynamics of DMPC membranes containing up to 20% Chol over a temperature range from 25 °C to 55 °C are compared and analyzed within the context of the aforementioned phase behavior. However, to achieve suitable contrast in scattering length density, it was necessary to use deuterated DMPC (d₅₄-DMPC), so complementary densitometry measurements were performed to verify the phase boundaries as well as determine the molecular volume needed for quantitative analysis of the SANS data. To aid in the interpretation, SANS data were fitted to a smeared 6-slab model representation of the bilayer to determine the membrane’s molecular architecture. With this structural model, analysis of the NSE spectra shows that the κ_eff as well as η_m in the L_d phase scale with the area per lipid as proposed in our previous studies of mixed lipid membranes [2].

Paul Butler

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