Bending elasticities of model membranes: influences of temperature and sterol content

Abstract

Giant liposomes obtained by electroformation and observed by phase-contrast video microscopy show spontaneous deformations originating from Brownian motion that are characterized, in the case of quasispherical vesicles, by two parameters only, the membrane tension sigma and the bending elasticity k(c). For liposomes containing dimyristoyl phosphatidylcholine (DMPC) or a 10 mol% cholesterol/DMPC mixture, the mechanical property of the membrane, k(c), is shown to be temperature dependent on approaching the main (thermotropic) phase transition temperature T(m). In the case of DMPC/cholesterol bilayers, we also obtained evidence for a relation between the bending elasticity and the corresponding temperature/cholesterol molecular ratio phase diagram. Comparison of DMPC/cholesterol with DMPC/cholesterol sulfate bilayers at 30 degrees C containing 30% sterol ratio shows that k(c) is independent of the surface charge density of the bilayer. Finally, bending elasticities of red blood cell (RBC) total lipid extracts lead to a very low k(c) at 37 degrees C if we refer to DMPC/cholesterol bilayers. At 25 degrees C, the very low bending elasticity of a cholesterol-free RBC lipid extract seems to be related to a phase coexistence, as it can be observed by solid-state (31)P-NMR. At the same temperature, the cholesterol-containing RBC lipid extract membrane shows an increase in the bending constant comparable to the one observed for a high cholesterol ratio in DMPC membranes.

A Quantitative Coarse-Grain Model for Lipid Bilayers

“…Using K A and d HH from our previous results, we obtain κ b ) 21.9 ( 1.6k B T. Experiments have measured 16.7k B T from X-ray data, 85 13.5k B T from pipet aspiration, 7 31.4k B T from thermally excited shape fluctuation, 86 and 32.1k B T from all-optical measurement. 87 Our estimate lies inside the range of the experimental values.…”

Adsorbed liposome deformation studied with quartz crystal microbalance

“…At temperatures above the transition temperature, the bilayer is in the fluid phase, and κ is of the order of 1 × 10 −19 J. 26,28,29 As we have seen above, bilayer bending opposes deformation. We therefore expect that liposomes made of a lipid in the fluid phase (above the transition temperature) would deform to a greater extent than those made of a lipid in the gel phase (Figure 1).…”

“…The temperature dependence of the bending modulus in the P β phase has been reported by several investigators for DPPC and for DMPC. 26,29,35,36 The DPPC data illustrate this trend best; therefore, we compare our results with these data. The absolute values of the bending moduli in the gel phase vary, however: the values reported by Dimova et al 36 for DMPC reach ∼25-35 × 10 −19 J at ∼19…”

“…Second, in Figure 4(a), the equivalent film thickness h is plotted as a function of temperature together with the bilayer bending modulus for DPPC measured by Lee et al 26 and for DMPC measured by Yi et al 35 There is a clear correspondence between the behavior of the equivalent thickness and the bending modulus as a function of temperature: below the main transition temperature, when the lipid is in the P β phase, the bending modulus increases with decreasing temperature. 26,29,35,36 There is a corresponding increase in the equivalent thickness extracted from the QCM data. Identifying the equivalent thickness with the height of the adsorbed liposomes, we explain the observed correspondence in terms of the effect of the bending modulus on the adsorbed liposome deformation: higher bending modulus (stiffer liposomes) leads to smaller deformation and larger equivalent thickness.…”

“…Indeed, the bending stiffness should display viscoelastic dispersion like any other mechanical property in soft matter experiments, and it is not reasonable to expect the values determined from low-frequency experiments on giant liposomes 26,28,29,36 to be applicable at the time scales probed by the MHz frequencies of QCM. κ is expected to be much higher at MHz frequencies than at low frequencies, and this can be corroborated by comparing the outcome of FEM calculations with our experiments.…”