Comparison of Line Tension Measurement Methods for Lipid Monolayers at Liquid–Liquid Coexistence

Abstract

Several methods of measuring the line tension between phase-separated liquid-ordered–liquid -disordered domains in phospholipid–cholesterol systems have been proposed. These experimental techniques are typically internally self-consistent, but the measured line tension values vary widely among these techniques. To date, no measurement of line tension has utilized multiple experimental techniques to look at the same monolayer system. Here we compare two nonperturbative methods, Fourier analysis of boundary fluctuations (BA) and one proposed by Israelachvili involving the analysis of domain size distributions (SD), to extract the line tension in a 70 mol % DMPC/30 mol % dihydrocholesterol (DChol) mixture as a function of surface pressure. We show that BA predicts the expected variation in line tension measurements consistent with the theoretical critical exponent whereas SD does not. From this comparison, we conclude that the size distribution of monolayer domains is metastable and primarily determined by the kinetics of domain nucleation and subsequent aging.

Influence of spontaneous curvature on the line tension of phase-coexisting domains in a lipid monolayer: A Landau-Ginzburg model

“…The line tension σ corresponding to the domain boundary of a lipid monolayer at the air-water interface has been determined based on a variety of methods, 3 including shape relaxation of domains, 2,13 boundary fluctuations, [14][15][16] and analysis of domain size distributions. 12,17 Using a domain shape relaxation method that employs non-homogeneous electrostatic fields, Bischof et al 2 have found a correlation between line tension σ and spontaneous curvature c 0 of the involved lipids. For binary dchol-phospholipid mixtures, they observed larger σ for phospholipids with a large negative c 0 (such as phosphatidylethanolamine) and smaller σ for lipids with small or vanishing c 0 .…”

“…5 In the heterogeneous environments of mixed monolayers and bilayers, line tension plays an important role for the shapes and sizes of the lipid domains, 1,6 the nucleation kinetics, 7,8 the capture zone of each domain, 9 and the formation and stability of lipid rafts. [10][11][12] The Journal of Chemical Physics…”

Spontaneous evolution of equilibrium morphology in phospholipid-cholesterol monolayers

“…λ is the energy per unit length of the monolayer domain boundaries and is the two-dimensional (2D) analog of surface tension in three dimensions ( 5 , 12 , 13 ). Differences in lipid composition, chain length, tilt, or local order at the domain boundaries determine the magnitude of λ ( 5 , 14 ). Experimental measurements of λ are typically done by analysis of spontaneous or imposed domain shape fluctuations; such analyses have been limited to immiscible liquid-liquid (L o -L d ) domains as fluctuations are too small to measure at solid-liquid (L C -L E ) domain boundaries.…”

“…However, over the past 40 years of visualizing monolayer domains ( 28 ), there have been few, if any, experimental observations of equilibrium circular domain size distributions in either L o -L d or L C -L E phase-separated monolayers that are quantitatively consistent with the predictions of Eq. 2 ( 5 , 14 , 25 , 27 , 29 , 30 ). Instead, circular domains are polydisperse and vary from experiment to experiment even for the same monolayer composition at a given surface pressure and temperature.…”

“…An equilibrium size distribution based on Eq. 2 should be reproducible at a fixed monolayer composition, surface pressure, and temperature and should also be independent of how those conditions are approached ( 14 ). Hence, it is unclear if there are fundamental limitations to the continuum theory or if there exist additional contributions to the energy yet to be understood.…”