Line Tension at Fluid Membrane Domain Boundaries Measured by Micropipette Aspiration

Aiwei Tian, Corinne Johnson, Wendy Wang, and Tobias Baumgart

Phys. Rev. Lett. 98, 208102 – Published 16 May 2007

Abstract

Line tension is a determinant of fluid phase domain formation kinetics and morphology in lipid bilayer membranes, which are models for biological membrane heterogeneity. We describe the first direct measurement of this line tension by micropipette aspiration. Our data are analyzed with a model that does not rely on independently measured (and composition dependent) secondary parameters, such as bending stiffness or membrane viscosities. Line tension is strongly composition dependent and decreases towards a critical consolute point in a quasiternary room temperature phase diagram.

Received 23 December 2006

DOI:https://doi.org/10.1103/PhysRevLett.98.208102

Authors & Affiliations

Aiwei Tian, Corinne Johnson, Wendy Wang, and Tobias Baumgart^*

Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA

^*Author to whom correspondence should be addressed. Current address: Department of Chemistry, University of Pennsylvania, Philadelphia, PA, USA. Email address: baumgart@sas.upenn.edu

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Vol. 98, Iss. 20 — 18 May 2007

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Images

Figure 1
(a) Demonstration of a typical line tension measurement. One domain (typically the $L_{o}$ domain, as depicted in green) of a dumbbell shaped vesicle was aspirated with a micropipette. The $L_{o}$ domain was labeled with CTB-Alexa 488 and the $L_{d}$ domain was labeled by Texas Red-DPPE (TR-PE). $R_{p}$ and $R_{b}$ are the radii of the pipette and the phase boundary; $R_{1}$ and $R_{2}$ are curvature radii of partially aspirated and nonaspirated domain; $ψ_{1}$ and $ψ_{2}$ are the tangent angles immediately before and after the phase boundary; the tip of the aspirated vesicle projection is marked with a vertical arrow. (b)–(e) Time-lapse series of an experiment where one vesicle was aspirated using four different aspiration pressures: (b) 2.7 Pa, (c) 1.9 Pa, (d) 1.0 Pa, and (e) 0.9 Pa (all values $\pm 0.2 Pa$ ). For these aspiration pressures, the line tension obtained from vesicle geometry, pipette diameter, and aspiration pressure was roughly the same: (b) $3.1 \pm 0.2 pN$ , (c) $3.0 \pm 0.3 pN$ , (d) $3.0 \pm 0.6 pN$ . (e) Vesicle beyond critical releasing instability. Scale bar: $5 μ m$ .Reuse & Permissions
Figure 2
Partial room temperature phase diagram of the DOPC:Chol:ESM mixture. The circles demonstrate the upper boundary of the liquid coexistence region. Open circles indicate homogeneous vesicles; pie diagrams indicate phase separated vesicles, in which gray color quantifies the average area fraction of the disordered phase. From the area fractions and the phase boundary the approximate location of an upper critical consolute point can be identified. Five compositions were chosen including this area as well as four more compositions lying on a trajectory orthogonal to the expected tie line directions. The ratios of DOPC:Chol:ESM for each composition from the top to bottom are (1): $0.34 ∶ 0.4 ∶ 0.26$ ; (2): $0.34 ∶ 0.33 ∶ 0.33$ ; (3): $0.34 ∶ 0.28 ∶ 0.38$ ; (4): $0.34 ∶ 0.23 ∶ 0.43$ ; (5): $0.34 ∶ 0.16 ∶ 0.5$ , respectively, as indicated by crosses.Reuse & Permissions
Figure 3
(a)–(e) Histograms of line tension values for the five compositions (1)–(5). 20 vesicles were measured for each composition. (f) The average value of line tensions are plotted vs the mole ratio of cholesterol and ESM. From left to right are the compositions (1)–(5), and the corresponding average line tension values are 0.5, 0.9, 1.3, 2.3, 3.3 pN, respectively. (g) Histogram of line tensions of 20 vesicles with composition $DPhyPC : Chol : DPPC = 3 ∶ 2.75 ∶ 4.25$ . The average value is 0.7 pN.Reuse & Permissions

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