Rate Constant of Tension-Induced Pore Formation in Lipid Membranes

Abstract

We investigated the effects of tension induced by micropipet aspiration on giant unilamellar vesicles (GUVs) composed of dioleoylphosphatidylglycerol (DOPG) and dioleoylphosphatidylcholine (DOPC). We analyzed the time course of the fraction of intact GUVs among all of the GUVs under constant tension σ and obtained the rate constants of pore formation kp(σ). To determine kp, we developed an approach using the mean first passage time. The fitting of the theoretical curves of kp versus σ to the experimental data determined the line tension of a prepore, Γ. The value of Γ of a DOPG/DOPC bilayer was smaller than that of a DOPC bilayer.

Communication: Activation energy of tension-induced pore formation in lipid membranes

“…Giant unilamellar vesicles (GUVs) composed of lipid membranes have been used to elucidate the kinetics and mechanism of tension-induced pore formation. [1][2][3][4][5][6] The classical theory of tension-induced pore formation 7,8 was based on the theory of how soap films rupture. 9 According to the classical theory, once a pre-pore with radius r is formed in the membrane, the total free energy of the system changes by an additional free energy component (called the free energy of a pre-pore U(r, σ)) consisting of two terms: one term (−πr 2 σ) associated with lateral tension (σ), favoring expansion of the pre-pore, and the other term (2πrΓ) associated with the line tension (Γ) of the pre-pore edge, favoring pre-pore closure.…”

“…Thermal fluctuation in the lateral density of a lipid membrane induces a pre-pore. 5 If the radius of a pre-pore a) Author to whom correspondence should be addressed. is less than the critical radius, r a (= Γ/σ), it closes quickly.…”

“…This classical theory has been used to explain several phenomena, such as the rate of closure and opening of pores 1,2 and the tension dependence of the rate constant of tension-induced pore formation. 5,6 Evans and colleagues found that at a fast loading rate (i.e., a rapid rate of increase in tension), the classical theory could not explain their results regarding the tension-induced rupture of a GUV. Based on their results, they proposed that the rupture of a GUV (i.e., pore formation in a GUV) can be modeled as a sequence of two thermally activated transitions (i.e., they proposed the existence of a metastable intermediate state).…”

“…DOPC-GUVs were prepared by natural swelling in water (MilliQ) containing 0.1 M sucrose. 5 DOPC-GUV suspensions (300 µL) (0.1 M sucrose in water as the internal solution; 0.1 M glucose in water as the external solution) were transferred into a hand-made microchamber. 10 GUVs were observed using an inverted fluorescence phasecontrast and differential interference contrast (DIC) microscope (IX-71, Olympus, Tokyo, Japan) at various temperatures controlled via a stage thermocontrol system (ThermoPlate CBU, TP-CH110R-C, Tokai Hit, Shizuoka, Japan).…”

“…Phasecontrast and DIC images of GUVs were recorded using a charge-coupled device (CCD) camera (CS230B, Olympus). The rate constant of constant tension-induced pore formation in a GUV was determined using the method of Levadny et al 5 To apply tension, σ, to the lipid membranes of single GUVs, we used the micropipette aspiration method. 11 σ can be described as a function of the difference in pressure between the outside and the inside of a micropipette, ∆P, as follows: 11…”