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Biophysical Journal 84:998-1009 (2003)
© 2003 The Biophysical Society

Material Studies of Lipid Vesicles in the L{alpha} and L{alpha}-Gel Coexistence Regimes

Scott D. Shoemaker and T. Kyle Vanderlick

Department of Chemical Engineering, Princeton University, Princeton, New Jersey 08544 USA

Correspondence: Address reprint requests to T. Kyle Vanderlick, Tel.: 609-258-4891; Fax: 609-258-0211; E-mail: vandertk{at}princeton.edu.

In this work, we utilize micropipette aspiration and fluorescence imaging to examine the material properties of lipid vesicles made from mixtures of palmitoyloleoylphosphocholine (POPC) and dipalmitoylphosphatidylcholine (DPPC). At elevated temperatures/low DPPC fractions, these lipids are in a miscible liquid crystalline (L{alpha}) state, whereas at lower temperatures/higher DPPC fractions they phase-separate into L{alpha} and gel phases. We show that the elastic modulus, K, and critical tension, {tau}c, of L{alpha} vesicles are independent of DPPC fraction. However, as the sample temperature is increased from 15°C to 45°C, we measure decreases in both K and {tau}c of 20% and 50%, respectively. The elasticity change is likely driven by a change in interfacial tension. We describe the reduction in critical tension using a simple model of thermally activated membrane pores. Vesicles with two-phase coexistence exhibit material properties that differ from L{alpha} vesicles including critical tensions that are 20–40% lower. Fluorescence imaging of phase coexistent POPC/DPPC vesicles shows that the DPPC-rich domains exist in an extended network structure that exhibits characteristics of a solid. This gel network explains many of the unusual material properties of two-phase membranes.




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