Line Tension in Lipid Monolayers with Liquid-Liquid Phase Coexistence

Abstract

When two phases coexist in a membrane, there is an energy penalty for the generation on the 1D-border between each phase, which is measured trough a parameter called “line tension”. In analogy to surface tension, line tension (λ) is a consequence of the different energetic of molecules in the border of a 2D domain. Its influence becomes noticeable when a domain is driven out of the more stable shape, since λ induces the restoration of its original shape.The line tension has been determined in monolayers and bilayers using several techniques. In this work we present a novel manner of determining λ in monolayers with liquid/liquid coexistence that overcomes some of the drawbacks of the previously described techniques. Using this technique, we determined the line tension of binary mixtures of different lipids and a molecule similar to cholesterol but less oxidable. We analyze the effect of the hydrocarbon chain length and of the polar head-group. The results indicate that an increment from 12 to 16 carbons in the hydrocarbon chain does not influence the value of λ. On the contrary, the nature of the polar head-group has an important influence on λ; PE presents high λ values while PG presents low values. PS and PC show intermediate values. The results are discussed in relation to the chemical structure of the lipids, their interaction with cholesterol and the mechanical properties of the lipid membranes that they form.