Two-Dimensional Polymerization of Lipid Bilayers: Effect of Lipid Lateral Diffusion on the Rate and Degree of Polymerization

Abstract

Hydrated amphiphiles can yield quite complex lyotropic liquid crystals such as the lamellar (bilayer) and nonlamellar phases. Lamellar structures can be solidlike or liquid crystalline. An important characteristic of these lamellar phases is the lateral diffusion of the lipids which increases by ca. 102 at the main phase transition, Tm. The rate (Rp) and degree (Xn) of polymerization were determined for polymerizable lipids in these two phases. A determination of the effect of temperature between 25 and 45 °C on the Rp of redox-initiated polymerization of mono-SorbPC bilayers showed a discontinuity near the Tm. The calculated activation energy, Ea, and frequency factor, A, for the polymerization at temperatures below Tm are 10 kcal/mol and 107, respectively. A similar calculation for the polymerization at temperatures above the Tm gave an Ea = 24 kcal/mol and A = 1016. The degree of polymerization, relative to poly(methyl methacrylate) standards, for bilayers of mono-SorbPC at temperatures above and below the Tm were 43 ± 3 and 51 ± 4, respectively. A comparable study of the polymerization of mono-AcrylPC bilayers found Xn of 198 ± 8 and 235 ± 9, respectively, showing that even in the slow diffusion regime at temperatures below the Tm relatively large polymers can be obtained. When the temperature increase spans the main phase transition, Tm, both the rate and the degree of polymerization are moderately increased. A most useful aspect of these results is the similarity of polymer size produced by redox polymerizations at temperatures both above and below the Tm. These studies in hydrated bilayers provide a clear indication that polymerizable lipids, such as the acryloyl, dienoyl, and sorbyl lipids, could also be usefully polymerized in condensed monolayers or multilayers at surfaces to create polymeric films composed of relatively long chains with high conversion from monomer to polymer.