Texture of Membrane Gel Domains

Abstract

In this work1 we investigate the texture of gel (g) domains in supported binary lipid membranes. Lateral organization of lipid bilayer membranes is a topic of fundamental and biological importance. Whereas questions related to the size and composition of fluid domains are well studied, the possibility of texture in condensed solid/gel domains has received limited attention. Gel domains are expected to be prominent in skin membranes and in ceramide domains during apoptosis. When using polarized light for two-photon excitation of the lipid probe Laurdan, the emission intensity is highly sensitive to the angle between the polarization and the tilt orientation of lipid acyl chains. By imaging the intensity variations as a function of the polarization angle, we map the lateral variations of the lipid tilt within domains. Results reveal that gel domains are composed of distinct subdomains with different lipid tilt directions. Vortex structures centered at the domain core can be observed. Texture patterns of the same type have historically been associated with the presence of hexatic order in monolayers. The hexatic phase is an intermediate phase between the crystal and fluid states, having short range positional order and long range orientation order. The present results provide some support for the notion that hexatic order may persist in bilayers. Using the generalized polarization (GP) function of Laurdan, we demonstrate that although gel domains have heterogeneous texture, the membrane phase state is uniform within domains.[1] U. Bernchou et. al. ‘Texture of Lipid Bilayer Domains’. J. Am. Chem. Soc., 2009. DOI: 10.1021/ja903375m