Texture defects in lipid membrane domains

Important aspects of lateral organization in biomembranes can be addressed in model systems. Recently, it has become clear from polarized fluorescence imaging and X-ray scattering that gel domains in bilayers may contain orientational texture related to the tilted acyl chains. Such internal structure of domains can be imaged in polarized 2-photon fluorescence microscopy using the Laurdan probe that aligns with the lipids. By imaging intensity variations as a function of the polarization angle, we map the lateral variations within domains. A Fourier analysis of the signal enables the texture to be obtained in single pixels. Here we show that gel domains display a complex pattern containing line defects and a pair of |m| = 1/2 point disclinations in the domain center. We perform a detailed image analysis of the line and point defects using gradient calculations. In contrast to results from Langmuir monolayers, the membrane texture vary radially and is continuous in the center and segmented near the periphery. A possible explanation is that bilayer domains are grown thermally whereas Langmuir monolayer domains grow isothermally upon compression. The defect lines show significant variation in the angle jump across the lines. We simulate the structure of the central disclination pair and the fit to the experimental data yields an offset angle of 60.5 degrees +/- 6.5 degrees. This indicates that the central defect has a conserved structure with an intermediate character between bend and splay.