Imaging cerebroside-rich domains for phase and shape characterization in binary and ternary mixtures

The objective of this paper is to review phase behavior and shape characterization of cerebroside-rich domains in binary and ternary lipid bilayers, as obtained by microscopy techniques. These lipid mixtures provide a format to examine molecular (e.g. headgroup, tail unsaturation, and tail hydroxylation) and thermodynamic (e.g. temperature and mole percentages) factors that determine phase behavior, molecular partitioning, crystal/atomic scale structure, and microstructure/shape (particularly of phase-separated domains). Microscopy can provide excellent spatial (often with high resolution) characterization of cerebroside-rich domains (and their surroundings) to identify, describe or infer with high certainty these characteristics. In the introduction to this review we review briefly the molecular structure, phase behavior, and intermolecular interactions of cerebrosides, in comparison to ceramides and sphingomyelins and in some binary and biological systems. The bulk of the review is then devoted to microscopy investigations of cerebroside-rich domain microstructure and shape dynamics in binary and ternary (one component is cholesterol) systems. Quantitative and/or high-resolution microscopy techniques have been used to interrogate cerebroside-rich domains such as freeze-fracture electron microscopy, atomic force microscopy, imaging elipsometry, two-photon fluorescence microscopy, and LAURDAN generalized polarization in addition to the laboratory workhorse technique of epifluorescence microscopy that allows a quick often qualitative assessment of microstructure and dynamics. We particularly focus on the information these microscopy investigations have revealed with respect to phase behavior, cholesterol partitioning, domain shape, and determinants of domain shape.