Domain Shapes, Coarsening, and Random Patterns in Ternary Membranes

Abstract

A number of morphological and statistical aspects of domain formation in singly and doubly supported ternary membranes have been investigated. Such ternary membranes produce macroscopic phase separation in two fluid phases and are widely used as raft models. We find that membrane interactions with the support surface can have a critical influence on the domain shapes if measures are not taken to screen these interactions. Combined AFM and fluorescence microscopy demonstrate small (500 nm) irregular domains and incomplete formation of much larger (5 microm) round domains. These kinetically trapped structures are the result of interactions between the membrane and the support surface, and they can be effectively removed by employing doubly supported membranes under physiological salt concentrations. These decoupled supported membranes display macroscopic round domains that are easily perturbed by fluid shear flow. The system allows a quantitative characterization of domain coarsening upon being cooled into the coexistence region. We determine the domain growth exponent alpha = 0.31, which is in close agreement with the theoretical value of 1/3. Analysis of the spatial domain pattern in terms of Voronoi polygons demonstrates a close similarity to equilibrated cellular structures with a maximized configurational entropy.

Effects of passive phospholipid flip-flop and asymmetric external fields on bilayer phase equilibria

“…6A), by measuring a total thickness mismatch via ellipsometry at least double the height difference found in atomic force microscopy (AFM) [54]. In contrast, [55] used x-ray diffraction and found a similar total thickness mismatch in free-floating vesicles compared to the AFM height difference [54], suggesting a picture more like Fig. 6C.…”

Remodeling of membrane compartments: some consequences of membrane fluidity

“…Using fluorescence microscopy, phase separation of ternary mixtures into liquidordered and liquid-disordered domains has been studied for a variety of membrane systems including giant vesicles (Dietrich et al, 2001;Baumgart et al, 2003;Veatch and Keller, 2003;Bacia et al, 2005;Dimova et al, 2007;Semrau et al, 2008), solid-supported membranes (Garg et al, 2007;Jensen et al, 2007;Kiessling et al, 2009), hole-spanning (or black lipid) membranes (Collins and Keller, 2008), as well as pore-spanning membranes (Orth et al, 2012). Furthermore, phase separation into liquid-ordered and liquid-disordered domains has also been observed in giant plasma membrane vesicles that contain a wide assortment of lipids and proteins (Baumgart et al, 2007;Veatch et al, 2008).…”

Model Answers to Lipid Membrane Questions

“…Proximal lipid bilayers on solid supports are significantly influenced by interactions with the substrate, and their thermodynamics and lateral organization and order is affected by the support (Jensen et al 2007;Keller et al 2005). Hence, caution should be exerted when comparing data for supported bilayers with corresponding data for free-standing bilayers.…”

“…The typical system is a ternary mixture including cholesterol together with a lipid with a high melting point (e.g., a saturated lipid or sphingolipid) and one with a low melting point (e.g., a monounsaturated or polyunsaturated lipid). Many ternary phase diagrams have been worked out (Veatch and Keller 2003;Jensen et al 2007;Goñi et al 2008), and paradoxically the identification of the liquid-ordered phase in coexistence with a fluid lipid phase has been much more easy in ternary systems compared to binary lipid -cholesterol systems Mouritsen 2010). …”