Functional and Structural Aspects of Gramicidin-Lipid Interactions
Lipids in isolated form show polymorphism. Depending on the type of lipid and experimental conditions, different macromolecular aggregate structures are formed. for membrane lipids hydrated in excess water, under physiological conditions of ionic strength, pH and temperature, three major liquid-crystalline phases are observed. These are: the lamellar phase, the inverted (type II) hexagonal phase (Fig. 1), and the normal (type I) micellar phase. The shape-structure concept of lipid polymorphism as originally proposed by Israelachvili et al. (1976) and adapted by Cullis and De Kruijff (1979) explains remarkably well most of the current data on lipid polymorphism (Fig. 2). If the surface area in the head group region matches that of the hydrocarbon region (such as in phosphatidylcholines) the molecule has an overall cylindrical shape and prefers to organize in extended bilayers. When the head group area exceeds the hydrocarbon area, as for instance in case of detergent-like lipids such as lysophosphatidylcholines, the molecule on average is cone shaped and prefers an organization with a convex (type I) surface curvature such as found in micelles or in the related normal (type I) hexagonal phase occurring for these lipids at low water content. Phosphatidylethanolamines are key representatives of the opposite case of an excess hydrocarbon area. This results in the concave (type II) surface curvature as found in the HII phase. Head group hydration and inter- and intramolecular interactions are important determinants for the shape of the lipid molecules in the shape-structure concept. The strictiy regulated (Goldfine et al., 1987; Wieslander et al., 1981) presence of large amounts of type II lipids (lipids which in hydration prefer to organize in type II structures) and their abundance in virtually all biomembranes suggests important structural and functional roles for these types of lipids.
KeywordsChemical Shift Anisotropy Lipid Organization Bilayer Formation Gramicidin Channel Transbilayer Movement
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