Several barrier epithelia such as renal collecting duct, urinary bladder, and gastric mucosa maintain high osmotic pH and solute gradients between body compartments and the blood by means of apical membranes of exceptionally low permeabilities. Although the mechanisms underlying these low permeabilities have been only poorly defined, low fluidity of the apical membrane has been postulated. The solubility diffusion model predicts that lower membrane fluidity will reduce permeability by reducing the ability of permeant molecules to diffuse through the lipid bilayer. However, litde data compare membrane fluidity with permeability properties, and it is unclear whether fluidity determines permeability to all, or only some substances. We therefore studied the permeabilities of a series of artificial large unilamellar vesicles (LUV) of eight different compositions, exhibiting a range of fluidities encountered in biological membranes. Cholesterol and sphingomyelin content and acyl chain saturation were varied to create a range of fluidities. LUV anisotropy was measured as steady state fluorescence polarization of the lipophilic probe DPH. LUV permeabilities were determined by monitoring concentration-dependent or pH-sensitive quenching of entrapped carboxyfluorescein on a stopped-flow fluorimeter. The relation between DPH anisotropy and permeability to water, urea, acetamide, and NH3 was well fit in each instance by single exponential functions (r > 0.96), with lower fluidity corresponding to lower permeability. By contrast, proton permeability correlated only weakly with fluidity. We conclude that membrane fluidity determines permeability to most nonionic substances and that transmembrane proton flux occurs in a manner distinct from flux of other substances.

We report measurements of the geometrical a In 1986, Helfrich (10) proposed a simple elastic model in which the optimal shape of a helical ribbon is controlled by a balance between bending of the ribbon and torsion of its edges, leading to a universal pitch angle of 450 for an elastically isotropic ribbon. Two years later Helfrich and Prost (11) generalized the bending free energy for anisotropic bilayers. To account for the lack of mirror reflection symmetry of bilayers of chiral molecules, they also added a term linear in the surface curvature to the elastic free energy (11).Despite these refinements, the analysis of the pitch angle was made only for an isotropic bilayer and the pitch angle remained 45°. In 1990, Ou-Yang and Liu (12) modeled the The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact. helical ribbon as a cholesteric liquid crystal layer and also obtained the pitch angle of 45°.Herein, we describe experimentally observed helical ribbon structures with either of two distinctive pitch angles: 540 or 110. We also present a form for the elastic free energy of anisotropic chiral bilayers that provides analytical expressions for the geometrical features and growth of the helical ribbon structures observed in our biles. MATERIALS AND METHODSAll model biles were composed of the common bile salt sodium taurocholate, lecithin, and cholesterol in a molar ratio of 97.5:0.8:1.7. Sodium taurocholate (Sigma) was recrystallized and found to be >98% pure by HPLC and TLC. Cholesterol (Nu Chek Prep, Elysian, MN) was >99% pure by TLC, GC, and HPLC. Grade I egg yolk lecithin (Lipid Products, Nutley, U.K.) was >99% pure by TLC. Synthetic sn-1-16:0-sn-2-18:1 lecithin (lecithin I) and sn-1-16:0-sn-2-16:0 lecithin (lecithin II) were obtained from Avanti Polar Lipids. Model biles A, B, C, and D contained lecithin I, a 1:1 mixture of lecithin I and lecithin II, egg yolk lecithin, and lecithin II, respectively. Native bile was a generous gift from M. Cahalane (Beth Israel Hospital, Boston) (2). Using TLC, no degradation of the components of bile was observed during our experiments.At a total lipid concentration of 70 mg/ml (0.15 M NaCl/3 mM NaN3), all four model biles formed micellar solutions with a mean hydrodynamic radius of 17 ± 5 A (2, 13).Meticulous precautions were taken to reduce the nucleating effect of dust. A 5-ml vial with a Teflon septum cap was acid-cleaned, rinsed, filled with Milli-Q water (Millipore), Abbreviation: ChM, cholesterol monohydrate crystal.§Present address:

The self-assembly of helical ribbons is examined in a variety of multicomponent enantiomerically pure systems that contain a bile salt or a nonionic detergent, a phosphatidylcholine or a fatty acid, and a steroid analog of cholesterol. In almost all systems, two different pitch types of helical ribbons are observed: high pitch, with a pitch angle of 54 ؎ 2°, and low pitch, with a pitch angle of 11 ؎ 2°. Although the majority of these helices are right-handed, a small proportion of left-handed helices is observed. Additionally, a third type of helical ribbon, with a pitch angle in the range 30-47°, is occasionally found. These experimental findings suggest that the helical ribbons are crystalline rather than liquid crystal in nature and also suggest that molecular chirality may not be the determining factor in helix formation. The large yields of helices produced will permit a systematic investigation of their individual kinetic evolution and their elastic moduli.Interest in molecular self-assembly of helical structures is driven by both technological and medical applications. Helices are often precursors in the growth of tubules (1-4), which can be used as a controlled release system for drug delivery in medicine and as templates for microelectronics and magnetic applications (5). The morphology of the tubules must be rationally optimized for each application. Therefore it is important to understand the role of the various constituent molecules in the formation of these structures.Helical ribbons have been observed in a variety of systems composed of chiral amphiphiles. Although the diameters and lengths of the helical structures varied from system to system, the pitch angle observed in all systems was either 45°(6-8) or Ϸ60°(9-12). In almost all cases, helical ribbons in enantiomerically pure systems were either right-or left-handed (6,8,13). Recently, however, Thomas et al. studied an enantiomerically pure phosphonate analogue of 1,2-bis(10,12-tricosadiynoyl)-sn-glycero-3-phosphocholine and related compounds, which self-assembled into a mixture of right-and left-handed helical ribbons (10, 11).A biologically important system in which helical ribbons form is model bile, consisting of a mixture of three types of chiral molecules in water: a bile salt, a phosphatidylcholine, and cholesterol (4, 14-21). Helical ribbons are metastable intermediates in the process of cholesterol crystallization in bile (2,19,20), which precedes cholesterol gallstone formation (4,(18)(19)(20)(21)(22)(23). In contrast to all other systems studied, two pitch types of helical ribbons are observed in bile: high pitch, with a pitch angle of 54 Ϯ 2°, and low pitch, with a pitch angle of 11 Ϯ 2°. To date, the production of these two helical pitch types has been thought to be a property unique to model biles. Indeed, previous work showed that all three components of model bile are required for helical ribbons to form. In the absence of the phosphatidylcholine, only needle-like crystals form (4, 21), whereas without the bile salt only...