Phospholipid monolayer vesicles

E. Kalb, S. Frey, and L. K. Tamm, Formation of supported planar bilayers by fusion of vesicles to supported phospholipid monolayers, Biochim. Biophys. Acta 1103, 307-316 (1992). 

A question of interest here is the origin of the DMPC molecules building up the bilayer, considering the low monomer concentration in the DMPC suspension and the small volume of the drop in the cell. However, as indicated in Section 3.4.3, NBF can be formed only at close packing at the interface (r ). A possible mechanism is the vesicle degradation at the surfaces, i.e. at the solution/air interface. An evidence of this mechanism are the kinetic studies of insoluble phospholipid monolayer of Ivanova et al. [291]. Nevertheless, NBF formation from vesicle suspensions needs further research. 

A very suitable method for measurement of the lateral diffusion of molecules adsorbed at the foam film surfaces is Fluorescence Recovery after Photobleaching (FRAP) ([491-496], see also Chapter 2). Measurements of the lateral diffusion in phospholipid microscopic foam films, including black foam films, are of particular interest as they provide an alternative model system for the study of molecular mobility in biological membranes in addition to phospholipid monolayers at the air/water interface, BLMs, single unilamellar vesicles, and multilamellar vesicles. 

Surfactants having two alkyl chains can pack in a similar manner to the phospholipids (see Box 6.4 for examples). Vesicle formation by the dialkyldimethylammonium cationic surfactants has been studied extensively. As with liposomes, sonication of the turbid solution formed when the surfactant is dispersed in water leads ultimately to the formation of optically transparent solutions which may contain single-compartment vesicles. For example, sonication of dioctadecyldimethyl-ammonium chloride for 30 s gives a turbid solution containing bilayer vesicles of 250-450 nm diameter, while sonication for 15 min produces a clear solution containing monolayer vesicles of diameter 100-150 nm. The main use of such systems has been as membrane models rather than as drug delivery vehicles because of the toxicity of ionic surfactants. 

IR spectroscopy of the X-receptor protein signal peptide in phospholipid monolayers shows that the peptide affects the packing of the lipid hydrocarbon tails (M. S. Briggs, R. A. Dluhy, D. G. Cornell, and L. M. Gierasch, unpublished results). In samples formed at the same surface pressure, the lipid tails are oriented differendy in the presence and absence of signal peptide. A phospholipase assay for structural defects in phospholipid bilayers (Jain et al., 1984) indicates that the X-receptor protein signal peptide interacts with vesicles to induce such defects. The peptides perturb the lipid structure at lower mole fractions than do various lysophospholipids. These data provide yet another indication that signal peptides interact with and perturb lipid complexes. 

With the advent of polymersomes and vesicles of other nonconventionar (i.e., not phospholipid-like) amphiphiles, numerous examples of monolayer vesicles in water have been reported. Typically, monolayer vesicles are prepared from small molecules with a hydrophobic core and two hydrophilic head groups (bolaform amphiphiles, see below) or from triblock-copolymers with two hydrophilic terminal blocks. The molecule must have a cylindrical or rectangular shape, so that it can arrange into a monolayer. 

Small amphiphiles need not necessarily have a phospholipid-like structure with two tails and one head group. For example, bolaform (or bipolar) amphiphiles are amphiphilic molecules that contain two head groups separated by an extended hydrophobic chain. Bolaform amphiphiles form monolayer vesicles in which each amphiphile extends across the monolayer membrane, exposing both head groups to water and sheltering the hydrophobic chain from water. However, also these types of... 

Hybrid phospholipid bilayers consisting of an outer phospholipid monolayer on a thiol SAMs (formed by liposome or vesicle fusion or by the Langmuir-Blodgett technique) have been prepared by this approach. These bilayers exhibit extremely low capacitance values, so that they can be used in sensor devices to test ions and lipophilic molecules. Thiolipids can also be used as an alternative to directly form the hybrid bilayer on the metal surface (Figure 3). ... 

However, hybrid bilayers are not suitable for protein incorporation because water is needed at the inner part of the bilayer to avoid protein denaturalization. In order to avoid this problem, Au surfaces were functionalized with a short hydroxylated dithiol (dithiothreitol, DTT) which adopts a lying down configuration with the OH groups exposed to the environment. Vesicle fusion on these DTT surfaces allows phospholipid bilayer formation with a water layer between the DTT SAM and the inner phospholipid monolayer. The phospholipid bilayer exhibits good fluidity as has been shown by in situ AFM (atomic force microscopy) imaging. These bilayers have been formed on both planar and nanostructured [SERS (surface enhanced Raman spectroscopy) active] gold surfaces. ... 

A back-to-back arrangement of phospholipid monolayers, often forming a closed vesicle or membrane. 

Using the phospholipid DMPC, it was formed a planar supported adlayer structures by vesicle fusion. Lipid bilayer formation proceeds on a hydro-xythiol-terminated Au surface. Phospholipid monolayers form on hydro-xythiol-terminated gold surfaces that have been treated with aqueous POCI3 and ZrOCl2 prior to lipid deposition, providing an interface that interacts... 

The interfacial stability of membrane lipids is a delicate balance of the amphipatic properties. To measure how the oxydation of cholesterol affects its membrane stability radiolabelled oxysterols were incorporated in phospholipid monolayers and their rate of release from the interface was determined (19). In the absence of vesicles there is no release measurable. The addition of serum high density lipoprotein or small unilamellar vesicles to the subphase brings about a hardly measurable release of cholesterol ( 0.5% h ). Much higher rates are found for 7-ketocholesterol, 7B-hydroxycholesterol, 7a-hydroxycholesterol, and 25-hydroxycholesterol in this order. This order is similar to their interaction with DOPC, that is, the most cholesterol-like oxysterol 7-ketocholesterol shows the lowest transfer rate and the oxysterol with the greater distance between the hydroxyl groups, 25-hydroxycholesterol, the highest transfer rate. The transfer measured is consistent with the involvement of a water soluble intermediate. 

Many complex systems have been spread on liquid interfaces for a variety of reasons. We begin this chapter with a discussion of the behavior of synthetic polymers at the liquid-air interface. Most of these systems are linear macromolecules however, rigid-rod polymers and more complex structures are of interest for potential optoelectronic applications. Biological macromolecules are spread at the liquid-vapor interface to fabricate sensors and other biomedical devices. In addition, the study of proteins at the air-water interface yields important information on enzymatic recognition, and membrane protein behavior. We touch on other biological systems, namely, phospholipids and cholesterol monolayers. These systems are so widely and routinely studied these days that they were also mentioned in some detail in Chapter IV. The closely related matter of bilayers and vesicles is also briefly addressed. 

Sonication of MLV dispersions above the Tc of the lipids results in the formation of SUV (Saunders, et al., 1962). Sonication can be performed with a bath sonicator (Papahadjopoulos and Watkins, 1967) or a probe sonicator (Huang, 1969). During sonication the MLV structure is broken down and small unilamellar vesicles with a high radius of curvature are formed. In case of SUV with diameters of about 20 nm (maximum radius of curvature), the outer monolayer can contain over 50% of the phospholipids and in the case of lipid... 

Polymerization in Bilayers. Upon irradiation with UV light the monomer vesicles are transferred to polymer vesicles (Figure 12.). In the case of the diyne monomers (2,5-9,12,13,14) the polyreaction can again be followed by the color change via blue to red except phospholipids (5,6), which turn red without going through the blue intermediate as observed in monolayers. The VIS spectra of these polymer vesicle dispersions are qualitatively identical to those of the polymer monolayers (Figure 13.). 

Figure 1. Various physical states of phospholipids in aqueous solution. Note the following features (a) phospholipids residing at the air/water interface are arranged such that their polar head groups maximize contact with the aqueous environment, whereas apolar side chains extend outward toward the air (b) solitary phospholipid molecules remain in equilibrium with various monolayer and bilayer structures (c) bilayer vesicles and micelles remain in equilibrium with solitary phospholipid molecules, provided that the total lipid content exceeds the critical micelle concentration.

Langmuir-Blodgett films, 36 195-197 Langmuir monolayers, 36 192-195 phospholipid vesicles, 36 182-187 encapsulated cations, 36 183-184... 

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