Lipid monolayers shape transitions

Theoretical models of the film viscosity lead to values about 10 times smaller than those often observed [113, 114]. It may be that the experimental phenomenology is not that supposed in derivations such as those of Eqs. rV-20 and IV-22. Alternatively, it may be that virtually all of the measured surface viscosity is developed in the substrate through its interactions with the film (note Fig. IV-3). Recent hydrodynamic calculations of shape transitions in lipid domains by Stone and McConnell indicate that the transition rate depends only on the subphase viscosity [115]. Brownian motion of lipid monolayer domains also follow a fluid mechanical model wherein the mobility is independent of film viscosity but depends on the viscosity of the subphase [116]. This contrasts with the supposition that there is little coupling between the monolayer and the subphase [117] complete explanation of the film viscosity remains unresolved. 

Fig. IV-19. Fluorescence micrographs showing the shape transitions in monolayers of dimyristoylphosphatidylcholine (DMPC) (84%) and dihydrocholesterol (15%) and a lipid containing the dye, Texas Red. (From Ref. 228.)...

An essential component of cell membranes are the lipids, lecithins, or phosphatidylcholines (PC). The typical ir-a behavior shown in Fig. XV-6 is similar to that for the simple fatty-acid monolayers (see Fig. IV-16) and has been modeled theoretically [36]. Branched hydrocarbons tails tend to expand the mono-layer [38], but generally the phase behavior is described by a fluid-gel transition at the plateau [39] and a semicrystalline phase at low a. As illustrated in Fig. XV-7, the areas of the dense phase may initially be highly branched, but they anneal to a circular shape on recompression [40]. The theoretical evaluation of these shape transitions is discussed in Section IV-4F. 

Figure 10.1 Schematic of two distinct pathways from the lamellar phase to the columnar inverted hexagonal i i phase of cationic liposome/DNA (CL/DNA) complexes. Along Pathway 1 the natural curvature C0=l/Ro of the cationic lipid monolayer is driven negative by the addition of the helper-lipid DOPE. This is shown schematically (middle top) where the cationic li DOT(4P is cylindrically shaped while DOPE is cone-like leading to the negative curvature. Along pathway II the to j transition is induced by the addition of a new class of helper-lipids consisting of mixtures...

De Koker R, McConnell HM. Circle to dogbone shapes and shape transitions of lipid monolayer domains. J. Phys. Chem. 1993 97 13419-13424. 

AFM was used to investigate whether a series of related molecules could be analyzed when present as SAM. The system chosen was the cholesterol molecule. The molecule cholesterol is a very important biological lipid. A cholesterol molecule with one hydroxyl group is known to oxidize into a variety of structures. These oxidized cholesterol products play an important role in many biological diseases, such as blood clots. The AFM data of the collapsed film of cholesterol (when spread on the surface of water) shows that two-dimensional crystallization takes place with very characteristic butterfly shapes. This shows, for the first time in the literature, that not all lipid monolayers collapse to give a transition from monolayer to trilayer. This shows that the collapsed state may be a two-dimensional crystal phase. 

S. Hartel, Mi. Eanani, and B. Maggio, Shape transitions and lattice structuring of ceramide-enriched domains generated by sphingomyelinase in lipid monolayers, Biophys. J., 88, 287-304 (2005). 

Several techniques have been developed over the last 15 years to visually probe the morphology of surfactant monolayers at the air-water interface. In fluorescence microscopy, a small amount of fluorescently labeled surfactant molecules is added to a monolayer due to steric effects these tagged molecules tend to partition into less-ordered phases, which results in a visual contrast between coexisting phases [25-29]. Fluorescence microscopy has been used to determine domain sizes and shapes during phase transitions [25,28,30]. Polarized fluorescence microscopy (PFM) provides additional information on the lipid hydrocarbon chain ordering within condensed monolayers, especially in areas where the lipid hydrocarbon chains are tilted with respect to the surface normal [31,32]. The interaction of the electric field vector of the polarized light with the absorption dipole moment of the... 

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