Membrane superstructure

Figure 6.5 Four snapshots of a grainy membrane superstructure as obtained from Monte Carlo simulation of a membrane disk (with zero height and slope along the circular circumference). Contour lines are shown. The sides of the square fiWes represent 66 nm. The bending elastic moduli and the temperature of the simulation are k = 0.5 x 10 J, k = 1 X lO- Jm icj =-0,9 X 10 Jm = 0.9 x IO-"" Jm temperature = 470 K.

Dam]anovich S, Gaspar R and Pier C 1997 Dynamic receptor superstructures at the plasma membrane Q. Rev. Biophys. 30 67-106... 

The combined features of structural adaptation in a specific hybrid nanospace and of a dynamic supramolecular selection process make the dynamic-site membranes, presented in the third part, of general interest for the development of a specific approach toward nanomembranes of increasing structural selectivity. From the conceptual point of view these membranes express a synergistic adaptative behavior the addition of the most suitable alkali ion drives a constitutional evolution of the membrane toward the selection and amplification of a specific transport crown-ether superstructure in the presence of the solute that promoted its generation in the first place. It embodies a constitutional selfreorganization (self-adaptation) of the membrane configuration producing an adaptative response in the presence of its solute. This is the first example of dynamic smart membranes where a solute induces the preparation of its own selective membrane. 

Helfrich, W. (1995). Handbook of Biological Physics. Tension-Induced Mutual Adhesion and a Conjectured Superstructure of Lipid Membranes. Chapter 14. Elsevier, Amsterdam. 

Lipofullerenes such as 35-37 self-assemble within lipid bilayers into rod-like structures of nanoscopic dimensions [61, 62]. These anisotropic superstructures may be important for future membrane technology. Significantly, lipofullerenes 35 and 37 have very low melting points, 22 and 67 °C (DSC, heating scan), respectively, with 35 being the first liquid fuUerene derivative at room temperature. 

Approaches to artificial ion channels have, for instance, made use of macrocyclic units [6.72,6.74] (see also below), of peptide [8.183-8.185] and cyclic peptide [8.186] components, of non-peptidic polymers [8.187] and of various amphiphilic molecules [6.11, 8.188, 8.189]. The properties of such molecules incorporated in bilayer membranes may be studied by techniques such as ion conductance [6.69], patch-clamp [8.190] or NMR [8.191, 8.192] measurements. However, the nature of the superstructure formed and the mechanism of ion passage (carrier, channel, pore, defect) are difficult to determine and often remain a matter of conjecture. 

The amphiphilic helical structure of most antibacterial peptides is a prerequisite for theu propensity to form chaimels across the double-layered biological membranes (12). In aqueous solution, positions a and d of the a-helical heptad repeat (a, b, c, d, e, f g) ( 3) require hydrophobic residues for the onset of the widespread antiparallel dimer (or multimer) superstructure (a-helix coiled coU) (14—16) (Fig. 2). The hydrophilic positions e and g, immediately on the back, reinforce the dimer stability... 

As a third example, consider the HDA process studied extensively by Douglas (1988). The superstructure for this process is shown in Fig. 17, which is based on a preliminary qualitative analysis of alternatives described in Douglas (1988). Given the basic options considered for the selection of reactors and the use of membrane separators, as well as a restricted set of alternatives for the... 

The inspection of the superstructure illustrated by the Fig. 7.17 emphasises again the role of the first-separation step. Sharp recovery will minimise the interactions between subsystems. On the other hand, the selection of the separation techniques and the design of units should bring flexibility to prevent bottlenecks. For example, adsorption or membranes should not be used in the first split, but could be certainly considered in the synthesis of the subsystem for gas separations. 

Nakashima, N., Asakuma, S., Kunitake, T. (1985). Optical microscopic study of helical superstructures of chiral bilayer membranes, J. Am. Chem. Soc., 1107 509. 

The transmission of chirality in membranes and other types of aggregates formed by surfactants was widely studied. The superstructures of bilayers formed by gemini dicationic surfactants based on quaternary ammonium centers can be modulated through the employ of enantiomeric tartrate salts. The twisted ribbons (Fig. 4) are formed when an enantiomeric excess of one of the tartrates is present, with the period (helical pitch) shorten-... 

The name ionophore suggests a "bearer" or carrier function. Cations are typically transported across a membrane by a carrier or a channel mechanism. Channel function is less well understood but is generally thought to involve the formation of a transmembrane pore through which ions pass. Effectively, a channel is a scaffold or superstructure that facilitates this process. In contrast, the ionophores discussed here typically function by a carrier mechanism. An ionophore is usually soluble in a hydro-phobic or membrane phase. The ionophore (host) captures the guest ion at the aqueous-hydrophobic interface. The complex diffuses across the hydrophobic membrane or barrier phase. At the opposite interface, guest release occurs, passing the ion into the second aqueous phase. The ionophore or host molecule then diffuses back to the opposite interface, where the process is repeated until equilibrium is reached. 

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