Bilayered clusters

Fig. 6-31. Coordination structure of adsorbed water molecules on an interface of metal electrodes (a) hydrogen-bonded clusters, (b) bilayer clusters of adsorbed water molecules, (c) a superficial ( 3 x V ) KdO lattice of adsorbed water molecules on a (111) surface plane of face-centered cubic metals. (HsOli = first la] r of adsorbed water molecules. [From Thiel-Madey, 1987.]...

Actually, when a mixed solution of two ionic precursors M and M is irradiated or chemically reduced, both situations of alloyed or bilayered cluster formation may be encountered without clear prediction [102,173]. Moreover, an unambiguous characterization of the intimate structure of nanometric mixed clusters is quite difficult and requires appropriate methods, applied at different steps (or different doses) of the mixed cluster construction. 

The alloyed or layered character of a small bimetallic cluster structure is generally quite difficult to conclude experimentally [102,179]. Even if the surface plasmon transitions of both pure metals are specific (with one possibly in the UV), the unknown spectra of alloyed or bilayered clusters are both expected in the same intermediate region. The... 

Because the dynamics observed by means of pulse radiolysis indicated that the displacement process was not instantaneous, it was suggested that very short, intense irradiation, with a dose sufficient to achieve the complete reduction of all the ions, could efficiently prevent the segregation, due to electron transfer between the metals. Therefore, the method could enable the formation of alloyed clusters, of major interest for various applications, particularly catalysis. The positive influence of high dose rates, which quench the atoms in an alloyed cluster, has been demonstrated a bilayered cluster would be obtained from the same system by irradiation at a lower dose rate. " Moreover, as for monometallic clusters (Section 3.13.4.3), the high dose rate favors nucleation rather than growth, and the final sizes of the alloyed clusters are particularly small. " " ... 

Lipid bilayer (Section 26 4) Arrangement of two layers of phospholipids that constitutes cell membranes The polar termini are located at the inner and outer membrane-water interfaces and the lipophilic hydrocarbon tails cluster on the inside... 

Lipids in model systems are often found in asymmetric clusters (see Figure 9.8). Such behavior is referred to as a phase separation, which arises either spontaneously or as the result of some extraneous influence. Phase separations can be induced in model membranes by divalent cations, which interact with negatively charged moieties on the surface of the bilayer. For example, Ca induces phase separations in membranes formed from phosphatidylserine (PS)... 

The surface consists of terraces of a height of 330 30 pm. Within error limits, this is the value that would be expected for Ge(lll) bilayers. Furthermore, we were able to observe that the electrodeposition gave rise to a less ordered surface structure with nanoclusters, transforming over a timescale of about 1 hour into a layered structure. With GeBr4 a transformation of clusters into such a layered surface was only partly seen with GeGl4 this transformation could not be observed. 

The aliphatic tails form a hydrocarbon region with properties not too different from the hydrocarbon core of bilayers. The clusters have an interfacial zone... 

Figure 7.22b shows that hydrophilic molecules, those with log Kj < 1, are much more permeable in octanol than in olive oil. The same may be said in comparison to 2% DOPC and dodecane. Octanol appears to enhance the permeability of hydrophilic molecules, compared to that of DOPC, dodecane, and olive oil. This is dramatically evident in Fig. 7.7, and is confirmed in Figs. 7.8c and 7.22b. The mechanism is not precisely known, but it is reasonable to suspect a shuttle service may be provided by the water clusters in octanol-based PAMPA (perhaps like an inverted micelle equivalent of endocytosis). Thus, it appears that charged molecules can be substantially permeable in the octanol PAMPA. However, do charged molecules permeate phospholipid bilayers to any appreciable extent We will return to this question later, and will cite evidence at least for a partial answer. 

Fig. 20. Diagram showing a singlelength channel and a doublelength channel formed across a phospholipid bilayer by a circular cluster of nystatin or amphotericin B aggregates...

The fluidity of lipid bilayers permits dynamic interactions among membrane proteins. For example, the interactions of a neurotransmitter or hormone with its receptor can dissociate a transducer protein, which in turn will diffuse to interact with other effector proteins (Ch. 19). A given effector protein, such as adenylyl cyclase, may respond differently to different receptors because of mediation by different transducers. These dynamic interactions require rapid protein diffusion within the plane of the membrane bilayer. Receptor occupation can initiate extensive redistribution of membrane proteins, as exemplified by the clustering of membrane antigens consequent to binding bivalent antibodies [8]. In contrast to these examples of lateral mobility, the surface distribution of integral membrane proteins can be fixed by interactions with other proteins. Membranes may also be partitioned into local spatial domains consisting of networks... 

The transmembrane domain in the RPTK is a hydrophobic segment of 22-26 amino acids inserted in the cell membrane. It is flanked by a proline-rich region in the N-terminus and a cluster of basic amino acids in the C-ter-minus. This combination of structures secures the transmembrane domain within the lipid bilayer. There is a low degree of homology in the transmembrane domain, even between two closely related RPTKs, suggesting that the primary sequence contains little information for signal transduction. 

Bilayer-forming amphiphiles are cast with hydrophobic polymer(e.g. poly(vinyl chloride)) from organic solvents. A large cluster of the bilayer membrane are formed as phase separated micro domains in the polymer matrix [36]. 

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