Phase Separation and Domain Formation

Many ESR studies (see Section 3.3.6) using different systems have shown that phase separation in lipid layers may lead to a domain-like lateral structure. The area of domain formation can be extended over several hundred angstroms. In this context, charge-induced domain formation in biomembranes is of special interest for the medicinal chemist. In particular, the addition of Ca2+ to negatively charged lipids leads to domain formation [106]. Each lipid component is expected to have a characteristic spontaneous curvature. The Ca2+-induced domains lead to protrusions in the 

The last point has been demonstrated in several contexts, most notably in protein kinase C [109, 110] and phospholipases [111-113], in coupling of G-proteins to their receptors [114], and in the activation of pancreatic lipase by colipase [115]. 

It has also been observed that sorting and transport of lipids and proteins can be mediated by the formation of small lipid-based domains, so-called rafts. The tenden- 

The physical and functional properties of natural and artificial membranes which have been discussed in brief in this section have been the subject of extensive investigations. Several books and reviews have been published on these topics [8, 126], The described features of membranes contribute to the physicochemical properties that make biological membranes highly structured fluids, in both space and time. They confer on the membranes particular structural, dynamic, and functional properties. 

For the medicinal chemist it is of interest to note that such phenomena, i.e. phase separation and domain formation, can also be induced in artificial membranes by cationic amphiphilic drugs. An increase in the microheterogeneity of biological membranes and in consequence a decisive change in membrane function in a defined area must, therefore, be considered. It is mediated through indirect physicochemical interaction with amphiphilic drags. 

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PHMB, alexidine Phase separation and domain formation of membrane lipids... 

The observed phase separation in the surroundings of integrated membrane proteins is of importance for their toctioning. According to Carruthers [44], phase separation and domain formation create an area of rigid phospholipids around the proteins in an otherwise fluid surrounding. 

Due to the incompatibility (different polarity and chemical nature) between HS and SS, phase separation occurs in most PUs. The degree of phase separation and domain formation depends on the HS and SS nature and sizes, on the type of the diisocyanate and polyol employed to produce prepolymers, on the type of the chain extender, and on the molecular weight of the SS. It is also influenced by the hydrogen bond formation between the urethane linkages, by the manufacturing process, and reaction conditions [3-6]. 

Phase separation and the formation of domains in blends and block copolymers results from the thermodynamic incompatibility of the constituents. The mixing of two phases is governed by the thermodynamic relation... 

The drop in the mutual diffusion coefficients of all the components would explain theoretically why even at such high temperatures the monomer and solvent would seem to be frozen in the system, as seen from NMR studies. This would have a profound effect on reactivity of the system, because it would also mean a reduction in conversion rate for the FRRPP system, which was indeed observed. One thing to note is that the explanation here applies to the point when the FRRPP system has already phase separated. Nanoscale domain formation at the early stages of FRRPP process requires a dilute regime explanation. 

Both blends. PLLA/PMA and PLLA/PMMA, presented some degree of miscibility in the melt but after cooling if an adequate thermal treatment is applied, the crystallization tendency of PLLA acted as the drivir.g force for phase separation and the formation of segregated crystalline micro-domains. 

The formation of a dual-phase morphology in PEI-epoxy IPNs is explained in the following way [90,171]. Two cases were discussed. First, when phase separation is induced by the curing reaction, PEI-modified epoxy shows bimodal UCST behavior (Fig. 30). At composition (p (volume fraction of component 1) phase separation starts at conversion x when the curing temperature is Ti. Because of the spinodal mechanism of decomposition and due to the lower viscosity of the medium, the system will have macroscale phase separation and domains with volume fraction cp[ and. As the reaction proceeds and the conversion X2 is reached, an abrupt change in the equilibrium composition of the PEI-rich phase occurs from cp l to (p " in a very short time. This abrupt change is similar to the effect of the two-... 

Phase behavior of lipid mixtures is a much more difficult problem, due to nonideal mixing of lipid components. Ideal mixing implies like and unlike lipids have the same intermolecular interactions, while nonideal mixing results from differential interactions between lipid types. If the difference is too great, the two components will phase separate. While phase separation and lateral domain formation have been observed in many experiments, we lack a molecular-level physical description of the interactions between specific lipids that cause the macroscopic behavior. The chemical potential of a lipid determines phase separation, as phase coexistence implies the chemical potential of each type of lipid is equal in all phases of the system [3,4],... 

This review emphasizes an intriguing and potentially useful aspect of the polymerization of lipid assemblies, i.e. polymerization and domain formation within an ensemble of molecules that is usually composed of more than one amphiphile. General aspects of domain formation in binary lipid mixtures and the polymerization of lipid bilayers are discussed in Sects. 1.1 and 1.2, respectively. More detailed reviews of these topics are available as noted. The mutual interactions of lipid domains and lipid polymerization are described in the subsequent sections. Given the proper circumstances the polymerization of lipid monolayers or bilayers can lock in the phase separation of lipids, i.e. pre-existing lipid domains within the ensemble as described in Sect. 2. Section 3 reviews the evidence for the polymerization-initiated phase separation of polymeric domains from the unpolymerized lipids. 

Phase-separated metal-containing block co-polymers formed by ROMP offer interesting possibilities for the controlled formation of semiconductor and metal nanoclusters, which are of intense interest as a result of their size-dependent electronic and optical properties, as well as their catalytic behavior. Zinc-containing block co-polymers generated by ROMP have been shown to form ZnS nanoclusters within the phase-separated organozinc domains upon treatment with gaseous The cluster sizes generated were up to 30 A and their small size led to quantum size effects. For example,... 

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