Oligomeric amphiphiles

Crystallization-induced phase separation can occur for concentrated solutions (gels) of diblocks [58,59]. SAXS/WAXS experiments on short PM-PEO [PM=poly(methylene) i.e. alkyl chain] diblocks revealed that crystallization of PEO occurs at low temperature in sufficiently concentrated gels (>ca. 50% copolymer). This led to a semicrystalline lamellar structure coexisting with the cubic micellar phase which can be supercooled from high temperatures where PEO is molten. These experiments on oligomeric amphiphilic diblocks establish a connection to the crystallization behaviour of related nonionic surfactants. 

Lattice models seek to describe microemulsion structural domains down to the near-molecular level. These include models of amphiphilicity, imposed by mean field attractive and repulsive interactions applied to simplified diatomic or oligomeric amphiphiles, oil and water. Recent versions of these models include bending energy contributions for surfactants meeting at angles to approximate realistic molecular features and accurately capture microemulsion phase diagram. In all cases, microemulsion phase behavior is most accurately captured when the models consider key physical attributes (either exphc-itly or implicitly), including the balance between entropic (which tend to disperse oil-water into ever finer domains) and interfacial (which drive phase separation and place limits on domain curvature) contributions. 

ChEs present several amphiphilic and soluble homo-and hetero-oligomeric molecular forms in tissues and body fluids, with different tissue distributions (Fig. 1). 

Nickel(O) complexes are extremely effective for the dimerization and oligomerization of conjugated dienes [8,9]. Two molecules of 1,3-butadiene readily undergo oxidative cyclization with a Ni(0) metal to form bis-allylnickel species. Palladium(O) complexes also form bis-allylpalladium species of structural similarity (Scheme 2). The bis-allylpalladium complexes show amphiphilic reactivity and serve as an allyl cation equivalent in the presence of appropriate nucleophiles, and also serve as an allyl anion equivalent in the presence of appropriate electrophiles. Characteristically, the bis-allylnickel species is known to date only as a nucleophile toward carbonyl compounds (Eq. 1) [10,11],... 

Fig. 8 Proposed model for gramicidin S in a membrane according to the orientational constraints obtained from and N-NMR. The upright backbone alignment (r 80°) and slant of the /3-sheets (p -45°) are compatible with the formation of an oligomeric /3-barrel that is stabilized by hydrogen bonds (dotted lines). A The oligomer is depicted sideways from within the lipid bilayer interior (showing only backbone atoms for clarity, but with hydrophobic side chains added to one of the monomers). Atomic coordinates of GS were taken from a monomeric structure [4], and the two DMPC lipid molecules are drawn to scale (from a molecular dynamics simulation coordinate file). The bilayer cross-section is coloured yellow in its hydrophobic core, red in the amphiphilic regions, and light blue near the aqueous surface. B Illustrates a top view of the putative pore, although the number of monomers remains speculative...

Multihydroxyl containing monomeric or oligomeric p-cyclodextrins (PCD) such as those attained by grafting with glycidyl ethers of protected polyols (glycerol and pentitols) appeared rather promising components for their amphiphilic character, connected to the presence of an hydrophobic pocket and an external hydrophilic shell with an amplified number of hydroxyl groups. 

Beside the conventional surfactants with one polar head group and one nonpolar tail, dimeric and oligomeric surfactants have attracted considerable interest in academia and industry [521], Dimeric surfactants, also called Gemini surfactants, are made up of two amphiphilic moieties connected closely to the head group by a spacer group (Fig. 12.1). In bolaform surfactants the connection is in the middle of the alkyl chain or close to the end, so that they can be considered as two polar head groups connected by a long hydrophobic chain. 

Perfluoroalkyl groups are also introduced into block copolymers with methacrylates, acrylates, and styrene (B-46 to B-53), which can be synthesized in scCC>2 or in the bulk.95,315 Amphiphilic block copolymers based on glycopolymer segments (B-54 and B-55) are synthesized by copper-catalyzed polymerizations.321,322 Comonomers with a polyhedral oligomeric silsesquioxane unit afforded hybrid polymers between organic and inorganic components (B-56 and B-57).326... 

Phospholamban is a homopentamer, each subunit consisting of 52 amino acids. The subunits consist of a 30-residue N-terminal hydrophilic region containing the phosphorylation sites, and a C-terminal hydro-phobic domain responsible for oligomerization and for anchoring phospholamban in the membrane. The N-terminal domain is a basic amphiphilic a-helix, which resembles the calmodulin-binding domain of many proteins (Chiesi efa/., 1991). Phospholamban is phosphorylated at serine-16 by cAK and cGK and at threonine-17 by Ca +/calmodulin-dependent protein... 

Fig.1. Barrel-stave poration complexes proposed for class 11 bacteriocins. Complexes maybe formed between one or two amphiphilic peptides which oligomerize and form membrane pores and ion channels [21]...

Oligomeric surfactants have been prepared for use as stalnlizers in emulsion polymerization. Functionalized co-oligomars of acrylonitrile and acrylic acid have been used as surfactants in the emulsion polymoization of butadiene, styrene and itaconic acid [169], Diblock copolymers (oxazoline based) with narrow MMD appeax to be excellmt emulsifiers for the inverse emulsion polymerization of styraie [170], and amphiphilic oligomeric diblock macromonomers (PEO-PPO type) have been relied as steric stabilize in regular styrene emulsion polymraizarion [171]. 

Many amphiphilic block copolymers and related oligomeric materials exhibit structure formation at a nanometre scale. By careful selection of the solvent, phase separation can be achieved allowing one block to be dissolved and the other to be immiscible. The head group(s) of a surfactant or the hydrophilic... 

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