Entrapment steric

The induction of steric effects by the pore walls was first demonstrated with heterogeneous catalysts, prepared from metal carbonyl clusters such as Rh6(CO)16, Ru3(CO)12, or Ir4(CO)12, which were synthesized in situ after a cation exchange process under CO in the large pores of zeolites such as HY, NaY, or 13X.25,26 The zeolite-entrapped carbonyl clusters are stable towards oxidation-reduction cycles this is in sharp contrast to the behavior of the same clusters supported on non-porous inorganic oxides. At high temperatures these metal carbonyl clusters aggregate to small metal particles, whose size is restricted by the dimensions of the zeolitic framework. Moreover, for a number of reactions, the size of the pores controls the size of the products formed thus a higher selectivity to the lower hydrocarbons has been reported for the Fischer Tropsch reaction. 

In the early 1960s it became evident that the reaction environment had an important role in dictating the course of photochemical conversions acting on the course of the relaxation processes and stabilizing photoproducts.17 A constrained medium such as that of a porous matrix or a micelle provides the restricted environment to stop any bimolecular processes that could lead to degradation of products. These effects, however, are subtle. For instance, confinement of a molecule within a host instead of leading to inhibition of reactions of the trapped substrate often results in enhanced reactivity and selectivity because confinement does not mean steric inhibition of all motions of the entrapped host molecule which may eventually enjoy less restriction of some motions than in common solvents. 

Plasmid DNA can be efficiently entrapped in liposomes. Encapsulation using the SPLP approach relies on the presence of a cationic lipid and a steric... 

Figure 6 Encapsulation of plasmid DNA (pDNA) in small sterically stabilized liposomes [stabilized plasmid-lipid particles (SPLP)] using a detergent dialysis procedure. (A) Entrapped pDNA-to-lipid ratio as a function of the initial pDNA-to-lipid ratio (mg/mg). The initial lipid concentration was lOmg/mL. (B) Cryo-electron micrograph showing the structure of SPLP. The location of the plasmid is indicated by the striated pattern superimposed on the liposomes. The bar represents 100 nm.

Figure 1 Schematic structures of micelle and liposome, their formation and loading with a contrast agent, (a) A micelle is formed spontaneously in aqueous media from an amphiphilic compound (1) that consists of distinct hydrophilic (2) and hydrophobic (3) moieties. Hydrophobic moieties form the micelle core (4). Contrast agent (asterisk gamma- or MR-active metal-loaded chelating group, or heavy element, such as iodine or bromine) can be directly coupled to the hydrophobic moiety within the micelle core (5), or incorporated into the micelle as an individual monomeric (6) or polymeric (7) amphiphilic unit, (b) A liposome can be prepared from individual phospholipid molecules (1) that consists of a bilayered membrane (2) and internal aqueous compartment (3). Contrast agent (asterisk) can be entrapped in the inner water space of the liposome as a soluble entity (4) or incorporated into the liposome membrane as a part of monomeric (5) or polymeric (6) amphiphilic unit (similar to that in case of micelle). Additionally, liposomes can be sterically protected by amphiphilic derivatization with PEG or PEG-like polymer (7) [1].

Electrostatic and non-electrostatic biopolymer complexes can also be used as effective steric stabilizers of double (multiple) emulsions. In this type of emulsion, the droplets of one liquid are dispersed within larger droplets of a second immiscible liquid (the dispersion medium for the smaller droplets of the first liquid). In practice, it is found that the so-called direct water-in-oil-in-water (W/O/W) double emulsions are more common than inverse oil-in-water-in-oil (O/W/O) emulsions (Grigoriev and Miller, 2009). In a specific example, some W/O/W double emulsions with polyglycerol polyricinoleate (PGPR) as the primary emulsifier and WPI-polysaccharide complexes as the secondary emulsifying agent were found to be efficient storage carriers for sustained release of entrapped vitamin Bi (Benichou et al., 2002). 

For the synthesis of LSGM, two different synthesis concepts were applied. In both routes, nitrate salts of each constituent cation were selected as the cation source. PVA ( Steric entrapment method14), CA, TA, EDTA, or Pechini precursors (with 60% CA - 40% and EG, or 90% CA - 10% EG mixtures) were used as the organic carrier materials in solution. In the synthesis of LSGM with EDTA as the carrier material, nitric acid was the solvent. In the second production route, nitrate salt of each constituent cation was dissolved in distilled water without any organic molecule. 

The importance of the chelate effect combined with the construction of multidentate ligands is well known in lanthanide chemistry. This is expressed in the rich coordination chemistry of / -diketonates [88] or complexes with Schiff bases [89] and macrocyclic polyethers [90] where lanthanide cations achieve steric saturation by high coordination numbers. Entrapment of the cation in a macrocyclic cavity results in greater complex stability. However, simply functionalized ligands such as ethanolamines can also supply a suitable ligand sphere [91-93],... 

In microporous supports or zeolites, catalyst immobilization is possible by steric inclusion or entrapment of the active transition metal complex. As catalyst retention requires the encapsulation of a relatively large complex into cages only accessible through windows of molecular dimensions, the term ship-in-a-bottle has been coined for this methodology. Intrinsically, the size of the window not only determines the retention of the complex, but also limits the substrate size that can be used. The sensitivity to diffusion limitations of zeolite-based catalysis remains unchanged with the ship-in-a-bottle approach. In many cases, complex deformation upon heterogenization may occur. 

Entrapment of guests within self-assembled capsules provides a fourth method for the construction of chiral supramolecular aggregates. In fact, confinement of guests into cavities with a proper shape and size limits their available space and allows the control over their reciprocal positioning. This reflects directly into their mutual electronic and steric interaction and, as a consequence, in their stereorecognition (Fig. 4D). 

The formation of these dinuclear complexes can be impeded by entrapment of the Mn(BPY)22+ complexes in the structure of zeolite Y. Preferably, Mn(BPY)2+ is assembled via ship-in-a-bottle synthesis in zeohte Y, through BPY adsorption on a NaY zeolite partially exchanged with Mn2+. Because a single zeolite Y supercage can contain only one Mn(BPY)2+ complex, the formation of dinuclear complexes is impossible for steric reasons. The reaction of H2C>2 with the zeolite-entrapped Mn(BPY)2+ complex does not lead to the same vigorous peroxide decomposition as occurs in solution. Instead, H2O2 is heterolytically activated on the Mn complex with civ-bipyridine ligands to form a Mn(IV)=0 or Mn(V)=0 species. The latter is a... 

Our approach was to enlarge the intrazeolitic cavities in order to generate superior hosts for bulky homogeneous chiral catalysts. Mesopores created this way are completely surrounded by micropores and offer additional advantages. The entrapped metal complex can move freely and is more accessible during catalysis and even sterically demanding transition states can be formed within the individual pores. 

A low content of F-68 in the final product was not surprising. Uncharged chains of F-68 are merely mechanically entrapped during the nanoparticle assembly process, which involves charged molecules of the reactants. The inclusion of F-68, however, is crucial for particle steric stabilization. 

Various strategies have been pursued in order to immobilise chiral epoxidation catalysts and these encompass covalent attachment to solid supports,[41] steric occlusion in nanosized cages of zeolites,[42 44] entrapment in a polydimethylsiloxane membrane145,461 and fluorous biphasic systems.1471 However, these approaches frequently require tedious ligand modifications and often lead to a marked decrease in both selectivity and activity of the transition metal catalyst. 

In summary, it is dear that the zeolite is a novel host for the entrapment of molecules and the rigidity and the charged nature of the framework allow for steric and electrostatic effects on the encapsulated molecules. Isolation of entrapped molecules can also influence their reactivity. The interest in zeolites as hosts for electron-transfer reactions stems from a combination of properties, including... 

Self assembly using amphiphilic components to trap drug in solution within a hydrophobic or hydrophilic core has been explored for decades for the delivery of drugs. The most notable example is that of the liposome, which, however, suffered from low entrapment efficiency, leakage, and rapid clearance by the reticuloendothelial system of the liver. This rapid clearance was reduced by the use of PEGylated phospholipids to form of the so-called sterically stabilized or stealth liposomes. ... 

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