Shape encapsulation compounds

One advantage of liposome is theirsingularability to take up material of all shapes and sizes regardles of their individual chemical peculiarities. The most relevant to this discussion is for lipid-soluble compounds, which are incorporated into the lipid bilayer of the liposome. Thus, the maximum amount taken up into liposomes is directly proportional to the quantity of membrane components, but is independent of the size of the liposomes used, except in a few cases where the structure o the encapsulated compound may restrict the Lnal size of the liposome. Theoretically, any type of liposome is suitable for the incorporation of lipid-soluble substances, but practical considerations make some choices more preferable over others. MLVs are the best choice for sustained release whereas SUVs are particularly suitable for rapid intermembrane transfer. 

Microcapsules are non-dense spherical shaped volumes. They are formed by a shell (polymeric wall in this case) with an empty volume inside that can be used to encapsulate compounds. The structure of the shell (a thin, dense layer, or a thick, porous layer, etc.), together with its nature (chemistry, material), determines the release rate of the encapsulated compound. There are several ways to obtain microcapsules. Some of these methods are based only on physical phenomena, certain are based on polymerization reactions and others combine both physical and chemical procedures. Most authors agree in classifying them all in two different groups chemical processes (like in-situ polymerization or desolvation in liquid media) and mechanical processes (e.g. spray drying or electrostatic deposition) [44]. A novel technology for microcapsule production, based on the employment of microdevices in continuous mode, has been presented [45]. Immersion precipitation is used in this case, in a way similar to that explained for flat membranes. 

Figure 13.14 Hollow-structured calcium phosphate nanospheres can be transferred into pin-shaped crystallites under ultrasonic treatment. In this way the release of encapsulated compounds can be triggered on/off and its kinetics precisely regulated by the power density, duty cycle and application time of the ultrasound. Calcium phosphate nanospheres and ultrasonic treatment can be...

A (Figure 4.9). The diameter of such a neck, 2.3 A, is sufficiently large for a linear C-C chain to pass, but too small to also be an equilibrium adsorption position. The largest compound allowed inside the pores is a linear molecule limited in length to four carbon atoms due to the distance between two subsequent necks [103]. Another example of shape-selective behavior is found in a Zn-based MOF able to encapsulate linear hexane while branched hexanes are blocked [104]. 

Up to 1999, only metal atoms [1-5], metal clusters [6,7], metal nitrides [55-57], and noble gas atoms [58-60] were observed to be encaged inside C60, C70, or various sizes of higher fullerenes. The experimental evidence for carbon atoms or metal-carbon compounds (carbides) being encapsulated inside fullerenes had not yet been observed. In 2000, Shinohara et al. succeeded in the first production, isolation, and spectroscopic characterization of a scandium carbide endohedral fullerene (Sc2C2) C84. Following this, the first experimental evidence based on synchrotron X-ray diffraction was presented and revealed that the Sc carbide is encapsulated in the form of a lozenge-shaped Sc2C2 cluster inside the D2d-C84 fullerene [8]. 

The easiest combination of two chiral guests in the cylindrical capsule comes by the double encapsulation of racemic species with adequate size, shape and polarity to be accommodated in pairs, giving two diastereoisomeric complexes, the homochiral couple (R) - (R)/(S) - (S) and the heterochiral (R) -(S) combination. ( )-trans-1,2-Cyclohcxancdiol has all the above requisites and showed a 1.2 ratio between the two complexes in favor of the heterochiral combination [60]. This observation may be related to the preference in nature for centrosymmetric crystals or, alternatively stated, the higher melting points of racemates vs. enantiopure compounds where the resolution is driven by the less soluble pair [61,62], In the cylindrical capsule a single couple of chiral molecules is extrapolated from the bulk and the interactions between the two is governed by the shape of the cavity and their goodness of fit within the cavity. 

Container molecules in general show an increasing number of applications and so do the container molecules based on imine type ligands. Many different shapes of open or nearly closed ones could already be synthesised. Those cages are known to encapsulate different types of guest molecules. This encapsulation can be selective and permanent or reversible. The container molecules described are also used for stabilisation of different compounds such as the allotrope P4. They can be used as gas or optical sensors. One of the described cages can also be opened and closed selectively. 

Fig. 5.15. Pyrogallarenes and resorcinarenes which form hexameric capsules through hydrogen bonding. The tetramethyl resorcinarene serves as a control compound. At the bottom, three different guest cations are shown, which carry charges to make the capsule MS-detectable. Right Computer model of the Ru(bpy)32 guest encapsulated in the cavity of the hexamer. Each monomer is shown with a different color. It can nicely be seen that the Ru complex not only fits size-wise, but also with respect to its symmetry and shape.

One end of the -barrel is closed by the amino-terminal peptide segment that runs across its bottom between fhe two short loops connecting strands B/C and F/ G, respectively, before it enters into /i-slrarid A. Dense packing of side chains in this region and wifhin the adjacent interior of fhe barrel structure leads to fhe formation of a hydrophobic core. The other end of fhe / -barrel is open to the solvent and forms a characteristic pocket. In the case of RBP, retinol is encapsulated as a ligand and protrudes into the barrel by almost half of its depth. The entrance to the pocket is formed by a set of four loops, which connect fhe eight antiparallel strands in a pairwise fashion. Because of the chalice-like shape of fhe protein (Fig. 8.2) and since many members of fhis family complex lipophilic compounds, the term lipocalins was proposed [25]. 

The cage compound 50 exhibited a remarkable ability to encapsulate C-shaped molecules such as cis-azobenzene 53 and cw-stilbene 54 derivatives. " ... 

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