Vesicles hollow

Block copolymer vesicles, or polymersomes, are of continued interest for their ability to encapsulate aqueous compartments within relatively robust polymer bilayer shells (Fig. 7) [66, 67]. Eisenberg and coworkers were the first to report the formation of block copolymer vesicles from the self-assembly of polystyrene-h-poly(acrylic acid) (PS-h-PAA) block copolymers. They also have described the formation of a wide range of vesicle architectures in solution from the self-assembly of five different block copolymers PS-h-PAA. PS-h-PMMA, PB-h-PAA, polystyrene-h-poly(4-vinyIpyridinium methyl iodide), and polystyrene-h-(4-vinylpyridinium decyl iodide) [68]. Small uniform vesicles, large polydisperse vesicles, entrapped vesicles, hollow concentric vesicles, onions, and vesicles with hollow tubes in the walls have been observed and the formation mechanism discussed. Since vesicles could be prepared with low glass transition polymers such as PB [69, 70] and PPO [71], it has been established than these structures are thermodynamically stable and not trapped by the glassy nature of the hydrophobic part. 

Eqs. 64-65 describe the theoretical scattering as a function of Q, which must be solved for each object or particle. Here, we will show some typical examples for different morphologies, the form factors for a sphere, cylinder, polymer chain, and a vesicle (hollow shell). The results for these structures are the following ... 

The interiors of rhodopseudomonad bacteria are filled with photosynthetic vesicles, which are hollow, membrane-enveloped spheres. The photosynthetic reaction centers are embedded in the membrane of these vesicles. One end of the protein complex faces the Inside of the vesicle, which is known as the periplasmic side the other end faces the cytoplasm of the cell. Around each reaction center there are about 100 small membrane proteins, the antenna pigment protein molecules, which will be described later in this chapter. Each of these contains several bound chlorophyll molecules that catch photons over a wide area and funnel them to the reaction center. By this arrangement the reaction center can utilize about 300 times more photons than those that directly strike the special pair of chlorophyll molecules at the heart of the reaction center. 

There are a variety of routes currently utilized to fabricate a wide range of hollow capsules of various compositions. Among the more traditional methods are nozzle reactor processes, emnlsion/phase-separation procednres (often combined with sol-gel processing), and sacrificial core techniques [78], Self-assembly is an elegant and attractive approach for the preparation of hollow capsules. Vesicles [79,80], dendrimers [81,82], and block hollow copolymer spheres [83,84] are all examples of self-assembled hollow containers that are promising for the encapsnlation of various materials. 

Fig. 2.18 Vesicle structures with silica wall (A) mesostructured silica vesicle (B) hollow capsule composed of silica particles prepared by LbL assembly.

Figure 6.4 The preparation of nanostructured materials in solution evolves from (a) the classic examples of suspension, dispersion, or emulsion polymerization, to the methods that include the covalent crosslinking of select domains within supramolecular polymer assemblies (b) core crosslinking of polymer micelles (c) shell crosslinking of polymer micelles (SCKs) (d) nanocages from core-eroded SCKs (e) shaved hollow nanospheres from outer shell/core-eroded vesicles.

Figure 6.5 Illustrations of nanoscale spherical assemblies resulting from block copolymer phase separation in solution are shown, along with the chemical compositions that have been employed to generate each of the nanostructures (a) core crosslinked polymer micelles (b) shell crosslinked polymer micelles (SCKs) with glassy cores (c) SCKs with fluid cores (d) SCKs with crystalline cores (e) nanocages, produced from removal of the core of SCKs (f) SCKs with the crosslinked shell shielded from solution by an additional layer of surface-attached linear polymer chains (g) crosslinked vesicles (h) shaved hollow nanospheres produced from cleavage of the internally and externally attached linear polymer chains from the structure of (g)...

Thyroid epithelial cells synthesize and secrete T4 and T3 and make up the functional units of thyroid glandular tissue, the thyroid follicles. Thyroid follicles are hollow vesicles formed by a single layer of epithelial cells that are filled with colloid. T4,T3, and iodine are stored in the follicular colloid. T4 and T3 are derived from tyrosyl residues of the protein thyroglobulin (Tg). Thyroid follicular cells synthesize and secrete Tg into the follicular lumen. Thyroid follicular cells also remove iodide (I ) from the blood and concentrate it within the follicular lumen. Within the follicles, some of the tyrosyl residues of Tg are iodinated, and a few specific pairs of iodoty-rosyl residues may be coupled to form T4 and T3. Thus, T4, T3, and iodine (in the form of iodinated tyrosyl residues) are found within the peptide structure of the Tg that is stored in the follicular lumen. 

One way is to label the pre-existing vesicles, and then follow the destiny of the label in the vesicle size distribution. The label that has been used to this aim is ferritin, which has been entrapped into vesicles. Ferritin is an iron-storage protein in plants and mammals, and consists of a hollow protein shell of c. 12 nm containing... 

The process utilizing supramolecular organization involves pore expansion in silicas. A schematic view of such micelles built from the pure surfactant and those involving in addition n-alkane is shown in Figure 4.9. Another example of pore creation provides a cross-linking polymerization of monomers within the surfactant bilayer [30]. As a result vesicle-templated hollow spheres are created. Dendrimers like that shown in Figure 4.10 exhibit some similarity to micellar structures and can host smaller molecules inside themselves [2c]. Divers functionalized dendrimers that are thought to present numerous prospective applications will be presented in Section 7.6. 

Liposomes are characteristic hollow spherical aggregates or vesicles that form spontaneously when phospholipids are dispersed in water. This is a function of their low solubilities in both oil and water and results from the hydrophobic nature of the twin acyl tails and the strongly hydrophilic polar head group which are the main characteristics of phospholipids (Figure 9.1). As a result, phospholipid molecules, when dispersed in water form double layers where the phospholipids align themselves, tail-to-tail and head-to-head (Figure 9.5)... 

At low concentrations, a hollow vesicle results with usually just one double layer and, as the concentration is increased, the number of double layers can increase in a transition from unilamellar vesicles to multilamellar structures. Since the hydro-plilic head groups are exposed on the inside as well as the outside of the vesicular structure this provides an opportunity to entrap hydrophilic guest drug molecules both inside the center of the vesicle and, if multilamellar, between the phospholipid bilayers as well. On the other hand, hydrophobic molecules can become incorporated in the hydrophobic regions of the bilayers where the hydrophobic tails overlap. 

The hollow interior of dodecahedrane and other organic cage compounds described in section 4.9 is much too small to envelop atoms, ions, or molecules. Tight closed-shell macromolecules have been obtained from vesicles by several research groups, by polymerization of amphiphiles possessing double or triple bonds within the membrane or at the head groups. Smaller, but well-defined, closed-shell containers have been obtained by two other methods described below, namely by directed synthesis and by formation of closed-shell all-carbon molecules in graphite vapor. 

Neurotransmitters are produced in our brains from the contents of our diets by means of a many-step process. First, nutrients (labeled i in Fig. i—i), such as amino acids, sugar, fats, and peptides (strings of amino acids bound together), are extracted and absorbed from the food we eat and are transported out of the arterial blood supply to the brain—that is, they are actively carried through the blood—brain barrier and transported into the neurons. Enzymes (2) convert these nutrients into different neurotransmitters. The neurotransmitter molecules are actively transported into what are called synaptic vesicles (3), or very tiny spheres with hollow centers into which about 10,000 molecules of a typical neurotransmitter can be stored for later release from a neuron. 

A final biomedical use for polyphosphazenes is as components in microspheres, vesicles, and micelles for use in drug-delivery applications. Microspheres are pseudo-spherical constructs that range in size from 1 to 600 microns. Vesicles (lipozomes) are hollow, water-filled bilayer spheres with diameters that range from 0.03 tolO microns. Micelles typically have diameters near 1 micron (100 nanometers). Idealized representations of these three structures are shown in Figure 3.23, together with the location of trapped drug molecules. 

FIGURE 7.30. Different morphologies found upon self-assembly of PS-fr-PAA copolymers, (a) micellar spheres (b) micellar rods (c) lamellae (d) vesicles (e) hexagonally packed hollow hoop (HHH) structures (f) LCMs and (g) the internal structure of a LCM. Reproduced with permission from the Canadian Chemical Society. 

Figure 3.30 Synthesis route of PMO magnetic hollow spheres (a) hydrophobic, stearic acid-capped Fes04 nanoparticles in an organic phase are treated with CTAB to produce (b) water-dispersible nanoparticles, (c) Formation of FC4 vesicles leads to encapsulation of the CTAB-stabilized Fe304 nanoparticles, (d) Addition of BTME/CTAB forms the outer ethane-bridged PMO shell surrounding the vesicles. (See color insert.)...

Tubulin is a self-assembling protein that forms microtubules (MTs) within the cell. MTs are involved in a variety of cell functions including vesicle movement, chromosome segregation, and cell motility. MTs are assemblies of heterodimeric proteins, a/p-tubulins. The protein consists of two subunits, a modified a-tubulin and a modified P-tubulin. The two monomers have about 40% of the same amino acid sequences and their structures are similar except for a few differences in the loops. Each monomer contains a pair of central P sheets surrounded by a helices. The tubulin monomers self-assemble into hollow structures, termed microtubules. 

A bilayered structure in the form of a closed, hollow sphere is also possible [Fig. 6-3(a)]. This type of structure is called a vesicle. The primary concept of a vesicle is two sheets of lipid with their hydrocarbon chains opposed [a bilayer. Fig. 6-3(b)]. An isolated bilayer cannot exist as such in water, because exposed hydrocarbon tails would exist at the edges of the sheet however, this situation is obviated by the sheet s curving to form a self-sealed, hollow sphere. 

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