Micelles and liposomes

Micelles are formed at sufficiently high eoncentrations of amphiphilic or oppositely charged bloek copolymers. Dibloek eopolymers are most commonly used, but triblock copolymers and dendrimers have also been investigated. While the 

Despite typically including PEG as a hydrophilic block for the reduction of protein adsorption, the stabiUly of block copolymer micelles can be affected by proteins, even under well-controlled in vitro conditions. Toncheva et al. found that the stability of PEG-6-poly(ortho ester)-6-PEG micelles in presence of bovine serum albumin depended most on the length of the PEG blocks (Toncheva et al. 2003). A study by Liu et al. found that bovine serum albumin did not affect the stability of PEG-i-poly(5-benzyloxytrimethylene carbonate) micelles, but the dmg release was accelerated in the presence of protein (Liu et al. 2005). 

Some pH-responsive micelle formulations have been developed. A simple example is a triblock copolymer of PEG-Z)-PDMAEMA-f)-PDEAEMA which 

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Fukuzawa, K. Gebicki, J. M. Oxidation of alpha-tocopherol in micelles and liposomes by the hydroxyl, perhydroxyl, and superoxide free radicals. Arch. Biochem. Biophys. 1983, 226, 242-251. 

Shown in Figure 1 are the principal schemes for micelle and liposome formation and loading with various reporter moieties that might be covalently or noncovalently incorporated into different compartments of these particulate carriers. Although micelles may be loaded with a contrast agent only into the core in the process of micelle assembly, liposomes may incorporate contrast agents in both the internal water compartment and the bilayer membrane. 

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].

Proton Relaxivity of Various Paramagnetic Micelles and Liposomes... 

In water, phosphatidic acid (PA) is 2 negatively charged and may combine with divalent ions to form salts that may precipitate. PA also stabilizes micelles and liposomes because its negative charge prevents fusion with each other. 

Waissig, V., K. R. Whiteman, and V. P. Torchilin. 1998. Accumulation of protein-loaded long-circulating micelles and liposomes in subcutaneous Lewis lung carcinoma in Wraem. Res15 1552-1556. 

Further work on the structural requirements for the antioxidant effect has been performed with micelles and liposomes [ 104]. This group concluded that the antioxidant properties of vitamin E and its model compound without the phytyl side-chain were similar within micelles, liposomes and homogeneous solution but that the phytyl side-chain enhances the retention of vitamin E in liposomes and suppresses the transfer of the vitamin between liposomal membranes. 

DEGRADATION KINETICS IN INCLUSION COMPLEXES, MICELLES, AND LIPOSOMES... 

Both polymeric micelles and liposomes are being used as carrier systems for drugs. What is a common feature of these two carrier systems ... 

Micelles and Liposomes. If in a solution amphiphiles (also known as surfactants) exceed a certain concentration called the critical micelle concentration, then they aggregate spontaneously into micelles or liposomes (Fig. 4.12). 

Studies by Ringsdorf [19] on the ability of various amphiphilic molecules to form monolayers, multilayers, micelles, and liposomes, coupled with the potential uses of such systems in many different scientific areas, have caused him to comment [19] that self-organization and the construction of supramolecular systems is an interdisciplinary area which cannot be understood without the cooperation of different fields of science . In short, cooperativity requires cooperation ... 

Experiments with liposomes, which are more complex stmctures than micelles, point in the same direction. Liposomes consist of one (or several) lipid double layers (see Sect. 10.2), ordered concentrically around an aqueous interior. Blocher et al. (2000) carried out polycondensation reactions of amino acids and peptides using l-palmitoyl-2-oleoyl-.vn-glycero-3-phosphocholine liposomes hydrophobically activated amino acids gave chain lengths of up to 29. The linkage of dipeptides (e.g., H-Trp-Trp-OH) to give Trps-OH, i.e., an octapeptide, was also carried out using liposomes. Thus, a second possibility, as well as adsorption on mineral surfaces, was available for polymer formation via micelles and liposomes. 

Effects of ionizing radiation on lipid molecules have been understood by studying model systems which are simpler than the real biological membranes, such as PUFA micelles and liposomes. The formation of lipid oxidative modifications of PUFAs appears as a dynamic process initiated by hydroxyl free radicals generated by water radiolysis, amplified by a propagating-chain mechanism involving alkyl and peroxyl free radicals, and leading not only to hydroperoxides but also to a lot of other lipidic oxidized end-products. Kinetic data, such... 

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