Polymerizable forming lipids

While most vesicles are formed from double-tail amphiphiles such as lipids, they can also be made from some single chain fatty acids [73], surfactant-cosurfactant mixtures [71], and bola (two-headed) amphiphiles [74]. In addition to the more common spherical shells, tubular vesicles have been observed in DMPC-alcohol mixtures [70]. Polymerizable lipids allow photo- or chemical polymerization that can sometimes stabilize the vesicle [65] however, the structural change in the bilayer on polymerization can cause giant vesicles to bud into smaller shells [76]. Multivesicular liposomes are collections of hundreds of bilayer enclosed water-filled compartments that are suitable for localized drug delivery [77]. The structures of these water-in-water vesicles resemble those of foams (see Section XIV-7) with the polyhedral structure persisting down to molecular dimensions as shown in Fig. XV-11. 

Influence of subphase temperature, pH, and molecular structure of the lipids on their phase behavior can easily be studied by means of this method. The effect of chain length and structure of polymerizable and natural lecithins is illustrated in Figure 5. At 30°C distearoyllecithin is still fully in the condensed state (33), whereas butadiene lecithin (4), which carries the same numEer of C-atoms per alkyl chain, is already completely in the expanded state (34). Although diacetylene lecithin (6) bears 26 C-atoms per chain, it forms both an expanded and a condensed phase at 30°C. The reason for these marked differences is the disturbance of the packing of the hydrophobic side chains by the double and triple bonds of the polymerizable lipids. At 2°C, however, all three lecithins are in the condensed state. Chapman (27) reports about the surface pressure area isotherms of two homologs of (6) containing 23 and 25 C-atoms per chain. These compounds exhibit expanded phases even at subphase temperatures as low as 7°C. 

Since the fatty acid chains in each lipid were 18 carbons and 16 carbons, respectively, it is reasonable that they could form a mixed lipid phase. Furthermore the bis-dienoyl substitution of 15 favors the formation of crosslinked polymer networks. Ohno et al. showed that the dienoyl group associated with the sn-1 chain could be polymerized by lipid soluble initiators, e.g. AIBN, whereas the dienoyl in the sn-2 chain was unaffected by AIBN generated radicals. Conversely, radicals from a water-soluble initiator, e.g. azo-bis(2-amidinopropane) dihydrochloride (AAPD), caused the polymerization of the sn-2 chain dienoyl group, but not the sn-1 chain. These data provide clear evidence for the hypothesis of Lopez et al. that the same reactive group located in similar positions in the sn-1 and sn-2 chains of polymerizable 1,2-diacyl phospholipids are positionally inequivalent [23]. 

In 1985 Tyminski etal. [55, 56] reported that two-component lipid vesicles of a neutral phospholipid, e.g. DOPC, and a neutral polymerizable PC, bis-DenPC (15), formed stable homogeneous bilayer vesicles prior to photopolymerization. After photopolymerization of a homogeneous 1 1 molar lipid mixture, the lipid vesicles were titrated with bovine rhodopsin-octyl glucoside micelles in a manner that maintained the octyl glucoside concentration below the surfactant critical micelle concentration. Consequently there was insufficient surfactant to keep the membrane protein, rhodopsin, soluble in the aqueous buffer. These conditions favor the insertion of transmembrane proteins into lipid bilayers. After addition and incubation, the bilayer vesicles were purified on a... 

All four types of polymerizable lipids shown in Fig. 4 have been realized synthetically. In this context, one need not attempt to reproduce mother nature slavishly (Fendler 8)). Kunitake 9) was able to show that simple molecules like dialkyldimethyl-ammonium salts also form bilayer assemblies. Fuhrhop 10) and Kunitake U) could accomplish the formation of monolayer liposomes with molecules containing only one alkyl chain and two hydrophilic head groups. Acryloylic and methacryloylic groups (type a and d, Table 1), as well as diacetylenic, butadienic, vinylic and maleic acid groups (type b and c), have been used as polymerizable moieties. A compilation of amphiphilic, photopolymerizable molecules is given in Table 1. 

Aqueous dispersions of polymerizable lipids and surfactants can be polymerized by UV irradiation (Fig. 18). In the case of diacetylenic lipids the transition from monomeric to polymeric bilayers can be observed visually and spectroscopically. This was first discussed by Hub, 9) and Chapman 20). As in monomolecular layers (3.2.2) short irradiation brings about the blue conformation of the poly(diacetylene) chain. In contrast, upon prolonged irradiation or upon heating blue vesicles above the phase transition temperature of the monomeric hydrated lipid the red form of the polymer is formed 23,120). The visible spectra of the red form in monolayers and liposomes are qualitatively identical (Fig. 19). 

Due to the topochemical restrictions of diacetylene polymerization, diacetylenic lipids are solely polymerizable in the solid—analogous phase. During the polyreaction an area contraction occurs leading to a denser packing of the alkyl chains. In addition to surface pressure/area isotherms the polymerization behavior of diacetylenic lipids containing mixed films give information about the miscibility of the components forming the monolayer ... 

Phase-separated monolayers and liposomes were characterized by R. Elbert1011 who synthesized saturated and polymerizable fluorocarbon amphiphiles (59, 60, 61) and investigated their mixing behavior with CH2-analogues and natural lipids. In these systems the fluorocarbon compounds are incompatible with hydrocarbon lipids in a wide range of compositions and tend to form domains of pure fluorocarbon and hydrocarbon amphiphiles. The domains can be visualized by freeze-fracture electron microscopy. 

K. Dorn 105 > polymerized dialkylammonium lipids with the polymerizable methacryloyl moiety either in the head group (29) or at the end of one of the hydrophobic chains (5). GPC revealed Mw 1.9 x 106, Mn 3.5 x 105, Mw/Mn 5.4 for (29) and Mw 1.9 xl06,Mn 3.9 x 105, Mw/Mn 2.4 for (5). It was also found that Mw varies inversely with the time of sonication, i.e. in smaller liposomes lower-molecular-weight polymers are formed. In a following paper, K. Dorn 108 present data for the permeability of monomeric and polymeric vesicles from (29). 

In addition to enzymatic hydrolysis of natural lipids in polymeric membranes as discussed in chapter 4.2.2., other methods have been applied to trigger the release of vesicle-entrapped compounds as depicted in Fig. 37. Based on the investigations of phase-separated and only partially polymerized mixed liposomes 101, methods to uncork polymeric vesicles have been developed. One specific approach makes use of cleavable lipids such as the cystine derivative (63). From this fluorocarbon lipid mixed liposomes with the polymerizable dienoic acid-containing sulfolipid (58) were prepared in a molar ratio of 1 9 101115>. After polymerization of the matrix forming sulfolipids, stable spherically shaped vesicles are obtained as demonstrated in Fig. 54 by scanning electron microscopy 114>. 

Early in the development of liposomes, it was recognized that their plasma instability could be a serious detriment in certain applications. Consequently, there were efforts to Lrst incorporate polymerizable lipids into the liposomal bilayers, and then initiate polymerization by, for example, photolysis, to form interchain crosslinks to stabilize the bilayer. The most commonly used polymerizable lipids have been PCs-containing diacetylene or butadiene moieties in the tailgroups... 

Hupfer, B., Rinsdorf, H., and Schupp, H. (1983). Liposomes form polymerizable lipidsm. Phys. Lipids,... 

The components can be chosen so that the material is biocompatible, opening up possibilities for use in controlled-release drug-delivery and other medical and biological applications that call for nontoxicity. It is known that many biological lipids form bicontinuous cubic phases, and many of these have modifications with polymerizable groups, such as the monolinolein case discussed above. 

What happens if the lipid molecules of an artificial membrane themselves contain polymerizable groups and are polymerized after a vesicle membrane has been formed from the monomers Will the polymerization chain reaction run through the whole of a monolayer and will the polymer retain the vesicle structure Or will parallel ordered clusters be formed and will the vesicle be ruptured The answers to most of these questions are frustrating domains do... 

A traceless detection of Escherichia coli has been reported by Ahn and co-workers (Figure 10.2). Two polymerizable lipids (one containing biotin. Figure 10.2) and DMPC were used to form the liposomes. The liposomes were spotted on streptavidin... 

It should be mentioned, however, that there is a major difference between the BLM and multilayers formed by the L-B technique. A BLM, formed either by the conventional painting method or self-assembled on a substrate (e.g. a freshly cleaved metallic wire or agar gel) is a dynamic hquidhke structure that is capable of accommodating a host of modifiers. In this connection, efforts to stabilize BLMs by using polymerizable lipids have been successfiil. However, the electrochemical properties ofthese BLMs were greatly compromised [3]. 

Starting from this idea, a considerable number of polymerizable lipids have been synthesized and investigated (11) (3). When spread on a water surface in a Langmuir trough (a film balance used to measure force-area relationships of monolayers of amphiphilic substances), they behave like natural lipids, forming different types of monomolecular layers. Upon UV-irradiation, they polymerize in the monolayer retaining the ordered structure (Fig. 5a). What can be done in a monolayer also works in liposomal solutions (Fig. 5b) ... 

Lipids with the appropriate sugars as headgroups and with polymerizable units in the hydrophobic chain were synthesized, and liposomes from these monomers were formed and polymerized. Upon addition of Con A to the liposome solution, a precipitate formed that contained intact liposomes (as controlled by e.m.). If low molecular weight sugars which can compete with the liposomes for the binding sites of Con A were added, the liposomes re-dissolved (Fig. 8). Thus, in the case of a model system, it could be shown that recognition of polymeric liposomes by specific protein structures can be achieved (14). 

ATP Synthetase. Membrane-boimd proteins that utilize the proton gradient across the membrane to form ATP from ADP and phosphate are known as ATP synthetases. The synthetase complex, composed of a hydrophobic part (FO) and a water-soluble domain (FI, catalsdic center) from Rhodospirillum rubrum, was incorporated into hposomes of a polymerizable sulfolipid (235). Lipid-free synthetase has a very low enzymatic activity. If the sulfolipid liposome is partially photopolymerized and the protein complex is incorporated into the liposome, the observed enzymatic activity is comparable to that of the natural lipid membranes. 

It is worto noting that all toe amino acid-diacefylene lipid microstructures studied here could be polymerizable to form blue colored PDAs. However, only hydrophilic amino acid lipids can readily form bilayer vesicles and allow polymerization (77). The intensity of toe initial blue color, however, varies with headgroups. Amino acids with hydrophilic segments give toe darkest blue appearance, while hydrophobic amino acids (lie-) produce barely noticeable blue appearance. 

The knowledge of bulk properties of polymerizable lipids is not sufficient to evaluate their ability to form membranes or to predict their polymerization behaviour in these oriented systems. Their interaction with an aqueous phase... 

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