Polymerizable phospholipid

Hub HH, HupferB, Koch H, Ringsdorf H. Polyreactions in ordered systems. 20. Polymerizable phospholipid analogs—new stable biomembrane and cell models. Angew Chem Int Ed... 

Lawson GE, Lee Y, Singh A (2003) Formation of stable nanocapsules from polymerizable phospholipids. Langmuir 19 6401-6407... 

Polymerizable phospholipids, on the other hand, have been used in the stabilization of molecular assemblies and in the development of strategies to expand... 

Polymerizable phospholipid analogues have been described [176] which contain diacetylene moieties in the side chains diacetylenes being known to be... 

The scientific community was attracted to the study of liposomes due to the relatively simple procedure of their preparation. Moreover, if prepared from natural phospholipids, they are biocompatible, and possess low cytotoxicity, low immunogenicity, and biodegradability [304], Liposomes, however, have two main disadvantages the structural instability both in vitro and in vivo, and low cell specificity [304], To increase the stability, the structure of the phospholipid layer has been modified to include artificial lipids and/or cholesterol. Polymerizable vesicles have also been prepared [305]. It is obvious that the biocompatibility of these modified systems has to be addressed. 

Fig. 10. Structures of polymerizable dienoyl ammonium lipid and phospholipids.

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... 

Monolayers are best formed from water-insoluble molecules. This is expressed well by the title of Gaines s classic book Insoluble Monolayers at Liquid-Gas Interfaces [104]. Carboxylic acids (7-13 in Table 1, for example), sulfates, quaternary ammonium salts, alcohols, amides, and nitriles with carbon chains of 12 or longer meet this requirement well. Similarly, well-behaved monolayers have been formed from naturally occurring phospholipids (14-17 in Table 1, for example), as well as from their synthetic analogs (18,19 in Table 1, for example). More recently, polymerizable surfactants (1-4, 20, 21 in Table 1, for example) [55, 68, 72, 121], preformed polymers [68, 70, 72,122-127], liquid crystalline polymers [128], buckyballs [129, 130], gramicidin [131], and even silica beads [132] have been demonstrated to undergo monolayer formation on aqueous solutions. 

First, a mixture of synthetic or natural phospholipids, polymerizable lipids, and proteins can be converted to liposomes and then be polymerized. Second, polymerizable lipids are introduced into e.g. erythrocyte ghost cells by controlled hemolysis and subsequent polymerization as described by Zimmermann et al.61). This hemolysis technique is based on a reversible dielectric breakdown of the cell membrane. Dielectric breakdown provides a third possible path to the production of bi omembrane models. Zimmermann et al. could show that under certain conditions cells can be fused with other cells or liposomes61). Thus, lipids from artificial liposomes could be incorporated into a cell membrane. A fourth approach has been published by Chapman et al.20). Bacterial cells incorporate polymerizable diacetylene fatty acids into their membrane lipids. The diacetylene units can be photopolymerized in vivo. The investigations discussed in more detail below are based on approaches 1. and 3. 

Since cholesterol is an important component of many biological membranes mixtures of polymerizable lipids with this sterol are of great interest. In mixed monolayers of natural lipids with cholesterol a pronounced condensation effect , i.e. a reduction of the mean area per molecule of phospholipid is observed68. This influence of cholesterol on diacetylenic lecithin (18, n = 12), however, is not very significant (Fig. 32). Photopolymerization indicates phase separation in this system. Apparently due to the large hydrophobic interactions between the long hydrocarbon chains of... 

As an example of an asymmetric membrane integrated protein, the ATP synthetase complex (ATPase from Rhodospirillum Rubrum) was incorporated in liposomes of the polymerizable sulfolipid (22)24). The protein consists of a hydrophobic membrane integrated part (F0) and a water soluble moiety (Ft) carrying the catalytic site of the enzyme. The isolated ATP synthetase complex is almost completely inactive. Activity is substantially increased in the presence of a variety of amphiphiles, such as natural phospholipids and detergents. The presence of a bilayer structure is not a necessary condition for enhanced activity. Using soybean lecithin or diacetylenic sulfolipid (22) the maximal enzymatic activity is obtained at 500 lipid molecules/enzyme molecule. With soybean lecithin, the ATPase activity is increased 8-fold compared to a 5-fold increase in the presence of (22). There is a remarkable difference in ATPase activity depending on the liposome preparation technique (Fig. 41). If ATPase is incorporated in-... 

Natural lipids used for fusion experiments were mainly phospholipids with different chain lengths and their mixtures with cholesterol. As polymerizable lipids, butadienic derivatives with a phosphatidylcholine (19) and a dimethylammonium head group (26) were used in the fusion experiments. 

Freeman, F. J., Hayward, J. A., and Chapman, D. (1987). Permeability studies on liposomes formed from polymerizable diactylenic phospholipids and their potential applications as drug delivery systems. Biochim. Biophys. Acta, 924, 341-451. 

Efforts to stabilize BLMs by the use of polymerizable lipids have been successful, but the electrochemical properties of these membranes were greatly compromised and ion channel phenomena could not be observed [21]. Microfiltration and polycarbonate filters, polyimide mesh, and hydrated gels have been used successfully as stabilizing supports for the formation of black lipid films [22-25] and these systems were observed to retain their electrical and permeability characteristics [24]. Poly(octadec-l-ene-maleic anhydride) (PA-18) was found to be an excellent intermediate layer for interfacing phospholipids onto solid substrates, and is sufficiently hydrophilic to retain water for unimpeded ion transfer at the electrode-PA-18 interface [26]. Hydrostatic stabilization of solventless BLMs has been achieved by the transfer of two lipid monolayers onto the aperture of a closed cell compartment however, the use of a system for automatic digital control of the transmembrane pressure difference was necessary [27]. 

Finally, phospholipidic compounds carrying polymerizable groups such as IX and X have been shown very recently [28] to allow the preparation of stable monodisperse polystyrene latexes, with sizes in the range of 200-300 nm in diameter, carrying a phospholipid layer on their surface with yields higher than 75%. 

Two phospholipids, one saturated and one polymerizable, containing iminodiacetic acid functionality linked to their phosphate headgroup are synthesized to build metal chelated lipid assemblies. These lipids were non-ideally miscible with analogous phosphatidylcholine as determined by monolayer studies at air/water interface. The metal chelating lipids produced vesicles when mixed with polymerizable... 

The use of phospholipids to mimic cell walls has been one commercially successful coating strategy. To create a polymer surface with phospholipid-type properties, two routes are possible. The polymer surface can be modified by the attachment of the biological molecule of interest, as described in the modification of polymer surfaces with phosphorylcholine (72-74), amphiphilic molecules (75) or liposomes. Alternatively, phosphorylcholine moieties have been incorporated into artificial surfaces by using polymerizable precursors the most representative monomer is 2-methacryloxyethylphosphorylcholine (MFC) 16, 17). Block copolymers of phosphorylcholine with other hydrophobic comonomers like lauryl methacrylate (75) also have been found to be effective. 

Diacetylenes in phospholipid bilayers have been the subject of extensive studies in our laboratory, not only because of the highly conjugated polymers they form, but also because of their ability to transform bilayers into interesting microstructures. Consequent to our synthesis and characterization of several isomeric diacetylenic phospholipids, we have found that the polymerization in diacetylenic bilayers is not complete. In order to achieve participation of all diacetylenic lipid monomer in the polymerization process, diacetylenic phospholipid was mixed with a spacer lipid, which contained similar number of methylenes as were between the ester linkage and the diacetylene of the polymerizable lipid. Depending upon the composition of the mixtures different morphologies, ranging from tubules to liposomes, have been observed. Polymerization efficiency has been found to be dependent on the composition of the two lipids and in all cases the polymerization was more rapid and efficient than the pure diacetylenic system. We present the results on the polymerization properties of the diacetylenic phosphatidylcholines in the presence of a spacer lipid which is an acetylene-terminated phosphatidylcholine. 

---