Lipids, polymerizable reactive

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]. 

Fabricating a supported lipid bilayer in which both monolayers are composed of polymerizable lipids results in formation of a polymeric network in each monolayer. Furthermore, if the reactive groups are located at the termini of the acyl chains, the monolayers can be covalently linked, which is inherently more stable than a HBM in which lipid polymerization occurs in only one monolayer. 

In pursuit of enhanced liposomal stability, Ringsdorf, Regen, Chapman, and O Brien pioneered the use of polymerized liposomes. These liposomes were prepared from polymerizable lipid molecules. Polymerized liposomes demonstrated uniform size distribution and are considerably more stable compared to their unpolymerized counterparts. Various polymerizable groups (e.g., butadiene, dia-cetylene, vinyl, or methacryloyl ) have been used to achieve the polymerization of the lipid bilayers. These reactive groups on the lipid may be in the head group region, the hydrocarbon core, or at the hydrocarbon termini. 

OmpF has also been reconstituted into the membrane of PMOX-PDMS-PMOX triblock copolymer vesicles carrying polymerizable end groups (236). The triblock copolymer membrane had a thickness of 10 nm, which is two to three times thicker than a conventional lipid bilayer (237). Nevertheless, the channelforming OmpF could be reconstituted and remained functional, showing similar activities as in lipid membranes, despite the extreme thickness of the membranes. The functionality even remained after polymerization of the reactive end groups, which was demonstrated by monitoring the ampicillin hydrolysis of encapsulated /3-lactamase (156,218,237-240). 

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