Polymerizable phosphatidylcholines

Fig. 3. Structures of selected polymerizable phosphatidylcholine (PC) lipids. The fatty acid chain lengths may vary from 12 to over 20 atoms.

Novel polymerizable phosphatidylcholines were successfully synthesized by the research team of G. Just. To prepare a 32-membered macrocyclic diyne, the Eglinton modification of the Giaser coupiing was utilized. The diester-diyne starting material was slowly added to a refluxing solution containing 10 equivalents of cupric acetate in dry pyridine. The macrocycle was isolated in 54% yield after column chromatography. 

The realization that tubules may be formed on temperature reduction of polymerized SUVs, prepared from polymerizable diacetylenic phosphatidylcholines (21 where n = 7-16 and m = 5-11), represented a major breakthrough in obtaining the desired supramolecular structure [355-360]. In the initial experiments, 0.4- to 1.0-pm-diameter and 10- to 1000-pm-long tubules were prepared by the gradual lowering of the temperature (to about 38 °Q of 21 (m = 8, n = 9) SUVs [358]. The walk of the tubules had thickness of 10-40 nm and were coated by spiral ripples and helical bilayer strips. Many tubules contained trapped SUVs. Polymerization of the acetylenic moieties greatly enhanced the mechanical and thermal stabilities of the tubules [355-360]. 

Electric field-induced fusion has been applied to a vast variety of cells including human erythrocytes and liposomes made from asolectin and egg phosphatidylcholine. To what extent this method can be utilized for fusing polymerizable vesicles will be demonstrated in the following. 

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. 

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... 

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. 

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