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Spectra and physical properties of liposomes and mono layers of polymerizable phospho lipids containing di acetylene groups in one or both acyl chains

, : Spectra and physical properties of liposomes and mono layers of polymerizable phospho lipids containing di acetylene groups in one or both acyl chains. Biochemistry 22(13): 3194-3202

Phosphatidylcholine (PC) molecules were synthesized containing a diacetylene group in one (mixed-chain PC) or both acyl chains (identical-chain PC). Upon UV irradiation they form high MW colored polymers. Under identical irradiation conditions the maximum absorption in the visible region of polymerized aqueous dispersions extracted into chloroform-methanol is far greater for mixed-chain PC than for identical-chain PC. Under certain conditions irradiation of identical-chain PC results in polymer which is insoluble in organic solvents. Mixed-chain PC do not form insoluble polymer. The visible spectra of both classes of PC consist of 2 bands which vary with temperature. At all temperatures for identical-chain PC and at temperatures above C for C23 mixed-chain PC and C for C27 mixed-chain PC the thermochromism is fully reversible. When mixed-chain PC are allowed to warm after polymerization at low temperatures, an irreversible shift in the longer wavelength band is observed. This shift is correlated with a change from a sharp positive band in the CD [circular dichroism] of C27 mixed-chain Pc aqueous dispersions at C to a broad negative band at C. The thermochromism may be related to energetic barriers to rearrangement of the polymer crystal lattice. The differences in solubility, visible spectra and thermochromism of polymerized mixed-chain and identical-chain PC favor a model in which short linear segments of polymer are interconnected through the glycerol backbone of the lipid in identical-chain PC while long linear regions are found in mixed-chain PC. The physical properties of polymerizable phosphatidylcholines were examined at the air-water interface and in bulk aqueous dispersions. There is a phase transition in aqueous dispersions of both lipids and in monolayers of identical-chain PC. Monolayers of mixed-chain PC are not stable at low temperature. The phase transition temperatures (Tc) for mixed-chain PC aqueous dispersions are lower than for identical-chain PC with equivalent-length diacetylenic acyl chains. The Tc of polymerizable PC increases with diacetylenic acyl chain length, but not as rapidly as for saturated PC. Above Tc, the order parameter of nitroxide stearic acid spin labels decreases as the nitroxide group is placed closer to the terminal methyl group of the polymerizable PC. The order parameter of spin labels in polymerizable PC is smaller than in saturated PC. Polymerization of the dispersions does not affect the mobility of the spin labels. As expected for liposomes, aqueous dispersions before and after polymerization shrink when exposed to hypertonic solutions of sucrose. Lamellar structures are evident in freeze-fractured and negatively stained liposomes examined under the EM. Liposomes of polymerizable PC are more permeable to glycerol than DMPC [dimyristoylphosphatidylcholine] liposomes above Tc before and after polymerization, although polymerization reduced the permeability. Polymerization markedly enhances the stability of liposomes to precipitation.

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