Interaction of benzene with bilayers. Thermal and structural studies

Abstract

The thermal and structural properties of saturated phosphatidylcholine liposomes are significantly altered by benzene. Upon the addition of benzene, the liposomes first swell and then disperse into small multilamellar vesicles. At 20 degrees C these vesicles contain striations or ripples in the plane of the bilayer. Major changes in the thermal behavior of DSPC-benzene liposomes occur near mole ratios of 2:1 and 1:1. At a 2:1 mole ratio, the area under the main endothermic peak, delta Hm, essentially disappears; however, the total heat absorbed, delta Hf, remains approximately equal to that of the control. This occurs because for benzene mole fractions 0.12 less than x less than 0.50, benzene increases the apparent molar heat capacity, Cp, of the gel phase to about 1.2 kcal/(mol . deg). We interpret this increase in heat capacity to be due to an increase in the concentration of defects (or disorder) in the gel phase. At mole fractions of benzene between 0.5 and 0.9, the transition temperature decreases by 20-30 degrees C, and broad, multiple transitions are observed. From 0.5 less than or equal to x less than or equal to 0.9, the apparent molar heat capacity of the liquid-crystal phase increases to that of the defected rippled gel phase. The value of delta Hf approaches the heat of fusion for 2 mol of n-octadecane, suggesting that benzene uncouples the liquid-crystalline acyl chains. The lipids affected by benzene or "boundary lipids" have higher heat capacity than nonperturbed lipids. The apparent molar specific heat, Cp, of 1,2-distearoyl-sn-glycero-3-phosphorylcholine (and 1,2-dipalmitoyl-sn-glycero-3-phosphorylcholine) multilamellar vesicles is 0.20 +/- 0.05 kcal/(mol. deg) in the L beta', P beta, and L alpha phases. Cp fluctuates about this value in all three phases upon repeated phase transitions in the same sample. However, the value of Cp in the P beta (rippled) phase exhibits much greater fluctuations in Cp than that in the L alpha phase. We attribute these fluctuations to crystal packing defects.

Directed Assembly of Sub‐Nanometer Thin Organic Materials with Programmed‐Size Nanopores

“…We loaded a 1:1 mixture of 4‐ tert ‐butylstyrene and p‐ divinylbenzene in the bilayer interior and determined the molar ratio of monomers to lipids to be approximately 0.9, by extraction with hexane followed by gas chromatography (see Supporting Information). This ratio correlates well with previously reported maximum molar ratios of approximately 0.9 for alkanes in multilamellar liposomes,10 0.9 for benzene11 in multilamellar liposomes, and 0.85 for styrene in surfactant admicelles 12. The UV‐initiated polymerization was complete, as evidenced by the absence of unreacted monomers determined by gas chromatography.…”

Directed Assembly of Sub‐Nanometer Thin Organic Materials with Programmed‐Size Nanopores

“…extraction with hexane followed by gas chromatography (see Supporting Information). This ratio correlates well with previously reported maximum molar ratios of approximately 0.9 for alkanes in multilamellar liposomes, [10] 0.9 for benzene [11] in multilamellar liposomes, and 0.85 for styrene in surfactant admicelles. [12] The UV-initiated polymerization was complete, as evidenced by the absence of unreacted monomers determined by gas chromatography.…”

Increased adhesion between neutral lipid bilayers: interbilayer bridges formed by tannic acid

“…Negative staining of LUVs of EPC was accomplished by placing a drop of the dispersion (2 mg/ml) on a carbon-formvar grid (400 mesh), blotting with filter paper, adding a drop of a 2% uranyl acetate, and blotting again with filter paper. Freeze-fractured samples of EPC LUVs were prepared as described previously (Borovyagin and Sabelnikov, 1989;McDaniel et al, 1982). Dispersions were applied to copper plates and frozen at -1900C with liquid nitrogen-cooled propane.…”