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.degree. 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.Hf, 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 < x [mole fraction in the organic phase] < 0.50, benzene increases the apparent molar heat capacity, Cp, of the gel phase to about 1.2 kcal/(mol .cntdot. deg). This increase in heat capacity is probably 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.degree. C, and broad, multiple transitions are observed. From 0.5 .ltoreq. x .ltoreq. 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 capacity than nonperturbed lipids. The apparent molar specific heat, Cp, of 1,2-distearoyl-sn-glycero-3-phosphorylcholine [DSPC] (and 1,2-dipalmitoyl-sn-glycero-3-phosphorylcholine) multilamellar vesicles is 0.20 .+-. 0.05 kcal/(mol .cntdot. deg) in the L.beta.'', P.beta., and L.alpha. phases. Cp fluctuates about this value in all 3 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. These fluctuations are attributed to crystal packing defects.