Tannic acid (TA) is a naturally occurring polyphenolic compound that aggregates membranes and neutral phosolipid vesicles and precipitates many proteins. This study analyzes TA binding to lipid membranes and the ensuing aggregation. The optical density of dispersions of phosphatidylcholine (PC) vesicles increased upon the addition of TA and electron micrographs showed that TA caused the vesicles to aggregate and form stacks of tightly packed disks. Solution calorimetry showed that TA bound to PC bilayers with a molar binding enthalpy of -8.3 kcal/mol and zeta potential measurements revealed that TA imparted a small negative charge to PC vesicles. Monolayer studies showed that TA bound to PC with a dissociation constant of 1.5 microM and reduced the dipole potential by up to 250 mV. Both the increase in optical density and decrease in dipole potential produced by TA could be reversed by the addition of polyvinylpyrrolidone, a compound that chelates TA by providing H-bond acceptor groups. NMR, micropipette aspiration, and x-ray diffraction experiments showed that TA incorporated into liquid crystalline PC membranes, increasing the area per lipid molecule and decreasing the bilayer thickness by 2 to 4%. 2H-NMR quadrupole splitting measurements also showed that TA associated with a PC molecule for times much less than 10(-4) s. In gel phase bilayers, TA caused the hydrocarbon chains from apposing monolayers to fully interdigitate. X-ray diffraction measurements of both gel and liquid crystalline dispersions showed that TA, at a critical concentration of about 1 mM, reduced the fluid spacing between adjacent bilayers by 8-10 A. These data place severe constraints on how TA can pack between adjacent bilayers and cause vesicles to adhere. We conclude that TA promotes vesicle aggregation by reducing the fluid spacing between bilayers by the formation of transient interbilayer bridges by inserting its digallic acid residues into the interfacial regions of adjacent bilayers and spanning the interbilayer space.

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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.