Lipid monolayers: Surface viscosity of dipalmitoyl lecithin in relation to surface potential and ion binding

Interaction of an organic cation with Gibbs monolayers of n-hexadecyl phosphate

Surface phase behavior of n-hexadecyl phosphate (n-HDP) and its mixture with L-arginine (L-arg), which behaves as L-argininium cation (L-arg(+)) in aqueous solution, at a molar ratio 2:3 in Gibbs adsorption layers has been studied by film balance, Brewster angle microscopy (BAM) and surface tensiometry at 20 degrees C. The monolayers of n-HDP show three phases that are gas (G), intermediate (I) and liquid condensed (LC), and two phase transitions. A first-order G-I phase transition that is followed by a second-order I-LC phase transition is found in these monolayers. Although the monolayers of the mixtures containing n-HDP and L-arg show three phases, the nature of the middle phase is different from that of the n-HDP monolayers. The three phases observed for the mixed systems are G, liquid expanded (LE) and LC phases. A first-order G-LE phase transition is found at a low surface pressure at > or =10 degrees C. This transition is followed by another first-order LE-LC phase transition at a certain higher surface pressure. The first-order nature of the phase transitions for both the systems is confirmed by the presence of plateaus in the pi-t curves, which are accompanied by two surface phases. A second-order phase transition in the monolayers of n-HDP is indicated by a gradual change in the surface morphology, from a uniformly bright isotropic to an anisotropic mosaic textured phase, which is accompanied by a continuous change in the surface pressure. The domains formed during the first-order phase transition in the adsorption layers of n-HDP are circular and remain unaffected by changing the temperature. Although the domains of an LE phase are circular, those of an LC phase at the latter transition are fractal in the mixed system. A further branching of the arms of the fractal domains is found to occur by an increase in the temperature. All the results are explained by considering salt formation between anion from n-HDP and L-arg(+).

Laser Raman studies of molecular interactions with phosphatidylcholine multilayers. II. Effects of mono- and divalent ions on bilayer structure

Laser Raman spectroscopy was applied to characterize structural behavior of dipalmitoyl phosphatidylcholine multibilayer systems in the presence of several cations (K+, Na+, Cs+, Rb+, Ca2+, Mg2+, Cd2+, Ba2+) and anions Cl-, Br-, I-, NO-3, SO2-3, SO2-4, S2O2-3, S2O2-8). To evaluate the Raman-spectroscopical data quantitatively, characteristic intensity ratios, lateral and trans order parameters were used and compared. It was shown that the different trans order parameters are rather sensitive to ion-polar head group interactions and thus, they cannot give unequivocal information on the trans-gauche isomerization of hydrocarbon chains of phospholipids. The observed effects of ions on Raman spectra of phospholipid multilayers could not be explained simply on the basis of electrostatic interactions. The possible involvement of other factors (changes in polarizability, hydrogen bonds, etc.) is also discussed. It was demonstrated that the order parameters defined in different ways may result in different effectiveness sequences of ions. Of monopositive ions Na+ was found to be the most effective to influence the bilayer structure. For dipositive ions, of which Ca2+ proved to be the most effective, concentration-dependent effectiveness sequences were obtained. A plausible interpretation and some consequences of the concentration-dependent two-step binding of divalent cations were also outlined. Bilayered phospholipid structures turned out to be more responsive to anions than to most cations investigated. Interdependent actions of cations and anions, as well as the possible relevance of the charge distribution on anions are postulated.

Influence of cations, sialic acid and pH on the expansion of films of canine lung surfactant

Sialic acid (14.6 mug/mg protein) was quantitated in the non-cellular material removed from the lung of Beagle dogs by lavage. Sialic acid did not affect the dynamic surface tension properties of either the total alveolar lipid removed by lavage or of its major lipids, dipalmitoyl lecithin (DPL) and dipalmitoyl phosphatidyl glycerol (DPG). The presence of divalent cation (Ca2+, Mg2+, Mn2+ and Zn2+) or a lowered pH in the subphase medium lowered the surface tension during the expansion phase of the total alveolar lipid film when it was compressed and expanded on a Wilhelmy trough. Films of DPL behaved similarly, but no pH effect was observed with DPG monolayers. The cation effect manifested itself in the same direction as the value of the individual stability constants (Zn2+ greater than Mn2+ greater than Mg2+ greater than Ca2+) which suggests ionic binding of the cations to the phosphate group of the phospholipids. A physiological advantage of such an effect may lie in the conservation of the energetically favorable low surface tension state achieved during film compression with a minimum of surfactant lipid.