Janice M. Smaby

Phosphatidylcholine acyl unsaturation modulates the decrease in interfacial elasticity induced by cholesterol.

The effect of cholesterol on the interfacial elastic packing interactions of various molecular species of phosphatidylcholines (PCs) has been investigated by using a Langmuir-type film balance and analyzing the elastic area compressibility moduli (Cs(-1)) as a function of average cross-sectional molecular area. Emphasis was on the high surface pressure regions (pi > or = 30 mN/m) which are thought to mimic biomembrane conditions. Increasing levels of cholesterol generally caused the in-plane elasticity of the mixed monolayers to decrease. Yet, the magnitude of the cholesterol-induced changes was markedly dependent upon PC hydrocarbon structure. Among PC species with a saturated sn-1 chain but different sn-2 chain cis unsaturation levels [e.g., myristate (14:0), oleate (18:1delta9(c), linoleate (18:2delta9,12(c), arachidonate (20:4delta5,8,11,14(c), or docosahexenoate (22:6delta4,7,10,13,16,19(c)], the in-plane elasticity moduli of PC species with higher sn-2 unsaturation levels were less affected by high cholesterol mol fractions (e.g., >30 mol %) than were the more saturated PC species. The largest cholesterol-induced decreases in the in-plane elasticity were observed when both chains of PC were saturated (e.g., di-14:0 PC). When both acyl chains were identically unsaturated, the resulting PCs were 20-25% more elastic in the presence of cholesterol than when their sn-1 chains were long and saturated (e.g., palmitate). The mixing of cholesterol with PC was found to diminish the in-plane elasticity of the films beyond what was predicted from the additive behavior of the individual lipid components apportioned by mole and area fraction. Deviations from additivity were greatest for di-14:0 PC and were least for diarachidonoyl PC and didocosahexenoyl PC. In contrast to Cs(-1) analyses, sterol-induced area condensations were relatively unresponsive to subtle structural differences in the PCs at high surface pressures. Cs(-1) versus average area plots also indicated the presence of cholesterol concentration-dependent, low-pressure (<14 mN/m) phase boundaries that became more prominent as PC acyl chain unsaturation increased. Hence, area condensations measured at low surface pressures often do not accurately portray which lipid structural features are important in the lipid-sterol interactions that occur at high membrane-like surface pressures.

Sphingomyelin Interfacial Behavior: The Impact of Changing Acyl Chain Composition ☆

Sphingomyelins (SMs) containing homogeneous acyl chains with 12, 14, 16, 18, 24, or 26 carbons were synthesized and characterized using an automated Langmuir-type film balance. Surface pressure was monitored as a function of lipid molecular area at constant temperatures between 10 degrees C and 30 degrees C. SM containing lauroyl (12:0) acyl chains displayed only liquid-expanded behavior. Increasing the length of the saturated acyl chain (e.g., 14:0, 16:0, or 18:0) resulted in liquid-expanded to condensed two-dimensional phase transitions at many temperatures in the 10-30 degrees C range. Similar behavior was observed for SMs with lignoceroyl (24:0) or (cerotoyl) 26:0 acyl chains, but isotherms showed only condensed behavior at 10 and 15 degrees C. Insights into the physico-mechanical in-plane interactions occurring within the different SM phases and accompanying changes in SM phase state were provided by analyzing the interfacial area compressibility moduli. At similar surface pressures, SM fluid phases were less compressible than those of phosphatidylcholines with similar chain structures. The area per molecule and compressibility of SM condensed phases depended upon the length of the saturated acyl chain and upon spreading temperature. Spreading of SMs with very long saturated acyl chains at temperatures 30-35 degrees below T(m) resulted in condensed films with lower in-plane compressibilities, but consistently larger cross-sectional molecular areas than the condensed phases achieved by spreading at temperatures only 10-20 degrees below T(m). This behavior is discussed in terms of the enhancement of SM lateral aggregation by temperature reduction, a common approach used during domain isolation from biomembranes.

Surface dipole moments of lipids at the argon-water interface. Similarities among glycerol-ester-based lipids

Surface potential-surface pressure-area isotherms at the argon-buffer interface have been determined for 38 lipid species comprising 19 chemical classes. These lipids all exhibited a finite range of liquid-expanded surface pressure-area behavior. For most species, the linearity of surface potential with reciprocal area was excellent, but nonzero intercepts were obtained. This suggests a lipid-induced reorganization of interfacial water molecules which is area independent. The linearity of the data permits calculation of the surface dipole moment, mu perpendicular, for each lipid. The values of mu perpendicular for a series of oleoyl-containing acylglycerols, dioleoyl phosphatidylcholine, and dioleoyl phosphatidylethanolamine exhibit acylglycerol ester group mu perpendiculars which are generally consistent with known conformational properties of such lipids. The values are 132 mD for the perpendicular oleoyl glycerol-ester group and 252 mD for that in the kinked-chain conformation. Comparison of mu perpendiculars calculated using these values with homologues confirms the approximate independence of mu perpendicular from aliphatic chain length and permits identification of exceptions with possible conformational or orientational differences. Notably, diphytanoyl phosphatidylcholine shows a 45% larger mu perpendicular than predicted. Differences in mu perpendicular among lipid classes allow estimation of the electrical consequences of lipid metabolism and exchange. Calculations show that reactions such as the generation of 1,2-diacylglycerol from diacyl glycerophosphocholine or diacyl glycerophosphoinositol should produce surface potential changes of -127 and +42 mV, respectively. Thus, the two phospholipids are not simply alternative sources of diacylglycerol with respect to processes dependent on surface potential.

Application of a microcomputer-controlled film balance system to collection and analysis of data from mixed monolayers

Abstract A microprocessor-controlled film balance for studying insoluble monolayers at gas—liquid interfaces is described. The combination of component and software architecture employed makes the system versatile and efficient to operate, and yields data with resolution equaling that obtained from analog operation. The availability of data in digital form greatly facilitates analysis of families of isotherms and, in particular, allows direct calculation of derivatives of surface pressure—area isotherms for use in the automated identification of phase transitions. Collection and analysis of the surface pressure—area isotherms of oleyl alcohol—cholesteryl oleate mixtures using this system show that below 8 mN/m they exhibit ideal miscibility. A limiting molecular area for cholesteryl oleate of 82.4A˚2/molecule in the monolayer phase, obtained from phase transition pressure—composition data, is in agreement with an area range of 65–90A2 between 1 and 12.9 mN/m calculated from average area—composition data.

Acyl Chain-Length Asymmetry Alters the Interfacial Elastic Interactions of Phosphatidylcholines

Phosphatidylcholines (PCs) with stearoyl (18:0) sn-1 chains and variable-length, saturated sn-2 acyl chains were synthesized and investigated using a Langmuir-type film balance. Surface pressure was monitored as a function of lipid molecular area at various constant temperatures between 10 degrees C and 30 degrees C. Over this temperature range, 18:0-10:0 PC displayed only liquid-expanded behavior. In contrast, di-14:0 PC displayed liquid-expanded behavior at 24 degrees C and 30 degrees C, but two-dimensional phase transitions were evident at 20 degrees C, 15 degrees C, and 10 degrees C. The average molecular area of 18:0-10:0 PC was larger than that of liquid-expanded di-14:0 PC at equivalent surface pressures, and the shapes of their liquid expanded isotherms were somewhat dissimilar. Analysis of the elastic moduli of area compressibility (Cs(-1)) as a function of molecular area revealed shallower slopes in the semilog plots of 18:0-10:0 PC compared to di-14:0 PC. At membrane-like surface pressures (e.g., 30 mN/m), 18:0-10:0 PC was 20-25% more elastic (in an in-plane sense) than di-14:0 PC. Other PCs with varying degrees of chain-length asymmetry (18:0-8:0 PC, 18:0-12:0 PC, 18:0-14:0 PC, 18:0-16:0 PC) were also investigated to determine whether the higher in-plane elasticity of fluid-phase 18:0-10:0 PC is a common feature of PCs with asymmetrical chain lengths. Two-dimensional phase transitions in 18:0-14:0 PC and 18:0-16:0 PC prevented meaningful comparison with other fluid-phase PCs at 30 mN/m. However, the Cs(-1) values for fluid-phase 18:0-8:0 PC and 18:0-12:0 PC were similar to that of 18:0-10:0 PC (85-90 mN/m). These values showed chain-length asymmetrical PCs to have 20-25% greater in-plane elasticity than fluid-phase PCs with mono- or diunsaturated acyl chains.

The suitability of nichrome for measurement of gas-liquid interfacial tension by the wilhelmy method

Abstract Determination of surface tension by the Wilhelmy method is facilitated by having a zero contact angle between the Wilhelmy plate and the liquid phase. Compared to filter paper or platinum, nichrome was shown to exhibit minimal hysteresis between advancing and receding contact angles under several experimental conditions. Calibration with a series of solvents of known surface tension indicates that the receding contact angle is zero. Comparison of equilibrium spreading pressures for several lipids measured by the Wilhelmy and Langmuir methods showed proportionality with a slope near 1.0. Thus, nichrome is superior to other commonly used materials for measurements of surface tension at the gas-liquid interface.

Galactosylceramides with homogeneous acyl chains: the effect of acyl structure on intermolecular interactions occuring at the argon/buffered saline interface

Abstract Galactosylceramides (GalCers) containing homogeneous acyl chains with no, one or two double bonds were synthesized and characterized at the argon-buffered saline interface using a Langmuir film balance. Surface pressure was monitored as a function of molecular area at various fixed temperatures between 10 and 30†C. In this temperature range, isotherms of GalCer containing palmitoyl acyl chains show condensed behavior. Calculated compressibility values verify the liquid-condensed nature of the films. Replacement of the long saturated acyl chains with long monounsaturated residues dramatically modifies surface behavior. N-nervonoyl galactosylsphingosine (N-24:1Δ15 GalSph) and N-docosenoyl galactosylsphingosine (N-22:1Δ13 GalSph) show discontinuities in thier force-area isotherms (24°C) at 10 and 35 mN m−1 respectively. Compressibility data are consistent with the discontinuities being transitions from liquid-expanded to condensed monolayer states. As the overall chain length is decreased while keeping the cis double bond nine carbons from the terminal methyl group, progressively lower temperatures are needed to induced the surface phase transition. GalCer species with eicosenoyl and oleoyl acyl chains (e.g. N-20:1Δ11 GalSph, N-18:1Δ9 GalSph) show the tow-dimensional phase transition only if the temperature is lowered to the 10–15°C range. Changing the stereochemical configuration of the double bond without changing its position (e.g. N-18:1Δ9(t) GalSph) rigidifies the film and is energetically equivalent to lowering temperature by about 30°C. Introducing acyl chains that are short and saturated (e.g. N-10:0 GalSph) or that are long but contain two cis double bonds (e.g. N-18:2Δ9,12 GalSph) causes GalCer to display only liquid-expanded behavior over the entire temperature range (10–30°C) along with accompanying increases in compressibility, “lift-off” area and apparent collapse area. The results help to explain why changes in the acyl composition of membrane sphingolipids can be so disruptive in demyelinating diseases such as adrenoleukodystrophy.

Use of a microcomputer controlled film balance to study the behavior of unsaturated esters of cholesterol at the air/water interface

Abstract Using a microcomputer controlled Langmuir film balance, we have studied the behavior of cholesteryl myristoleate, oleate, linoleate, linolenate and arachidonate in mixtures with dioleoyl lecithin (PC). Software for analyzing surface pressure-area data was developed to detect phase transitions and to help determine both the number and type of surface phases formed and the state(s) of the cholesteryl esters (CEs) in each phase. Each of the CEs formed mixed surface films with PC and, for all but cholesteryl arachidonate, two surface phases are formed. The phase common to all CEs is monomolecular with the CE occupying an area equivalent to cholesterol plus fatty acid with some PC-CE condensation. Depending on surface pressure, its composition ranges from 0 to 0.5 mole fraction CE for all but cholesteryl arachidonate which is miscible in all proportions. The second mixed phase is multimolecular and exists in two miscible states. The preferentially formed state is comprised of CE and PC in an interfacial, monolayer-like configuration covered with a continuum of CE. At high CE/PC ratios and low pressures, a second state resembling a CE monolayer covered with CE as described above is also found. Overall, our data suggested that in vivo finite amounts of cholesteryl esters should be available at the lipid/water interface and that their absolute concentration and state will depend on both the acyl structure of the cholesteryl ester and the packing density of surface lipids. In particular, comparison of the phase behaviur of cholesteryl arachidonate with other unsaturated cholesteryl esters shows that in the monolayer phase it is available in higher concentrations than in other cholesteryl esters. This may be important in tissues in which it serves as a source of arachidonic acid.