Interaction of gentamicin with phosphatidylserine/phosphatidylcholine mixtures in adsorption monolayers and thin liquid films: morphology and thermodynamic properties

Gentamicin possesses strong adverse actions like oto and nephrotoxicity. The latter is a result of strong gentamicin–acid phospholipid interactions, resulting in cell fusion, fission, etc., ions as calcium interact with gentamicin and effectively deter its toxicity. In this work, the interactions of gentamicin and Ca²⁺ with phosphatidylserine/phosphatidylcholine (PS/PC) mixtures of different ratio are experimentally characterized. Special attention is paid to bridge thermodynamic and morphological properties of adsorption monolayers and thin liquid films (TLFs) composed of these lipid mixtures. Our results show that gentamicin decreases the stability of common black TLFs formed of pure PS coupled with suppression of lipid surface adsorption to the monolayers at the air–water interface; also, gentamicin reveals effects of lowering of lipid spreading on the interface and significant loss of material during monolayer cycling, increase of condensed phase, and organization of dense net-like domain monolayer texture. Gentamicin addition results in opposite effects for films formed of DPPC/PS (95:5) mixture. It increases the stability of Newton black TLFs formed by DPPC/PS correlated with faster and stronger surface adsorption and better surface spreading; also, gentamicin lowers the amount of condensed phase and organization of domains of smaller size. We also showed that Ca²⁺ itself decreases the stability of common black TLFs formed of PS accompanied with weaker surface adsorption, formation of higher amounts of condensed phase and organization of domains. In our experiments, Ca²⁺ softens, even deters, the effects of gentamicin on both PS and DPPC/PS films.     

Effects of specific versus nonspecific ionic interactions on the structure and lateral organization of lipopolysaccharides

We report x-ray reflectivity and grazing incidence x-ray diffraction measurements of lipopolysaccharide (LPS) monolayers at the water-air interface. Our investigations reveal that the structure and lateral ordering of the LPS molecules is very different from phospholipid systems and can be modulated by the ionic strength of the aqueous subphase in an ion-dependent manner. Our findings also indicate differential effects of monovalent and divalent ions on the two-dimensional ordering of lipid domains. Na⁺ ions interact unspecifically with LPS molecules based on their ability to efficiently screen the negative charges of the LPS molecules, whereas Ca²⁺ ions interact specifically by cross-linking adjacent molecules in the monolayer. At low lateral pressures, Na⁺ ions present in the subphase lead to a LPS monolayer structure ordered over large areas with high compressibility, nearly hexagonal packing of the hydrocarbon chains, and high density in the LPS headgroup region. At higher film pressures, the LPS monolayer becomes more rigid and results in a less perfect, oblique packing of the LPS hydrocarbon chains as well as a smaller lateral size of highly ordered domains on the monolayer. Furthermore, associated with the increased surface pressure, a conformational change of the sugar headgroups occurs, leading to a thickening of the entire LPS monolayer structure. The effect of Ca²⁺ ions in the subphase is to increase the rigidity of the LPS monolayer, leading to an oblique packing of the hydrocarbon chains already at low film pressures, an upright orientation of the sugar moieties, and much smaller sizes of ordered domains in the plane of the monolayer. In the presence of both Na⁺- and Ca²⁺ ions in the subphase, the screening effect of Na⁺ is predominant at low film pressures, whereas, at higher film pressures, the structure and lateral organization of LPS molecules is governed by the influence of Ca²⁺ ions. The unspecific charge-screening effect of the Na⁺ ions on the conformation of the sugar moiety becomes less dominant at biologically relevant lateral pressures.     

Lateral mobility of phospholipid molecules in thin liquid films

The Fluorescence Recovery After Photobleaching (FRAP) method was applied to measure the lateral mobility of the fluorescent lipid analog, dioctadecylindocarbocyanine perchlorate (Dil-C18), in microscopic thin liquid films (Foam Films (FFs)). The foam film structures were comprised of two phosphatidylcholine monolayers adsorbed at air/water interfaces which sandwiched a thin liquid core. Lateral diffusion of the DiI molecules in the plane of the monolayers was determined as a function of the thickness of the thin liquid core of the film between the FF monolayers. The results obtained indicated that the diffusion coefficient was strongly dependent both on the distance between the FF monolayers in the range 4 nm to 85 nm (corresponding to the FF thickness) and on the film type. The applicability of the FRAP method for studying the molecular mobility in phospholipid FFs was demonstrated. Considerable differences in the surface diffusion coefficient of Dil were observed, ranging between 2 × 10^–8 cm²/s and 22 × 10^–8 cm²/s in so called yellow, gray, common black and Newton black FFs. The effect of the presence of polyethylene glycol (PEG-400) in the liquid core of lecithin FFs on surface diffusion was also studied. The surface diffusion results from the FF studies were compared with data from black lipid membranes (BLMs). These structures are related in thickness terms but the molecular orientation in FFs is the reverse of that in BLMs. Correspondence to: Z. Lalchev     

Divalent cation-induced interaction of phospholipid vesicle and monolayer membranes

The effects of phospholipid vesicles and divalent cations in the subphase solution on the surface tension of phospholipid monolayer membranes were studied in order to elucidate the nature of the divalent cation-induced vesicle-membrane interaction. The monolayers were formed at the air/water interface. Various concentrations of unilamellar phospholipid (phosphatidylserine, phosphatidylcholine and their mixtures) vesicles and divalent cations (Mg²⁺, Ca²⁺, Mn²⁺, etc.) were introduced into the subphase solution of the monolayers. The changes of surface tension of monolayers were measured by the Wilhelmy plate (Teflon) method with respect to divalent ion concentrations and time.When a monolayer of phosphatidylserine and vesicles of phosphatidylserine/phosphatidylcholine (1 : 1) were used, there were critical concentrations of divalent cations to produce a large reduction in surface tension of the monolayer. These concentrations were 16 mM for Mg²⁺, 7 mM for Sr²⁺, 6 mM for Ca²⁺, 3.5 mM for Ba²⁺ and 1.8 mM for Mn²⁺. On the other hand, for a phosphatidylcholine monolayer and phosphatidylcholine vesicles, there was no change in surface tension of the monolayer up to 25 mM of any divalent ion used. When a phosphatidylserine monolayer and phosphatidylcholine vesicles were used, the order of divalent ions to effect the large reduction of surface tension was Mn²⁺ > Ca²⁺ > Mg²⁺ and their critical concentrations were in between the former two cases. The threshold concentrations also depended upon vesicle concentrations as well as the area/molecule of monolayers. For phosphatidylserine monolayers and phosphatidylserine/phosphatidylcholine (1 : 1) vesicles, above the critical concentrations of Mn²⁺ and Ca²⁺, the surface tension decreased to a value close to the equilibrium pressure of the monolayers within 0.5 h.This decrease in surface tension of the monolayers is interpreted partly as the consequence of fusion of the vesicles with the monolayer membranes. The     

Effect of Mg<Superscript>2+</Superscript> versus Ca<Superscript>2+</Superscript> on the behavior of Annexin A5 in a membrane-bound state

Annexin A5 (AnxA5) binds to negatively charged phospholipid membranes in a Ca²⁺ dependent manner. Several studies already demonstrate that Mg²⁺ ions cannot induce the binding. In this paper, quartz crystal microbalance with dissipation monitoring (QCM-D), Brewster angle microscopy (BAM), polarization modulation infrared reflection absorption spectroscopy (PMIRRAS) and molecular dynamics (MD) were performed to elucidate the high specificity of Ca²⁺ versus Mg²⁺ on AnxA5 binding to membrane models. In the presence of Ca²⁺, AnxA5 showed a strong interaction with lipids, the protein is adsorbed mainly in α-helix under the DMPS monolayer, with an orientation of the α-helices axes slightly tilted with respect to the normal of the phospholipid monolayer as revealed by PMIRRAS. The Ca²⁺ ions interact strongly with the phosphate group of the phospholipid monolayer. In the presence of Mg²⁺, instead of Ca²⁺, no interaction of AnxA5 with lipids was detected. Molecular dynamics simulations allow us to explain the high specificity of calcium. Ca²⁺ ions are well exposed and surrounded by labile water molecules at the surface of the protein, which then favour their binding to the phosphate group of the membrane, explaining their specificity. To the contrary, Mg²⁺ ions are embedded in the protein structure, with a smaller number of water molecules strongly bound. We conclude that the embedded Mg²⁺ ions inside the AnxA5 structure are not able to link the protein to the phosphate group of the phospholipids for this reason.     

Comparative study of the interaction of synthetic methionine-enkephalin and its amidated derivate with monolayers of zwitterionic and negatively charged phospholipids

Using Langmuir’s monolayer technique, the surface behavior and the interaction of the synthetic neuropeptide methionine-enkephalin (Met-enk) and its amidated derivate (Met-enk-NH₂) with monolayers of the zwitterionic dimyristoylphosphatidylcholine (DMPC) and the negatively charged dimyristoylphosphatidylglycerol (DMPG) were studied. The surface tension (γ, mN/m) of DMPG and DMPC monolayers as a function of time (after injection of the peptide under the interface) was detected. The decrease in γ values showed that there was a strong penetration effect of both types of Met-enk molecules into the monolayers, being significantly stronger for the amidated derivate, Met-enk-NH₂. We suggest that the interaction between the neuropeptides and DMPC was predominantly determined by peptides amphiphilicity, while the electrostatic forces play significant role for the insertion of the cationic Met-enk-NH₂ in DMPG monolayers, especially at high packing densities. Our results demonstrate the potential of lipid monolayers formed in Langmuir’s trough to be successfully used as an elegant and simple membrane models to study lipid–peptide interactions at the air/water interface.     

Phase diagram of mixed monolayers of stearic acid and dimyristoylphosphatidylcholine. Effect of the acid ionization

The aim of this work is to study the phase diagram of mixed monolayers composed of dimyristoylphosphatidylcholine (DMPC) and stearic acid (SA) at different ionic strength and bulk pH of the aqueous subphase. In this way, the effect of ionization of SA on the interaction and thus on phase separation with the DMPC matrix can be analyzed. To this purpose, we first determined the ionization state of pure SA monolayers as a function of the bulk subphase pH. The SA monolayers are nearly fully ionized at pH 10 and essentially neutral at pH 4 and the mixture of DMPC and SA was studied at those two pHs. We found that the DMPC-enriched phase admits more SA if the SA monolayer is in a liquid-expanded state, which is highly related to the acid ionization state, and thus to the bulk pH and ionic strength. At pH 4 the molecules hardly mix while at pH 10 the mixed monolayer with DMPC can admit between 30 and 100% of SA (depending on the lateral pressure) before phase separation is established. The addition of calcium ions to the subphase has a condensing effect on SA monolayers at all pHs and the solubility of SA in the DMPC matrix does not depend on the bulk pH in these conditions. The observed phase diagrams are independent on the manner in which the state of the mixed film is reached and may thus be considered states of apparent equilibrium.     

Thin liquid films and monolayers of DMPC mixed with PEG and phospholipid linked PEG

In this work thin liquid films (TLFs) and monolayers at the air/water interface formed by dimyristoylphosphatidylcholine (DMPC) and by DMPC mixed with poly ethylene glycols (PEGs) and dimyristoylphosphatidylethanolamine (DMPE) linked PEGs were studied. Film forming dispersions were composed of two types of particles: liposomes and micelles. TLFs stability, threshold concentration C _t (i.e., the minimum one for stable film formation), and hydrodynamic behavior were measured. At equivalent conditions, DMPC films were Newton black films (real bilayers), while DMPE-PEGs films were much thicker with free water between the monolayers. DMPE-PEG addition to DMPC films caused both C _t decrease (depending on PEG moiety length and Mw) and change of TLF formation mechanism. TLFs’ hydrodynamic behavior also strongly depended on DMPE-PEG content and Mw. It was observed that thinning of the DMPC and DMPE-PEGs films continued to different film types and thickness, being much thicker for the latter films. Addition of free PEGs (PEG-200/6000) did not alter TLF type or stability, but changed TLF thinning time, confirming that free PEGs with Mw<8000 could not penetrate in the membrane and alter “near-membrane” water layer viscosity. Monolayer studies showed improved formation kinetics of both adsorbed and spread films, decrease of surface tension (equilibrium and dynamic), and of film compression/decompression histeresis area in DMPE-PEGs monolayers compared with DMPC pure films. Our study shows that combining the models of phospholipid TLFs and monolayers provide the opportunity to investigate the properties of membrane surface and to clarify some mechanisms of its interactions with membrane-active agents.     

Aggregate formation in the intercalation of long-chain fatty acid esters into liposomes

Various hydrophobic benzenediacetic esters, the corresponding benzenedipropionic esters, and branched alkyl esters were intercalated into DMPC liposomes, where the molar ratio (n/n) of ester:DMPC was 1:5. In the case of the very long-chain derivatives, double carbonyl peaks were observed in the ¹³C NMR spectrum. This doubling phenomenon was observed only for the carbonyl peaks, whose chemical shift is most sensitive to solvent polarity, and disappeared when the ester:DMPC molar ratio drops below 1:15. This doubling reflects the presence of two populations in these samples: one group includes those molecules which are intercalated within the liposome and feel the polarity corresponding to the liposomal microenvironment; the other consists of aggregates of these long-chain derivatives located in the extra-liposomal aqueous phase.     

Interaction of monolayers of concanavalin A with mono- and polysaccharides A study by means of infrared-attenuated total reflectance spectroscopy

The effects of pH, Mn²⁺ and Ca²⁺ and urea denaturation on the interaction of monolayers of concanavalin A on saline with the polysaccharide dextran B-1355 and the monosaccharides methyl α-d-mannopyranoside and d-galactose have been investigated. Infrared absorption spectra of compressed monolayers of the protein and the protein-dextran complex coated on a germanium plate have been obtained by means of attenuated total reflectance spectroscopy. Except in one case of denaturation, the amide I absorption of concanavalin A peaked around 1631 cm^?1, indicating a predominance of the β-pleated sheet conformation, in agreement with its secondary structure in the solution and crystalline phases. The contribution to the absorbance of the concanavalin A-dextran films at 3300 cm^?1 due to absorption by the O-H stretching modes of the polysaccharide is a measure of its binding. Increasing the pH from 6.1 to 7.5 appreciably reduced the dextran binding, at pH 9.3 the binding was zero. Adding 1 mM Mn²⁺ and Ca²⁺ to the subphase at pH 7.5 restored both the dextran binding and the affinity of concanavalin A for methyl α-d-mannopyranoside to that of the native protein at pH 6.1. At this latter pH, the weak binding of dextran to monolayers of demetallized concanavalin A (apo-concanavalin A) was also restored to that for the native molecule by the addition of these divalents. This indicates the requirement of concanavalin A for these ions to maintain the integrity of the saccharide-binding site. The loss of dextran binding with urea denaturation was also observed. These results parallel those for solutions of the protein, indicating the validity of the monolayer system for the study of these interactions.     

Calcium association with phosphatidylserine Modification by cholesterol and phosphatidylcholine in monolayers and bilayers

We have examined the association of Ca²⁺ with phosphatidylserine/cholesterol and phosphatidylserine/ dimyristoylphosphatidylcholine mixed monolayers using a surface radiocounting technique. No Ca²⁺ association with pure monolayers of the uncharged molecules was observed. The Ca²⁺/phosphatidylserine surface ratio was approximately 1:2 in expanded monolayers of the pure anionic lipid and in phosphatidylserine/phosphatidylcholine mixtures. An increase in surface-associated Ca²⁺ to a number ratio of 1:1 was observed in phosphatidylserine/cholesterol films when the mole fraction of cholesterol was raised to 0.5 and above and the phospholipid number density held constant. We interpret these findings as a prevention of intermolecular salt formation by the sterol. Further support is provided by particle electrophoresis     

Gadolinium Ions Block Mechanosensitive Channels by Altering the Packing and Lateral Pressure of Anionic Lipids

Effects of polyvalent ions on the lateral packing of phospholipids have been known for decades, but the physiological consequences have not been systematically studied. Gd³⁺ is a relatively nonspecific agent that blocks mechano-gated channels with a variable affinity. In this study, we show that the large mechanosensitive channel MscL of Escherichia coli is effectively blocked by Gd³⁺ only when reconstituted with negatively charged phospholipids (e.g., PS). Taking this lead, we studied effects of Gd³⁺ on monolayers and unilamellar vesicles made of natural brain PS, DMPS, and its mixtures with DMPC. In monolayer experiments, we found that μM Gd³⁺ present in the subphase leads to ∼8% lateral compaction of brain PS (at 35 mN/m). Gd³⁺ more strongly shrinks and rigidifies DMPS films causing a spontaneous liquid expanded-to-compact transition to the limiting 40 Ų/mol. Pressure-area isotherms of uncharged DMPC were unaffected by Gd³⁺, and neutralization of DMPS surface by low pH did not produce strong compaction. Upshifts of surface potential isotherms of DMPS monolayers reflected changes in the diffuse double layer due to neutralization of headgroup charges by Gd³⁺, whereas the increased packing density produced up to a 200 mV change in the interfacial dipole potential. The slopes of surface potential versus reciprocal area predicted that Gd³⁺ induced a modest (∼18%) increase in the magnitude of the individual lipid dipoles in DMPS. Isothermal titration calorimetry indicated that binding of Gd³⁺ to DMPS liposomes in the gel state is endothermic, whereas binding to liquid crystalline liposomes produces heat consistent with the isothermal liquid-to-gel phase transition induced by the ion. Both titration curves suggested a K_b of ∼10⁶ M⁻¹. We conclude that anionic phospholipids serve as high-affinity receptors for Gd³⁺ ions, and the ion-induced compaction generates a lateral pressure increase estimated as tens of mN/m. This pressure can “squeeze” the channel and shift the equilibrium toward the closed state.     

Effects of pulmonary surfactant proteins SP-B and SP-C and calcium ions on the surface properties of hydrophobic fractions of lung surfactant

This study focused on two hydrophobic fractions (HF-A and HF-B) isolated from porcine lung surfactant (LS) that had similar phospholipid composition, but HF-A consisted of the hydrophobic LS specific proteins (SP-B and SP-C), in contrast to HF-B. Monolayers spread in a Langmuir trough were formed at the air/water interface of both fractions and the rate of adsorption-desorption and the respreading potential of the LS constituents was studied during six consecutive compression/decompression cycles of the monolayers. By drawing a comparison between the behavior of HF-A and HF-B monolayers on the subphase of 150 mm NaCl, either with or without additional Ca²⁺, we estimated the role of hydrophobic LS proteins and Ca²⁺ ions for LS surface activity. The results demonstrated much higher ability of the HF-A sample, compared to HF-B, to maintain lower surface tension (γ) during monolayer compression and its better respreading capacity during decompression. For instance, at a surface concentration corresponding to 80 Ų per phospholipid molecule, the HF-A monolayers showed a much lower γ _max value (surface tension at 100% of the trough area), being ca. 31.0 mN/m, compared to the HF-B monolayers (γ _max? 62.0 mN/m). The surface tension after compression to 20% of the initial area (γ _min) reached ca. 7.0 and 19.0 mN/m in the HF-A and HF-B monolayers, respectively. Better respreading of the HF-A monolayers compared to the HF-B monolayers was due to the faster adsorption and spreading of LS phospholipids during decompression, facilitated by the hydrophobic proteins. As the phospholipid composition of both fractions was similar, we showed that the hydrophobic surfactant proteins were responsible also for the prevention of the irreversible loss of material from the surface during monolayer compression/decompression. The effects observed demonstrated also that the hydrophobic surfactant proteins were the stronger determinant, compared with Ca²⁺ ions, for the surface tension decrease and respreading of the monolayers during film compression/decompression. For instance, when the HF-A monolayers were spread on a subphase with an additional 5 mm Ca²⁺ ion content, no significant changes were detected in the γ _min and γ _max values between the first and sixth cycle, compared to the monolayers spread on a subphase of 150 mm NaCl only. However, in the absence of positively charged SP-B and SP-C (HF-B sample) in highly compressed monolayers, Ca²⁺ ions were able to cause the effects shown by SP-B and SP-C, although to a less extent. The role of the electrostatic and hydrophobic interactions is discussed for the better respreading of LS components in the presence of LS proteins and Ca²⁺ ions.     

Ion Transport through Monolayers and Interfacial Films

Ion transport through monolayers and through several molecules of thick films at the mercury/water interface is discussed. The permeability of the monolayer is described by a rate constant, k_c. The permeability of a thin but not monomolecular film is expressed as a function of the thickness of the film, the diffusion coefficient of the permeant in the film, and the distribution coefficient between the film and the bulk of the solution. The rate constant k_c is expressed in terms of absolute rate processes. In the absence of specific interactions, the activation energy is composed of three terms: (a) electrostatic interaction between the permeating ion and the charged monolayer, (b) monolayer compression work of forming a hole for passage of the ions, and (c) energy of boundary line formation between the monolayer and the hole. The contribution of the third term is especially marked in condensed monolayers. Ions are bound weakly to the monolayers of the dipolar ion lecithin, which complicates the transport problem in this system. The retardation of oxygen reduction by the lecithin monolayer is of particular interest.     

A monolayer and freeze-etching study of charged phospholipids. I. Effects of ions and pH on the ionic properties of phosphatidylglycerol and lysylphosphatidylglycerol

A comparative study of the ionic properties of phosphatidylglycerol (PG) and lysylphosphatidylglycerol (LPG) has been carried out using monolayer and freeze-etch techniques. It is shown that the ionization state of the PG monolayer is strongly dependent on the subphase ionic strength. Mg⁺⁺ and Ca⁺⁺ induced a marked condensing effect. With Ca⁺⁺ a typical cylindrical structure could be observed by freeze etching, this structure being assumed to be generated by Ca⁺⁺ binding to PG. These phenomena were observed with both didodecanoyl-PG and PG from S. aureus. With respect to LPG it has been shown that the anion MoO₄⁼ gave a strong film condensation of didodecanoyl-LPG monolayers at pH 6 corresponding to a change in the bulk morphology from a transparent gel to a lamellar liposomal structure. A similar decrease in the molecular packing of S. aureus LPG was induced by MoO₄⁼, without a change in the freeze etch morphology of the dispersion. Evidence is presented on the important role that the polar head groups of these charged phospholipids and their ionic environment have on the overall molecular packing. Differential scanning calorimetric measurements demonstrated that the liquid-crystalline to gel transition of didodecanoyl-PG is strongly dependent on the cations in the suspension. These phenomena may be relevant to the physical state of lipids in biological membranes.     

The pH dependence of calcium adsorption onto anionic phospholipid monolayers

The adsorption of ⁴⁵Ca to monolayers of phosphatidylinositol and dicetylphosphoric acid has been measured as a function of subphase pH with simultaneous recordings of surface pressure and interfacial potential. Below pH 3 little calcium was adsorbed and the films are assumed to be unionized. With acid subphases between pH 3 and 6.5 adsorption of calcium occurred initially, but it was then gradually lost due to an ageing process in the films. This time dependent change in the properties of the film was independent of the presence of Ca²⁺, but was dependent on the H⁺ concentration in the subphase; it was however not due to an acid hydrolysis of the monolayer. Ca²⁺ was permanently adsorbed at pH values above 6.5 with an increasing affinity up to pH 11.     

Comparison of the interaction of tomatine with mixed monolayers containing phospholipid, egg sphingomyelin, and sterols

The interaction of the glycoalkaloid tomatine with monolayers of a phospholipid (dimyristoylphosphatidylcholine, DMPC), and sphingolipid (egg sphingomyelin), and cholesterol is compared. Using measurements of the surface pressure response as a function of the subphase concentration of tomatine, interfacial binding constants are estimated for mixed monolayers of DMPC and cholesterol and for those of egg sphingomyelin and cholesterol of mole ratio 7:3. The binding constants obtained suggest a stronger interaction of tomatine with DMPC and cholesterol mixed monolayers, reflecting easier displacement of cholesterol from its interaction with DMPC than from its interaction with egg sphingomyelin. Mixtures of tomatine and cholesterol are found to spread directly at the water-air interface and form stable monolayers, suggesting that cholesterol holds tomatine at the interface despite the absence of observed monolayer behavior for tomatine alone. The interaction of tomatine with DMPC and cholesterol monolayers is found to exhibit a pH dependence in agreement with previously reported results for its interaction with liposomes; in particular, the interaction is much less at pH 5 than at pH 7 or pH 9. It is found that while tomatine interacts strongly with monolayers containing sitosterol, it does not interact with monolayers containing sitosterol glucoside. The response of monolayers of varying composition of DMPC and cholesterol to tomatine is also examined. Brewster angle microscopy (BAM) reveals further evidence for formation of suspected islands of tomatine + cholesterol complexes upon interaction with mixed monolayers of lipid and sterol.     

Interaction of poly(l-lysine) and Ca2+ with stearic acid monolayers

Interaction of poly(l-lysine) and Ca²⁺ with stearic acid monolayers is studied at pH 9.1, 9.9 and 10.7. The competition between the condensation effect of Ca²⁺ and the expansion effect of the protein on the monolayer is seen to depend on surface pressure as well as pH. Ca²⁺ is much less effective in the competition when the poly(l-lysine) penetration into the monolayer is stabilized by electrostatic interactions.     

A physicochemical study of the interaction of phosphatidylinositol with buprenorphine and naloxone

The interactions of two opioid molecules (buprenorphine and naloxone) with phosphatidylinositol and phosphatidylcholine were studied in lipid monolayers at the air-water interface. The influence of Na⁺, Ca²⁺, and Mn²⁺ ions in these interactions has also been determined. Neither buprenorphine nor naloxone influence the ordered state of phosphatidylcholine monolayers. On the contrary, both opioid molecules interact specifically with phosphatidylinositol monolayers. The area/molecule of phosphatidylinositol spread on buprenorphine containing subphases is highly affected by this molecule and also by ions. The phosphatidylinositol/naloxone interactions are rather weak and less affected by ions.Abbreviations PI phosphatidylinositol - PC phosphatidylcholine - Bup Buprenorphine - Nx naloxone - ODS octadecyl silica - k capacity factor - logP partition coefficient between octanol and water     

Direct measurement of interaction forces in free thin liquid films stabilized with phosphatidylcholine

Microscopic foam films from suspensions of small unilamellar dimyristoylphosphatidylcholine (DMPC) vesicles have been obtained. The film formation is facilitated at temperatures above the gel-liquid crystalline transition of DMPC. A detailed study of the dependence of equilibrium thickness of DMPC foam films on electrolyte concentration at constant capillary pressure and a direct measurement of the disjoining pressure isotherm has been carried out. Formation of thick equilibrium horizontal microscopic films stabilized with DMPC at low external pressures and low electrolyte concentrations was found and interpreted as being due to the existence of long-range electrostatic interactions in these films. A diffuse electric layer potential of 36 mV has been calculated. The DMPC films have been compared to films obtained from non-ionic surfactant solutions where the long range electrostatic repulsion is explained as being due to specific adsorption of OH^– at the film interfaces. However, unlike the results obtained for surfactant films, in this study formation of common black films and thinning of the DMPC Newton film with pressure have been observed. Offprint requests to: D. Exerowa