A method for characterization of sea surface microlayer based on monolayer properties in presence and absence of phospholipids

Electrochemical impedance spectroscopy, ac voltammetry and fractal analysis were used to characterize model compounds, compound mixtures and extracted samples of sea surface microlayer (ssm) und underlying water (ulw). The reasons for carrying out this work were to use the scientific basis of these characterizations in future on-line analytical procedures of ssm. The mercury (Hg) drop electrode uncoated and coated with a monolayer of dioleoyl phosphatidylcholine (DOPC) was used as an experimental basis for investigation of the major sea surface film forming material. Firstly, the interaction of the uncoated and DOPC coated Hg electrode with model water insoluble compounds of increasing polarity was investigated. The compounds studied in order of increasing polarity were: nonadecane, stearic acid, cholesterol and cardiolipin. Subsequently the electrochemical response of the system to different ssm extracts was compared to signals observed with model compounds to demonstrate method selectivity. From the electrochemical results, it is observed that both the molecular structure and polarity of the investigated compounds have a role in their interaction with the uncoated and DOPC coated electrode. In the fractal analysis the increase above 2 of fractal dimension D imparted to the DOPC layer is related to the degree of apolarity of the additive model compound. Consistent with this, the more apolar hexane extracted ssm 2 imparts a fractal dimension D value of 2.45 when incorporated in DOPC layers. The electrochemical response to the ssm and ulw follows that characteristic of sterol compounds.     

Effect of membrane composition on surface states of ganglioside GM1/dipalmitoylphosphatidylcholine/dioleoylphosphatidylcholine monolayers

The surface states of ganglioside GM1 (GM1)/dipalmitoylphosphatidylcholine (DPPC)/dioleoylphosphatidylcholine (DOPC) monolayers having various compositions were investigated using atomic force microscopy (AFM), and the effect of the composition on the surface states of the membrane was examined. The AFM images for the ternary system showed a DPPC-rich phase containing GM1 in the DOPC matrix, which indicated that the morphology varied as the composition of the monolayers changed. The AFM images for the GM1/DPPC/DOPC monolayers having (2:9:9) and (4:18:9) molar ratios showed a percolation pattern similar to that observed for the GM1/DPPC (1:9) monolayer. The AFM image for the GM1/DPPC/DOPC (2:18:9) monolayer showed a dotted pattern with a high topography. Monolayers having a higher content of DOPC than DPPC and/or having a higher content of GM1 showed dot-like domains in the DPPC-rich phase containing GM1. In conclusion, the surface states of GM1/DPPC/DOPC monolayers changed depending on the composition. These results may be related to a diversity of GM1 in various organs.     

Total and free charge densities on mercury coated with self-assembled phosphatidylcholine and octadecanethiol monolayers and octadecanethiol/phosphatidylcholine bilayers

The thermodynamic ‘total’ charge density is the charge to be supplied to the electrode to keep the applied potential constant when the electrode surface is increased by unity, while the extrathermodynamic ‘free’ charge density is the charge actually experienced by the diffuse layer ions. The total charge density at dioleoylphosphatidylcholine (DOPC) and octadecanethiol (ODT) monolayers and mixed ODT/DOPC bilayers self-assembled on mercury from aqueous solutions was determined from chronocoulometric single potential steps to a final potential negative enough to cause complete desorption of the film. The effect of different alkali metal ions and of tetramethylammonium on DOPC desorption was examined. The total charge for ODT monolayers and ODT/DOPC bilayers, +56±3 μC cm⁻², agrees with the value obtained by integration of the current under the reductive desorption voltammetric peaks, only provided the scan rate is higher than 100 mV s⁻¹. An approximate model of the interface of the ODT-coated electrode, which accounts for partial charge transfer from sulfur to mercury and for the degree of dissociation of the sulfhydryl group upon self-assembly, was employed to estimate the free charge density.     

维生素E与磷脂酰胆碱的单分子膜研究

测定了下列气水界面单分子膜的表面压－平均分子面积等温线：（１）ｄ－α，ｄ－β，ｄ－γ－，和ｄ－δ－生育酚等４种维生素Ｅ与ＤＰＰＣ，ＤＯＰＣ及ＤＬＰＣ的混合物，（２）ｄ－α－生育酚等摩尔比的ＤＰＰＣ和ＤＬＰＣ的混合物，（３）胆固醇与ＤＰＰＣ，ＤＬＰＣ的混合物，讨论了维生素Ｅ色满环上甲基对其在ＰＣ单分子膜中物理化作用的影响，实验结果表明：（１）维生素Ｅ以ｄ－α〉ｄ－β－≈ｄ－γ〉ｄ－δ的次序引起的ＤＰ     

Glycoprotein adsorption into bilirubin/cholesterol mixed monolayers at the air-water interface

The adsorption of α₁-acid glycoprotein into bilirubin/cholesterol mixed monolayers with various component molar ratios is investigated using surface pressure-area (π-A) isotherms and (dπ/dA)-A curves. The results showed that the surface area per molecule increased after the adsorption/insertion of glycoprotein molecules into the monolayers. The compressibility of mixed monolayers increased as a result of hydrogen bonding between bilirubin and glycoprotein molecules, while the interactions between bilirubin and cholesterol are weakened. The adsorption of glycoprotein into a monolayer induced changes in molecular surface area depending on the surface pressure and molar fraction of bilirubin. The transmission electron microscopy of mixed monolayers confirmed the insertion of glycoprotein particles of spherical shape with an average diameter of about 80 nm into the monolayer. The text was submitted by the authors in English.     

An electrochemical study into the interaction between complement-derived peptides and DOPC mono- and bilayers

Electrochemical methods employing the hanging mercury drop electrode were used to study the interaction between variants of the complement-derived antimicrobial peptide CNY21 (CNYITELRRQH ARASHLGLAR) and dioleoyl phosphatidylcholine (DOPC) monolayers. Capacitance potential and impedance measurements showed that the CNY21 analogues investigated interact with DOPC monolayers coating the mercury drop. Increasing the peptide hydrophobicity by substituting the two histidine residues with leucine resulted in a deeper peptide penetration into the hydrophobic region of the DOPC monolayer, indicated by an increase in the dielectric constant of the lipid monolayer (Deltaepsilon = 2.0 after 15 min interaction). Increasing the peptide net charge from +3 to +5 by replacing the histidines by lysines, on the other hand, arrests the peptide in the lipid head group region. Reduction of electroactive ions (Tl+, Pb2+, Cd2+, and Eu3+) at the monolayer-coated electrode was employed to further characterize the types of defects induced by the peptides. All peptides studied permeabilize the monolayer to Tl+ to an appreciable extent, but this effect is more pronounced for the more hydrophobic peptide (CNY21L), which also allows penetration of larger ions and ions of higher valency. The results for the various ions indicate that charge repulsion rather than ion size is the determining factor for cation penetration through peptide-induced defects in the DOPC monolayer. The effects obtained for monolayers were compared to results obtained with bilayers from liposome leakage and circular dichroism studies for unilamellar DOPC vesicles, and in situ ellipsometry for supported DOPC bilayers. Trends in peptide-induced liposome leakage were similar to peptide effects on electrochemical impedance and permeability of electroactive ions for the monolayer system, demonstrating that formation of transmembrane pores alone does not constitute the mechanism of action for the peptides investigated. Instead, our results point to the importance of local packing defects in the lipid membrane in close proximity to the adsorbed peptide molecules.     

Membrane properties of binary and ternary systems of ganglioside GM1/dipalmitoylphosphatidylcholine/dioleoylphosphatidylcholine

The membrane properties of the ganglioside GM1 (GM1)/dioleoylphosphatidylcholine (DOPC) binary system and GM1/dipalmitoylphosphatidylcholine (DPPC)/DOPC ternary system were investigated using surface pressure measurements and atomic force microscopy (AFM), and the effect of surface pressure on the properties of the membranes was examined. Mixed GM1/DPPC/DOPC monolayers were deposited on mica using the Langmuir-Blodgett technique for AFM. GM1 and DOPC were immiscible and phase-separated. The AFM image of the GM1/DOPC (1:1) monolayer showed island-like GM1 domains embedded in the DOPC matrix. There was no morphological change on varying surface pressure. The surface pressure-area isotherm of the GM1/DPPC/DOPC (2:9:9) monolayer showed a two-step collapse as in the DPPC/DOPC (1:1) monolayer. The AFM image for the GM1/DPPC/DOPC monolayer showed DPPC and GM1 domains in the DOPC matrix, and the DPPC-rich phase containing GM1 showed a percolation pattern the same as the GM1/DPPC (1:9) monolayer. The percolation pattern in the GM1/DPPC/DOPC monolayer changed as the surface pressure was varied. The surface pressure-responsive change in morphology of GM1 was affected by the surrounding environment, suggesting that the GM1 localized in each organ has a specific role.     

Atomic force microscopy studies of ganglioside GM1alpha in dioleoylphosphatidylcholine/dipalmitoylphosphatidylcholine mixed monolayers and hybrid bilayers

The membrane states of the alpha-series ganglioside GM1alpha in 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC)/1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) mixed monolayers and hybrid bilayers were investigated using atomic force microscopy (AFM). The AFM image for the GM1alpha/DOPC/DPPC ternary monolayers showed the formation of GM1alpha-raft in the DOPC matrix. As increase of the surface pressure, GM1alpha are condensed in DPPC-rich domains; long and slender GM1alpha-rafts are separated from the DPPC-rich domains into the DOPC matrix. The GM1alpha/DOPC/DPPC ternary monolayers were deposited on mica coated with the first layer (1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine: DPPE) using the Langmuir-Schaeffer technique. The AFM image for the hybrid bilayers showed that same molecules were heterogeneously concentrated according to increase of the surface pressure to form GM1alpha-raft, DPPC-rich domain and DOPC matrix, being in agreement with the observation on the monolayer experiment. The found phenomenon implies that a binding of lectin to GM1alpha causes the increase of the surface pressure, the localization of GM1alpha and the succeeding formation of the raft as a first step of a specific signal transduction.     

Electrochemical modeling of the silica nanoparticle-biomembrane interaction

The interaction of amorphous colloidal silica (SiO(2)) nanoparticles of well-defined sizes with a dioleoyl phosphatidylcholine (DOPC) monolayer on a mercury (Hg) film electrode has been investigated. It was shown using electrochemical methods and microcalorimetry that particles interact with the monolayer, and the electrochemical data shows that the extent of interaction is inversely proportional to the particle size. Scanning electron microscopy (SEM) images of the electrode-supported monolayers following exposure to the particles shows that the nanoparticles bind to the DOPC monolayer irrespective of their size, forming a particle monolayer on the DOPC surface. A one-parameter model was developed to describe the electrochemical results where the fitted parameter is an interfacial layer thickness (3.2 nm). The model is based on the adsorptive interactions operating within this interfacial layer that are independent of the solution pH and solution ionic strength. The evidence implies that the most significant forces determining the interactions are van der Waals in character.     

L-α-二油酸磷酯酰胆碱和芦丁硬脂酸酯混合单分子膜的相容性

黄酮类化合物广泛存在于植物中,具有抗氧化、抗肿瘤和抗病毒等多种生物活性[1-3]。许多研究表明,具有相同苷元的黄酮类化合物比其糖苷具有更优秀的抗氧化活性,这是由于苷元亲脂性强能嵌入生物膜流水层的内核发挥作用,以及糖基的空间位阻减弱了黄酮化合物和生物分子的结合能力[4     

Relaxation phenomena in phospholipid monolayers at the air–water interface

In this work we are concerned with the study of long-term relaxation phenomena in dipalmitoyl phosphatidylcholine (DPPC) and dioleoyl phosphatidylcholine (DOPC) monolayers spread at the air–water interface as a function of the surface pressure and the aqueous phase pH (pH 5, 7, and 9). Long-term relaxation phenomena were determined in an automated Langmuir-type film balance at constant temperature (20 °C). Two kinds of experiments were performed to analyze relaxation mechanisms. In one, the surface pressure (π) was kept constant, and the area (A) was measured as a function of time (θ). In the second, the area was kept constant at monolayer collapse and the surface pressure was decreased. This decrease was measured as a function of time. Various relaxation mechanisms, including monolayer molecular loss by dissolution, collapse, and/or organization/reorganization changes, can be fitted to the results derived from these experiments. These relaxation mechanisms are pH and phospholipid dependent. In the discussion, special attention will be given to the effect of the relaxation phenomena on the hysteresis in π–A isotherms before and after the relaxation experiment. At π lower than the equilibrium spreading pressure (π_e) the relaxation phenomena are mainly due to the loss of DPPC or DOPC molecules by desorption into the bulk aqueous phase. The formation of interfacial macroscopic vesicles, which are dissolved into the bulk phase, makes the phospholipid monolayer molecular loss irreversible. At the collapse point (at π > π_e), the relaxation phenomena may be due either to collapse for DPPC and/or to a complex mechanism including competition between desorption and monolayer collapse for DOPC.     

The effects of substituent grafting on the interaction of pH-responsive polymers with phospholipid monolayers

pH-responsive amphiphilic polymers with suitable graftings have demonstrated highly efficient cell membrane activity and hence are promising applicants for drug-delivery. Grafting the hydrophobic amino acid l-phenylalanine and the hydrophilic methoxy poly(ethylene glycol) amine onto the pendant carboxylic acid moieties of a linear polyamide, poly(l-lysine isophthalamide), can effectively modify the amphiphilicity and conformation of the amphiphilic polymers. Here, the interactions of these polymers with phospholipid monolayers adsorbed on mercury (Hg) electrodes have been studied. AC voltammetry (ACV), rapid cyclic voltammetry (RCV), and electrochemical impedance spectroscopy (EIS) have been applied to monitor phospholipid monolayer associations with different polymer concentrations under different pH values. The polymers interact reversibly with the monolayer shown by altering the monolayer capacitance and inhibiting the phospholipid reorientation in electric field. Polymer grafting enhances the pH-mediated conformational change of the polymers which in turn increases their phospholipid monolayer activity. The most significant monolayer interactions have been observed with the polymer grafted with hydrophobic l-phenylalanine. A low level of PEGylation of the backbone also increases the monolayer activity. The polymer/DOPC interactions have been represented with an impedance model, which takes account of the interaction giving rise to an increase in monolayer capacitance and inhomogeneity and a Debye type dielectric relaxation. The extent of penetration of the polymers into the monolayer is inversely related to the electrical resistance they give rise to during the Debye relaxation. The cell membrane activities of these amphiphilic polymers have been successfully mirrored in this supported DOPC monolayer system, isolating the key parameters for biomembrane activities and giving insight into the mechanism of the interactions. The conclusions from this study provide strategic directions in material design catering to different requirements in biomedical applications.     

Interaction of gramicidin derivatives with phospholipid monolayers

A study of the interaction of gramicidin A (gA), tert-butyloxycarbonyl-gramicidin (g-BOC), and desformyl gramicidin (g-des) with dioleoyl phosphatidylcholine (DOPC) and DOPC/phosphatidylserine (PS) mixed monolayers on a mercury electrode is reported in this paper. Experiments were carried out in electrolytes KCl (0.1 mol dm(-3)) and Mg(NO3)2 (0.05 mol dm(-3)). The channel-forming properties of the gramicidins were studied by following the reduction of Tl(I) to Tl(Hg). The frequency dependence of the complex impedance of coated electrode surfaces in the presence and absence of the gramicidins was estimated between 65,000 and 0.1 Hz at potentials of -0.4 V versus Ag/AgCl with 3.5 mol dm(-3) KCl. Epifluorescence microscopy was used to qualitatively correlate the interaction of the gramicidin peptides with dipalmitoyl phosphatidylcholine (DPPC) and dipalmitoyl phosphatidylglycerol (DPPG) at the air-water interface. gA was shown to form Tl+ conducting channels in a DOPC monolayer, while g-BOC and g-des did not. In DOPC-30% PS (DOPC-0.3PS) layers, there is a marked increase in channel activity of all three gramicidin derivatives. None of the peptides facilitate the permeability of the DOPC-0.3PS layer to Cd2+. All three peptides interact with the layer as shown by capacitance-potential curves and impedance spectroscopy indicated by penetration of the peptide into the dielectric, an increase in surface "roughness", and an increased significance of low-frequency relaxations. The order of interaction is gA > g-des > g-BOC. The epifluorescence study of DPPC and DPPG layers at the air-water interface shows a selective action of the different gramicidins.     

Reduction of oxygen and hydrogen peroxide on electrodes with adsorbed monolayer of aliphatic compounds

Cathodic reduction of oxygen and hydrogen peroxide on amalgamated platinum electrodes, which are coated with monolayers of long-chain aliphatic compounds cetyl alcohol (CA) and stearic acid (SA), is retarded as compared with the same reactions on clean mercury (or amalgam) surface. The oxygen reduction kinetics differ from that on mercury. The difference is explained by that oxygen diffuses into the monolayer and is reduced in it at a certain distance from the metal surface and only at the limiting current the reaction is forced onto the monolayer surface. In contrast to the oxygen reduction, the hydrogen peroxide reduction kinetics on electrodes with SA and CA monolayers is much closer to that on mercury, but with some quantitative distinctions. All results favor the H₂O₂ reduction at the monolayer/solution interface. The difference in the behavior of O₂ and H₂O₂ is explained by different polarity of these molecules: it is significantly more difficult to penetrate the hydrocarbon monolayer for polar H₂O₂ molecule than for nonpolar O₂ molecule.     

Effect of n-alkanes and peptides on the phase equilibria in phosphatidylcholine—water systems

Abstract

The phase equilibria in phosphatidylcholine (PC)-n-alkane-²H₂O systems have been studied to elucidate the driving forces for the transition between a lamellar liquid-crystalline (L _α) phase and a reversed hexagonal (H _II) phase. A tentative phase diagram for the system dioleoyl-PC (DOPC)-n-dodecane-²H₂O was determined. DOPC forms an L _α phase up to at least 90°C in excess water. However, an H _II phase was formed at room temperature at both low and high water concentrations in DOPC-n-dodecane-²H₂O mixtures. The phase equilibria were also studied in PC-n-dodecane-²H₂O systems containing PC with different degrees of acyl chain unsaturation. The water and dodecane concentrations required to induce the formation of an H _II (or isotropic) phase increase in the order dilinoleoyl-PC ～ DOPC < 1-palmitoyl-2-oleoyl-PC < dipalmitoyl-PC. The effect of n-alkanes with different chain lengths (C₈–C₂₀) on the phase equilibria in DOPC-n-alkane-²H₂O mixtures was studied. Although the number of alkane carbon atoms added per DOPC molecule was kept constant, the ability of the alkanes to promote the formation of an H _II phase was strongly chain length dependent; the ability decreased when going from octane to eicosane. Finally, some PC-peptide-²H₂O systems were investigated. Gramicidin (hydrophobic) had a similar influence on the phase equilibria as the alkanes. Melittin (amphiphilic) induced the formation of an isotropic phase, while insulin and duramycin (water soluble) had no, or a very limited, ability to induce a non-lamellar phase, respectively. Our results are discussed in the light of simple physical models dealing with the self-assembly of amphiphiles.     

DC photoelectric signals from bacteriorhodopsin adsorbed on lipid monolayers and thiol/lipid bilayers supported by mercury

Purple membrane (PM) fragments were adsorbed on a dioleoylphosphatidylcholine (DOPC) monolayer and on a mixed alkanethiol/DOPC bilayer supported by mercury to investigate the kinetics of light-driven proton transport by bacteriorhodopsin (bR). The light-on and light-off capacitive currents on an alkanethiol/DOPC bilayer at pH 6.4 were interpreted on the basis of a simple equivalent circuit. The pH dependence of the biphasic decay kinetics of the light-on currents was analyzed to estimate the pK(a) values for the transitions releasing protons to, and taking up protons from, the solution. The linear dependence of the stationary light-on current of bR on a DOPC monolayer self-assembled on mercury upon the applied potential was interpreted on the basis of an equivalent circuit.     

DOPC/DPPC单层膜中分子间的相互作用及形态特征

利用Langmuir-Blodgett(LB)技术制备了不同表面压力下的1,2-二油酸-甘油-3-磷脂酰胆碱(DOPC)/1,2-二棕榈酸甘油-3-磷脂酰胆碱(DPPC)(摩尔比为1:1)和DOPC/DPPC/Chol(摩尔比为2:2:1)单层膜, 对单层膜内分子间的相互作用进行了热力学分析, 并用荧光显微镜和原子力显微镜对其形态进行了观测.热力学分析表明, DOPC与DPPC分子在单层膜结构中相互作用为排斥力, 诱导单层膜出现相变; DOPC, DPPC与胆固醇(Chol)间的相互作用均为吸引力, 当表面压力(π)大于18 mN/m时, DPPC与胆固醇的作用力大于DOPC.荧光显微镜观测表明, DOPC/DPPC单层膜出现明显相分离现象, 富含DPPC微区成“花形”结构, 且随着表面压力的升高微区逐渐增大, “花瓣”增多; 当胆固醇加入到DOPC/DPPC体系时, 单层膜相态由液相与凝胶相共存转变为液态无序相与液态有序相共存结构, 富含DPPC的微区形状从“花形”转变成“圆形”.原子力显微镜对单层膜的表征验证了荧光显微镜的观测结果, 表明胆固醇加入到DOPC/DPPC体系中对单层膜排列具有明显的影响, 压力和溶液状态等是影响脂膜结构的重要因素.     

Effect of hydrocarbon chain and pH on structural and topographical characteristics of phospholipid monolayers

Structural characteristics (structure, elasticity, topography, and film thickness) of dipalmitoyl phosphatidylcholine (DPPC) and dioleoyl phosphatidylcholine (DOPC) monolayers were determined at the air-water interface at 20 degrees C and pH values of 5, 7, and 9 by means of surface pressure (pi)-area (A) isotherms combined with Brewster angle microscopy (BAM) and atomic force microscopy (AFM). From the pi-A isotherms and the monolayer elasticity, we deduced that, during compression, DPPC monolayers present a structural polymorphism at the air-water interface, with the homogeneous liquid-expanded (LE) structure; the liquid-condensed structure (LC) showing film anisotropy and DPPC domains with heterogeneous structures; and, finally, a homogeneous structure when the close-packed film molecules were in the solid (S) structure at higher surface pressures. However, DOPC monolayers had a liquid-expanded (LE) structure under all experimental conditions, a consequence of weak molecular interactions because of the double bond of the hydrocarbon chain. DPPC and DOPC monolayer structures are practically the same at pH values of 5 and 7, but a more expanded structure in the monolayer with a lower elasticity was observed at pH 9. BAM and AFM images corroborate, at the microscopic and nanoscopic levels, respectively, the same structural polymorphism deduced from the pi-A isotherm for DPPC and the homogeneous structure for DOPC monolayers as a function of surface pressure and the aqueous-phase pH. The results also corroborate that the structural characteristics and topography of phospholipids (DPPC and DOPC) are highly dependent on the presence of a double bond in the hydrocarbon chain.     

Lipolysis of didecanoyl-lecithin/triolein mixed monolayers by phospholipaseA 2

A study of enzyme lipolysis by pancreatic phospholipaseA ₂ and by vipera berus phospholipaseA ₂ on monomolecular mixed films of didecanoyl-lecithin and triolein on an aqueous subphase of pH 8 has been carried out. The influence of the composition of the mixed film, the surface pressure of the film and the amount and type of the injected enzyme on the lipolysis rate were studied.In order to relate the lipolytic activity with the monolayer state, the compression isotherms of the didecanoyl-lecithin/triolein mixed monolayers have also been obtained.The resuls are compared to observations on lipolytic activity of phospholipaseA ₂ on the didecanoyl-lecithin/cholesterol mixed monolayers. Triolein improves the kinetic conditions of the lipolysis of lecithin films in a higher degree than cholesterol. Probably it increases the enzyme penetration by the fluidifying effect exerted on the lecithin monolayers.     

The magnitude of condensation induced by cholesterol on the mixtures of sphingomyelin with phosphatidylcholines-Study on ternary and quaternary systems

The studies on the condensing and ordering effect of cholesterol by application of the Langmuir monolayer technique are usually performed on binary lipid/cholesterol systems. The results concerning a quantitative analysis of these effects in multicomponent monolayers are very limited. In this work the condensing and ordering effect of cholesterol in ternary (SM/DSPC/Chol and SM/DOPC/Chol) and quaternary (SM/DSPC/DOPC/Chol) films was investigated. It was evidenced that the systems containing saturated PC (both SM/DSPC and SM/DSPC/Chol) are always more condensed and chain-ordered than the systems containing unsaturated PC (SM/DOPC and SM/DSPC/DOPC and their mixtures with cholesterol). However, the magnitude of condensation provoked by cholesterol at higher surface pressures is stronger on the monolayers containing unsaturated PC. The addition of cholesterol into SM/PC films induces the increase of chain-ordering however, the effectiveness of cholesterol as an ordering agent is determined by the presence/absence of unsaturated phospholipid. The magnitude of the effect of cholesterol on the investigated mixed monolayer was analyzed in the context of the influence of sterol on lipid chains (ordering, straightening and reorientation of chains) as well as the reorientation of polar heads.