Maxwell displacement current allows to study structural changes of gramicidin A in monolayers at the air-water interface

We applied methods of measurement Maxwell displacement current (MDC) pressure-area isotherms and dipole potential for analysis of the properties of gramicidin A (gA) and mixed gA/DMPC monolayers at an air-water interface. The MDC method allowed us to observe the kinetics of formation of secondary structure of gA in monolayers at an air-water interface. We showed, that secondary structure starts to form at rather low area per molecule at which gA monolayers are in gaseous state. Changes of the MDC during compression can be attributed to the reorientation of dipole moments in a gA double helix at area 7 nm(2)/molecule, followed by the formation of intertwined double helix of gA. The properties of gA in mixed monolayers depend on the molar fraction of gA/DMPC. At higher molar fractions of gA (around 0.5) the shape of the changes of dipole moment of mixed monolayer was similar to that for pure gA. The analysis of excess free energy in a gel (18( ) degrees C) and in a liquid-crystalline phase (28( ) degrees C) allowed us to show influence of the monolayer structural state on the interaction between gA and the phospholipids. In a gel state and at the gA/DMPC molar ratio below 0.17 the aggregates of gA were formed, while above this molar ratio gA interacts favorably with DMPC. In contrast, for DMPC in a liquid-crystalline state aggregation of gA was observed for all molar fractions studied. The effect of formation ordered structures between gA and DMPC is more pronounced at low temperatures.     

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.     

Interaction of the cationic peptide bactenecin with mixed phospholipid monolayers at the air-water interface

The initial mechanism by which antimicrobial peptides target microbes occurs via electrostatic interactions; however, the mechanism is not well understood. We investigate the interaction of the antimicrobial peptide bactenecin with a 50:50 w:w% 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1,2-dimyristoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (DMPG) phospholipid mixture at the air-water interface with different NaCl concentrations (0.01, 0.05, 0.1, 0.5 M) in the subphase. A larger shift of DPPC:DMPG isotherms was obtained for 0.1 M salt concentration at lower and higher pressures, demonstrating the influence of the negative charge of DMPG molecules and the screening of the electrostatic interaction by the salt concentration. Raman spectroscopy of monolayers demonstrated the presence of cysteine-cysteine bridges in bactenecin loops. The peptide adsorption in DPPC:DMPG monolayers observed by AFM images suggests a self-assembled aggregation process, starting with filament-like networks. Domains similar to carpets were formed and pore structures were obtained after a critical peptide concentration, according to the carpet model.     

Interface functional architecture using mixed thiol monolayers: Effect of the composition and spatial organization on the immobilization of low-molecular-weight ligands

A simple procedure is examined for forming a selective sensor coating using mixed self-assembling thiol monolayers functionalized by molecules of the sulfanilamide antibiotic, sulfamethoxazol (SMX). A model is proposed for the interface processes, which accounts for the complex dependence of the adsorption capacity of the coatings obtained on the surface density of the reactive groups and the low adsorption capacity at high reaction site concentrations. The model was supported by competitive inhibition analysis. __________ Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 44, No. 3, pp. 158–164, May–June, 2008.     

Pure and mixed monolayers of poly-L-methionine and poly-γy-methyl-L-glutamate with 1,2 dioleyl-L-α-phosphatidylcholine at air-water interface

Monolayers of poly-L-methionine (PM) and poly--methyl-L-glutamate (PMG), spread at an air-water interface, exhibit a high compressibility plateau region at 10 dyn/cm and 18 dyn/cm in their respective-A curves. This behaviour is related to a-helical conformation of the polypeptide molecules in the said interface.Comparative studies of the areas per residue observed in the spreading isotherms, with the areas calculated from molecular models, indicate that the polypeptide molecules should be associated in parallel, forming islets of at least 5-helix chains.Mixed monolayers corresponding to the poly-L-methionine (PM)-1,2 dioleyl-L--phosphatidylcholine (DOPC) and poly--methyl-L-glutamate (PMG)-1,2 dioleyl-L--phosphatidylcholine (DOPC) systems, follow, in every case, the additivity rule for monolayer composition, any indicating that its components are insoluble in the surface mixture. In the case of mixed monolayers of PM-DOPC, a certain degree of compatibility is possible among its components, as a modulation of the plateau region is observed in the spreading isotherms, while for the PMG-DOPC system the plateau region appears at the same surface pressure value as the isotherm corresponding to pure PMG polypeptide.Instituto de Química-Física Rocasolano, C.S.I.C. Madrid -6, España     

Structural and topographical characteristics of adsorbed WPI and monoglyceride mixed monolayers at the air-water interface

In this work we have analyzed the structural and topographical characteristics of mixed monolayers formed by an adsorbed whey protein isolate (WPI) and a spread monoglyceride monolayer (monopalmitin or monoolein) on the previously adsorbed protein film. Measurements of the surface pressure (pi)-area (A) isotherm were obtained at 20 degrees C and at pH 7 for protein-adsorbed films from water in a Wilhelmy-type film balance. Since the surface concentration (1/A) is actually unknown for the adsorbed monolayer, the values were derived by assuming that the A values for adsorbed and spread monolayers were equal at the collapse point of the mixed film. The pi-A isotherm deduced for adsorbed WPI monolayer in this work is practically the same as that obtained directly by spreading. For WPI-monoglyceride mixed films, the pi-A isotherms for adsorbed and spread monolayers at pi higher than the equilibrium surface pressure of WPI are practically coincident, a phenomenon which may be attributed to the protein displacement by the monoglyceride from the interface. At lower surface pressures, WPI and monoglyceride coexist at the interface and the adsorbed and spread pi-A isotherms (i.e., the monolayer structure of the mixed films) are different. Monopalmitin has a higher capacity than monoolein for the displacement of protein from the air-water interface. However, some degree of interactions exists between proteins and monoglycerides and these interactions are higher for adsorbed than for spread films. The topography of the monolayer corroborates these conclusions.     

Close-packed monolayers of charged Janus-type nanoparticles at the air-water interface

Two DNA-block copolymers, poly(caprolactone)-DNA and poly(methyl metacrylate)-DNA, were synthesized by conjugation of a short single strand of DNA (12 or 22 mer) to a single reactive group at one end of the synthetic polymer. These polymers self-assemble in water, without the need of any cosolvent, forming micelle-like aggregates that were imaged by TEM. The solution behavior of the bioconjugated polymers was investigated by surface tension measurements. In the direction of dilution, the surface tension was measured using a down-scaled Wilhelmy plate method. To proceed in the reverse direction (concentration), we measured the surface tension of a sessile drop during its evaporation. This latter method was firstly validated using ionic and non-ionic surfactants, including polymeric surfactants. It was then applied to investigate the unimer to micelles transition of the DNA-block copolymers. In all cases, a reversible transition was observed demonstrating the existence of a critical micellar concentration, close to 0.01 mmol L⁻¹ for all the conjugates. The CMC was only slightly influenced by the length of the hydrophilic DNA block.     

Interactions of Pluronics with phospholipid monolayers at the air-water interface

Pluronics are triblock copolymers which are extensively applied excipients shown to interact with cell membranes. The aim of our study was to apply monolayer techniques and epifluorescence microscopy to investigate the interaction behavior between selected Pluronics and phospholipid monolayers which serve as a model of cell membranes. The results showed that Pluronic L61 with hydrophobic proportions much larger than those of F68 demonstrated condensed film-like surface behavior while F68 exhibited more expanded behavior. The increments of surface pressure and the changes of image were more obvious in adding Pluronic L61 than F68 to the subphase of dipalmitoylphosphatidylcholine (DPPC) monolayers, which indicated that the interaction may be related to van der Waals forces and hydrophobic interaction. Pluronics selected with higher hydrophobicities demonstrated larger surface activities and penetration abilities while being added to the subphase of DPPC and dimyristoylphosphatidylcholine (DMPC) monolayers. Pluronic P85 and F68 were found to be squeezed to subphase at higher surface pressures, which may be attributed to their relatively higher hydrophilicities.     

Structural analysis of PEO-PBO copolymer monolayers at the air-water interface

X-ray reflectivity (XR) and grazing incidence X-ray diffraction (GIXD) have been used to examine an oxyethylene-b-oxybutylene (E(23)B(8)) copolymer film at the air-water interface. The XR data were fitted using both a one- and a two-layer model that outputted the film thickness, roughness, and electron density. The best fit to the experimental data was obtained using a two-layer model (representing the oxyethylene and oxybutylene blocks, respectively), which showed a rapid thickening of the copolymer film at pressures above 7 mN/m. The large roughness values found indicate a significant degree of intermixing between the blocks and back up the GIXD data, which showed no long range lateral ordering within the layer. It was found from the electron density model results that there is a large film densification at 7 mN/m, possibly suggesting conformational changes within the film, even though no such change occurs on the pressure-area isotherm at the same surface pressure.     

Adsorption of DNA to octadecylamine monolayers at the air-water interface

In this work, surface properties of octadecylamine (ODA) monolayers in the presence of different concentrations of calf thymus DNA in the aqueous subphase covering a range of 2-8μM have been investigated. The increase of DNA concentration is accompanied by a marked increment in the expansion of the corresponding isotherms. In addition, there is a change in the profile of the isotherms ranging from an abrupt liquid-solid transition for the lipid monolayer on pure water to a slow condensation of the monolayer in a liquid state when DNA is added to the subphase, demonstrating the effective adsorption of the polynucleotide to the long chain amine monolayer. Additional phase transitions appear in the isotherms upon addition of sufficient amount of DNA, revealing the existence of specific processes such as folding or squeezing out of the DNA. This system is, however, highly reversible during compression-expansion cycles due to the strong interaction between the two components. These results are also supported by Brewster Angle Microscopy (BAM) images showing significant changes in the morphology of the film. Integral reflectivity of the BAM microscope has been used to study both isotherms themselves and the kinetic process of DNA inclusion into the lipid-like ODA monolayer. This parameter has been proven to be very effective for quantification of the monolayer processes showing high consistency with the compressibility and kinetics results.     

Mixed monolayers of poly-alanine and lipids at the air-water interface

Summary The mixed monolayers of poly-alanine + stearyl alcohol and poly-alanine + cholesterol were studied at the air-water interface. In the mixed monolayers the surface pressure-area isotherms showed three collapse states. The first and the third collapse pressures were identical in magnitude with the collapse pressures of pure components. The intermediate collapse pressure in the poly-alanine + stearyl alcohol was found to be ca. 5 dyne/cm higher than that was observed in the poly-alanine + cholesterol system. Further, the mixed films in both systems were found to show no deviation from the ideality rule. The magnitude o f the intermediate collapse state is shown to be related to the van der Waals forces present in the lipid films.With 6 figures     

Structural and shear characteristics of adsorbed sodium caseinate and monoglyceride mixed monolayers at the air-water interface

The structural and shear characteristics of mixed monolayers formed by an adsorbed Na-caseinate film and a spread monoglyceride (monopalmitin or monoolein) on the previously adsorbed protein film have been analyzed. Measurements of the surface pressure (pi)-area (A) isotherm and surface shear viscosity (eta(s)) were obtained at 20 degrees C and at pH 7 in a modified Wilhelmy-type film balance. The structural and shear characteristics of the mixed films depend on the surface pressure and on the composition of the mixed film. At surface pressures lower than the equilibrium surface pressure of Na-caseinate (at pipi(e)(CS) have important repercussions on the shear characteristics of the mixed films.     

Dynamic organization of mixed Langmuir films of glucose oxidase and stearylamine at the air-water interface

The structure and the dynamic organization of a mixed Langmuir film of glucose oxidase and stearylamine at the air–water interface have been studied. The film has been first characterized at the air–water interface by surface pressure/area isotherms. The dynamics of the mixed film was studied by following the evolution of the film area at a constant pressure and the evolution of the pressure at a constant area. After transfer of the films on solid substrates, the chemical composition of the mixed film has been quantified by UV–vis and IR spectroscopies. These characterizations were carried out in order to study the incorporation of glucose oxidase into the stearylamine film, and its influence on the structural evolution of the film. From these results, the dynamic organization of this mixed film may be described. For short times, glucose oxidase molecules interact with stearylamine molecules in solution or at the interface; these interactions would lead to the formation of a complex between stearylamine and glucose oxidase molecules. For long times (at least 3 h), a homogeneous mixed film constituted essentially of this complex is obtained at the air–water interface. A detailed analysis by atomic force microscopy allowed us to support this model and the existence of the glucose oxidase/stearylamine complex.     

Interactions of anthracyclines with zwitterionic phospholipid monolayers at the air-water interface

The present note describes the use of surface pressure measurements (Langmuir monolayer technique) for the analysis of interactions of two different anthracyclines (adriamycin and daunorubicin) with a non-ionic, zwitterionic phospholipid monolayer, at the air-water interface. Because the surface membrane of the cell is the first barrier encountered by the anthracyclines in the treatment of cancer, drug-membrane interactions studied in model (monolayers or bilayers) and natural systems play an important role in the understanding of the bioactivity properties of these molecules. We report here the rate constants of the adsorption process of adriamycin and daunorubicin in the presence of a zwitterionic phospholipid monolayer at the air-water interface. Because interactions with the lipid monolayer strongly depend on the molecular packing of the lipid, we investigated this process at a relatively low surface pressure (7 mN/m), the interactions being favoured by the gaseous and liquid expanded structure of the lipid monolayer. The apparent molecular area of these molecules during the insertion into the lipid film and their interactions with the phospholipid polar head groups was evaluated and the estimated percentage of anthracyclines at the interface after adsorption into the lipid monolayer is briefly discussed. The rate constants for the adsorption and desorption process at the water-monolayer interface have been calculated on the basis of a single-exponential model. The observed difference of these parameters for daunorubicin and adriamycin suggests a different interaction of these anthracyclines during the adsorption to and/or penetration across the phospholipid monolayer.     

The miscibility of dodecyltrihydroxyethylammonium bromide with cationic, nonionic and anionic surfactants in mixed monolayers and micelles

In this paper were analyzed the surface properties of surfactants and the miscibility and interactions between components of adsorbed monolayers and micelles formed from mixed systems. The investigated compounds differ in the structure of the polar head and represented cationic (dodecyltrihydroxyethylammonium bromide—DTEAB, dodecyltrimethylammonium bromide DTMAB), anionic (sodium dodecyl sulfate—SDS), and nonionic (dodecyl-β-d-glucoside—DG) surfactant. The experiments were based on the measurements of the surface tension of the aqueous solutions of the investigated compounds and their mixtures (cationic/nonionic—DTEAB/DG, cationic/cationic—DTEAB/DTMAB and cationic/anionic—DTEAB/SDS). The composition of the mixed films and micelles as well as the free energies of mixing values, which are a measure of the molecular interactions, was calculated basing on the equations resulting from the Motomura theory. The obtained results indicate that all the investigated systems mix nonideally both in the monolayers and micelles. The magnitude of the deviations from ideal behavior is strongly dependent on the type of the investigated mixture and increases in the following order: DTEAB/DTMAB < DTEAB/DG  DTEAB/SDS.     

Structural characteristics of dipalmitoylphosphatidylcholine and hydrolysates from sunflower protein isolate mixed monolayers spread at the air-water interface

In this work, surface film balance and Brewster angle microscopy techniques have been used to analyze the structural characteristics (structure, topography, reflectivity, thickness, miscibility, and interactions) of hydrolysates from sunflower protein isolate (SPI) and dipalmitoylphosphatidylcholine (DPPC) mixed monolayers spread on the air-water interface. The degree of hydrolysis (DH) of SPI, low (5.62%), medium (23.5%), and high (46.3%), and the protein/DPPC mass fraction were analyzed as variables. The structural characteristics of the mixed monolayers deduced from the surface pressure (pi)-area (A) isotherms depend on the interfacial composition and degree of hydrolysis. At surface pressures lower than the equilibrium surface pressure of SPI hydrolysate (pi(e)(SPI hydrolysate)), both DPPC and protein are present in the mixed monolayer. At higher surface pressures (at pi > pi(e)(SPI hydrolysate)), collapsed protein residues may be displaced from the interface by DPPC molecules. The differences observed between pure SPI hydrolysates and DPPC in reflectivity (I) and monolayer thickness during monolayer compression have been used to analyze the topographical characteristics of SPI hydrolysates and DPPC mixed monolayers at the air-water interface. The topography, reflectivity, and thickness of mixed monolayers confirm at microscopic and nanoscopic levels the structural characteristics deduced from the pi-A isotherms.     

Crystalline self-assembly into monolayers of folded oligomers at the air-water interface

Insertion of the 1,3-bis(ethynylene)benzene unit as a rigid spacer into a linear alkyl chain, thus separating the two resulting stems by 9 A. induces chain folding at the air-water interface. These folded molecules self-assemble into crystalline monolayers at this interface, with the plane of the folding unit almost perpendicular to the water surface, as determined by synchrotron grazing-incidence X-ray diffraction. Three distinct molecular shapes, of the types U, inverted U, and M, were obtained in the two-dimensional crystalline state, depending upon the number of spacer units, and the number and position of the hydrophilic groups in the molecule. The molecules form ribbons with a higher crystal coherence in the direction of stacking between the molecular ribbons, and a lower coherence along the ribbon direction. A similar molecule, but with a spacer unit that imposes a 5 A separation between alkyl chains, yields the conventional herringbone arrangement.     

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     

Molecular recognition of cytosine- and guanine-functionalized nucleolipids in the mixed monolayers at the air-water interface and Langmuir-Blodgett films

Molecular recognition of mixed nucleolipids of 1-(2-octadecyloxycarbonylethyl)cytosine and 7-(2-octadecyloxycarbonylethyl)guanine in the monolayers at the air-water interface and Langmuir-Blodgett (LB) films has been investigated in detail using surface pressure/potential-area isotherms, infrared reflection-absorption spectroscopy (IRRAS), and Fourier transform infrared (FTIR) transmission spectroscopy, respectively. Prior to molecular recognition, the cytosine moieties in the monolayer were hydrogen bonded with an almost flat-on orientation, the alkyl chains were uniaxially oriented with respect to the film normal, the guanine moieties in the monolayer were stacked probably through pi-pi interaction with an end-on orientation, and the C-C-C planes of the alkyl chains were preferentially oriented parallel to the water surface. In the monolayer of equimolar mixture, molecular recognition between the cytosine and guanine moieties occurred together with the ring planes of base pairing and the C-C-C planes of the alkyl chains favorably oriented parallel to the water surface. The guanine moieties underwent an orientation change from an end-on mode before molecular recognition to a flat-on one after molecular recognition. The base pairing between the cytosine and guanine moieties in the monolayers was achieved since the N7-substituted guanine derivatives suppressed the formation of guanine tetramers. Both the IRRAS spectra of the monolayers and the FTIR spectra of the LB films presented the exact sites in the cytosine and guanine moieties for the formation of triple hydrogen bonds. The base pairing resulted in a change in molecular orientation and interaction, and the corresponding LB film exhibited a different phase transition behavior from a typical crystal transition for the cytosine-functionalized nucleolipids and an analogous glass transition for the guanine-functionalized nucleolipids. The thermal stability of the mixed LB film was improved in comparison to the LB films of pure components.