The study of the interaction of a model alpha-helical peptide with lipid bilayers and monolayers

We studied the interaction of the alpha-helical peptide acetyl-Lys(2)-Leu(24)-Lys(2)-amide (L(24)) with tethered bilayer lipid membranes (tBLM) and lipid monolayers formed at an air-water interface. The interaction of L(24) with tBLM resulted in adsorption of the peptide to the surface of the bilayer, characterized by a binding constant K(c)=2.4+/-0.6 microM(-1). The peptide L(24) an induced decrease of the elasticity modulus of the tBLM in a direction perpendicular to the membrane surface, E(radial). The decrease of E(radial) with increasing peptide concentration can be connected with a disordering effect of the peptide to the tBLM structure. The pure peptide formed a stable monolayer at the air/water interface. The pressure-area isotherms were characterized by a transition of the peptide monolayer, which probably corresponds of the partial intercalation of the alpha-helixes at higher surface pressure. Interaction of the peptide molecules with lipid monolayers resulted in an increase of the mean molecular area of phospholipids both in the gel and liquid crystalline states. With increasing peptide concentration, the temperature of the phase transition of the monolayer shifted toward lower temperatures. The analysis showed that the peptide-lipid monolayer is not an ideally miscible system and that the peptide molecules form aggregates in the monolayer.     

Nonionic surfactant sorption onto the bacterial cell surface: a multi-interaction isotherm

The adsorption of linear polyoxyethylene (POE) alcohol surfactants of the form CxEy onto the surface of a Sphingomonas sp. has been examined. For this study, the alkyl chain length (x) was fixed at 12 and the POE chain length (y) was varied, with y = 4, 7, 9, 10, and 23 ethylene oxide units. Langmuirian isotherms were observed for C12E4 and C12E23, and more complex isotherms were observed for the three intermediate POE chain length surfactants, with C12E7 and C12E9 exhibiting strong S-shaped isotherms. All isotherms showed plateaus near the critical micelle concentration (CMC) with the plateau decreasing with increasing POE chain length. A simple multi-interaction isotherm is proposed that models the sorption isotherm as the sum of two interactions. The first interaction describes monolayer adsorption, whereas the second interaction describes lateral interactions between sorbed surfactant molecules and the formation of surface aggregates. Varying ratios of these two interactions as a function of POE chain length gives rise to the variety of observed isotherm shapes. Results of the isotherm analysis suggest that lateral interactions dominate for surfactants with low POE chain lengths, and the lateral interactions decrease as the POE chain length is increased.     

Role of microenvironment in the mixed Langmuir-Blodgett films

This paper reports the pi-A isotherms and spectroscopic characteristics of mixed Langmuir and Langmuir-Blodgett (LB) films of nonamphiphilic carbazole (CA) molecules mixed with polymethyl methacrylate (PMMA) and stearic acid (SA). pi-A isotherm studies of mixed monolayer as well as the remarkable change in collapse pressure of the mixed monolayer isotherms definitely show that CA is incorporated into PMMA and SA matrices. However, CA is stacked in the PMMA/SA chains and forms microcrystalline aggregates, as is evidenced from the scanning electron micrograph picture. The nature of these aggregated species in the mixed LB films has been revealed by UV-vis absorption and fluorescence spectroscopic studies. The presence of two different kinds of band systems in the fluorescence spectra of the mixed LB films have been observed. This may be due to the formation of low-dimensional aggregates in the mixed LB films. Intensity distribution of different band systems is highly sensitive to the microenvironment of two different matrices as well as also on the film thickness.     

Effect of alkali halide salt additives on the structure of poly(vinyl alcohol) films cast from aqueous solutions

The molecular structures of poly(vinyl alcohol) films cast from polymer aqueous solutions (1 mol/L) containing 0.1 mol/L of LiCl, NaF, NaCl, KCl, CsCl, KBr, and KI salts are studied via FTIR spectroscopy. The addition of any of these salts except LiCl leads to an increase in the degree of crystallinity of poly(vinyl alcohol) in a film by a factor of 1.3–1.6. In contrast, LiCl significantly decreases the crystallinity of the polymer. It is found that, in the IR spectra of the films containing salt additives, the position of the maximum in the band of the stretching vibrations of OH groups of the polymer is shifted relative to its position in the IR spectra of the films free of salt additives. The magnitude and direction of this band shift depend on the types (anion or cation) and radii of ions comprising salts. The observed effects are interpreted in terms of existing ideas on the interactions of salt ions with the OH groups of water and other hydroxyl-containing molecules.     

Anion effects on calixarene monolayers: a Hofmeister series study

Due to their amphiphilic structure, calixarenes adsorb at the air/water interface and form stable Langmuir films. We have explored the effect of salts on calix[6]- and calix[8]arene spreading isotherms at the air/water interface. A wide range of different potassium salts was used in the subphase: KCl, KI, KBr, KSCN, KNO(3), CH(3)COOK, K(2)SO(4), and K(3)PO(4). The differences in Langmuir isotherms are due to the presence of different anions in the subphase, to the different conformations of the ligands at the interface, and to the different complexing affinities of calix[6]- and calix[8]arene for potassium ions. The two systems show a significant specific ion effect that can be discussed in terms of Hofmeister series. Characteristic monolayer parameters, e.g., limiting area (A(lim)), collapse pressure (pi(coll)), modulus of compressibility (C(s)(-1)), and surface potential (DeltaV), are discussed in terms of some physicochemical parameters that reflect dispersion forces: in particular, anion polarizabilities, lyotropic number (N), molar surface tension increment (sigma), and partial molar volume (nu(s)).     

Photoinduced self-structured surface pattern on a molecular azo glass film: structure-property relationship and wavelength correlation

In this study, three series of star-shaped molecular azo glasses were synthesized, and self-structured surface pattern formation on the azo compound films was studied by laser irradiation at different wavelengths. The molecular azo glasses were synthesized from three core precursors (Tr-AN, Tr-35AN, Tr-H35AN), which were prepared by ring-opening reactions between 1,3,5-triglycidyl isocyanurate and corresponding aniline derivatives. The star-shaped azo compounds were obtained through azo-coupling reactions between the core precursors and diazonium salts of 4-chloroaniline, 4-aminobenzonitrile, and 4-nitroaniline, respectively. By using the two-step reaction scheme, three series of azo compounds with different structures were obtained. The core precursors and azo compounds were characterized by using (1)H NMR, FT-IR, UV-vis, mass spectrometry, and thermal analyses. The self-structured surface pattern formation on films of the azo compounds was studied by irradiating the azo compound films with a normal-incident laser beam at different wavelengths (488, 532, and 589 nm). The results show that the photoinduced surface pattern formation behavior is closely related to the structure of the azo compounds, excitation wavelength, and light polarization conditions. The absorption band position of the π-π* transition is mainly determined by the electron-withdrawing groups on the azo chromophores. When the excitation wavelength is between λ(max) and the band tail at the longer wavelength side, the self-structured surface patterns can be more efficiently induced to form on the films. The 3,5-dimethyl substitution on azo chromophores inhibits the surface pattern formation for certain excitation wavelengths. Increasing molecular interaction also shows an effect of restraining the surface pattern formation. The irradiations with linearly and circularly polarized light cause significant differences in the alignment manner of the pillarlike structures and their saturated height.     

表面活性素单分子膜在空气/水界面的迟滞现象

表面活性素是一类具有较强表面活性的微生物脂肽类化合物,能在空气/水界面形成不溶性单分子膜.利用Langmuir膜天平测定了表面活性素单分子膜的压缩-扩张循环曲线,发现单分子膜在经历了“平台区”后出现较大的迟滞环,迟滞环的形状与亚相pH有关.将“平台区”的单分子膜转移到云母表面后,用原子力显微镜(AFM)和扫描电子显微镜(SEM)均观察到高度达几十至数百纳米的表面聚集体,说明表面活性素在单分子膜的“平台区”伴随着自聚集.研究结果表明,表面活性素单分子膜在空气/水界面的迟滞现象是分子浸入亚相和形成三维表面聚集体共同作用的结果.     

Interactions of L-arginine with Langmuir monolayers of di-n-dodecyl hydrogen phosphate at the air-water interface

The surface phase behavior of di-n-dodecyl hydrogen phosphate (DDP) in Langmuir monolayer and its interactions with L-arginine (L-arg) have been investigated by measuring pi-A isotherms with a film balance and observing monolayer morphology with a Brewster angle microscopy (BAM). The DDP monolayers on pure water show a first-order liquid expanded-liquid condensed (LE-LC) phase transition and form fingering LC domains having uniform brightness at different temperatures. At 15 degrees C, the pi-A isotherms on pure water and on different concentration solutions of L-arg show a limiting molecular area at approximately 0.50 nm(2)/molecule. With increasing the subphase concentration of L-arg up to 4.0 x 10(-4)M, the LE and the LE-LC coexistence regions shift to larger molecular areas and higher surface pressures, respectively. With a further increase in the concentration of L-arg beyond this critical concentration, these isotherms show little or no more expansion. These results have been explained by considering the fact that the L-arg undergoes complexation with the DDP to form L-arg-DDP that remains in equilibrium with the components at the air-water interface. As the concentration of L-arg in the subphase increases, the equilibrium shifts towards the complex. At a concentration of L-arg > or =4.0 x 10(-4)M, the DDP monolayers get saturated and show the characteristics of the new amphiphile, L-arg-DDP. BAM is applied to confirm the above results. When the concentration of the L-arg is <4.0 x 10(-4)M, domains always start forming at an area of approximately 0.64 nm(2)/molecule, which is the critical molecular area for the phase transition in the DDP monolayers on pure water. In contrast, when the monolayers are formed on a solution containing > or =4.0 x 10(-4)M L-arg, comparatively smaller size domains are formed after the appearance of a new cusp point at approximately 0.55 nm(2)/molecule. With an increase in the concentration of L-arg in the subphase, the size of the domains decreases indicating that the fraction of the DDP gradually decreases, whereas the fraction of the complex gradually increases. In addition, a very simple procedure for determination of the stability constant, which is 2.6 x 10(4)M(-1) at 15 degrees C, has been suggested.     

In situ Observation of the Photochromism in the Langmuir Monolayer of a Non—typical Amphiphilic Spiropyran Derivative at the Air／Water Interface

In situ photochromic process in the monolayer of aphotochromic spiropyran derivative without long alkyl chain,was investigated.The photochromism at the air/water interface under differnet surface pressures was studied by surface pressure-area isotherms,surface pressure-time curves,area-time curves and Brewster angle microscopy.Both forms of the compound were found to form monolayers at the air/water interface althouhg it does not have long alkyl chain.A large area expansion in the monolayer corresponding to a zreo^th order reaction was found at the initial stage of the UV light irradiation.A series of dynamic investigations revealed that at high pressure after phase transition in the monolayer,the surface pressure changes greatly umder alternative irradiation of UV and visible light.An obvious morphological change accompanying with the photochromism was observed in situ.     

Optical limiting of gold nanoparticle aggregates induced by electrolytes

The electronic absorption spectra and optical-limiting (OL) properties of gold nanoparticle (AuNP) aggregates induced by KCl and NaCl have been investigated using 4.1-ns laser pulses at 532 nm. Although the individual AuNP colloid shows no optical-limiting effect, the AuNP aggregates exhibit significant optical-limiting characteristics. With an increased concentration of KCl and NaCl, the surface plasmon resonance (SPR) band shifts to a longer wavelength, and the optical-limiting performance is enhanced. Both the electronic absorption and optical limiting are influenced by the particle size. The larger the individual nanoparticle, the further red-shifted the SPR band and the stronger the optical limiting. Optical limiting of aggregates induced by KCl is stronger than that of aggregates induced by NaCl. Mechanistic studies reveal that free-carrier absorption is the dominant contributor to the optical limiting, with negligible contribution from nonlinear scattering.     

Effects of temperature and chemical modification on polymer Langmuir films

We previously reported on a rheological study of Langmuir films of poly(tert-butyl methacrylate) (PtBMA), and this work describes further studies on this system. Here, surface pressure-area (Pi-A) isotherms and interfacial shear rheology experiments are performed to better understand the effects of two modifications: varying the temperature between 5 and 40 degrees C and introducing small amounts of carboxylic acid groups by partially hydrolyzing the polymer. Increased temperature produced isotherms with lower plateau surface pressures, Pip, and Pi values shifted to lower areas above Pip. Film properties transition from being primarily viscous to being dominated by elasticity as Pip is crossed for all temperatures studied, even as the value of Pip changes. Increasing the hydrolysis fraction leads to isotherms shifted to lower areas throughout the curve and higher Pip values. Both temperature variation and chemical modification are believed to affect the relative importance of polymer-polymer and polymer-subphase interactions.     

Discreteness-of-charge-effects in the shift of the metallation equilibrium of an amphiphilic porphyrin in a lipid monolayer

Summary The metallation equilibrium of an amphiphilic porphyrin in a mixed lipid monolayer may be changed at the air/water interface, independent of the overall surface potential. Langmuir-Blodgett assemblies of the same lipid monolayer are used to prove that the mean surface potential only shifts the porphyrins' protonation equilibrium, but not its metallation equilibrium. It is then shown that the choice of a cationic porphyrin embedded in a fatty acid lipid matrix (dominating the surface potential) allows the metallation equilibrium to be shifted by more than 4 pK-units as compared to the same system with either an electrically neutral porphyrin or lipid matrix. This is due to a molecular order in the mixed monolayer with peripheral positive charges dissociating the free base porphyrin and the fatty acid molecules sitting on top of the porphyrins' chromophores, facilitating the incorporation of a metalion. Evidence for this molecular arrangement is given by means of -A-Isotherms and reflection spectroscopy.     

卟啉衍生物复合单层膜的结构特征

四羧酸对称取代的卟啉衍生物5,10,15,20-tetra-4-oxy(aceticacid)phenylporphyrin(TAPP)及其与二甲基二(十六烷基)溴化铵(DMDHA)所形成的复合物在空气/水溶液界面上可形成稳定的单层膜,利用π-A等温线、UV-vis光谱及偏振光谱对单层膜的结构作用了研究,结果表明,纯TAPP的单层膜中,卟啉环间有强的相互作用,其间距约为0.55nm;1:1(摩尔比)的复合物单层膜中,环间距离与纯TAPP相比无明显变化,但聚集体间距离增加了;而1:4的复合物单层膜中,环与环间的相互作用减弱,环间距离及聚集体间的距离均明显增加。     

The interaction between G-quadruplex-forming oligonucleotide and cationic surfactant monolayer at the air/water interface

We report on the interactions between a 21-mer quadruplex-forming oligonucleotide bearing human telomere sequence of dG(3)(T(2)AG(3))(3) (G4 DNA) and a positively charged dioctadecyldimethylammonium bromide (DODAB) monolayer at the air-aqueous interface, studied by surface film balance measurements. In the presence of G4 DNA, the π-A isotherm of the cationic Langmuir film shifted to lower molecular areas when compared with the reference isotherm recorded on the subphase containing only 50 mM triethylamine-acetate (TEAA) buffer. The presence of quadruplex-stabilizing metal cations (K(+) or Na(+)) further affected profiles of π-A isotherms. Further insight into processes related to the G4 DNA-monolayer interactions was provided by recording time profiles of the surface pressure of monolayer at a constant mean molecular area. In these experiments G4 DNA and/or metal ions were sequentially injected under the monolayer surface. Results indicated that multistranded assemblies of G4 DNA were formed at the monolayer interface even in the absence of metal ions, which suggested that the charged cationic surface of Langmuir monolayer induced aggregation of guanine-rich DNA strands. The presence of sodium and potassium ions inhibited formation of multi-stranded assemblies through the competitive G-quadruplex formation but to different extent that might be related to the differences in stability and topology of both quadruplexes.     

Effect of organic solvents on J aggregation of pseudoisocyanine dye at mica/water interfaces: morphological transition from three-dimension to two-dimension

Morphological and spectroscopic properties of pseudoisocyanine (PIC) J aggregates produced at mica/solution interfaces have been characterized by absorption/fluorescence spectroscopy, fluorescence microscopy, and atomic force microscopy. Addition of organic solvents (1-propanol (PrOH) or 1,4-dioxane (Dox)) into aqueous solutions of the PIC dye induced a transition of the morphology of the interfacial J aggregates. The characteristic feature of this transition is the thickness (or height) change of the aggregate domain layers from three-dimensions to two-dimensions: The domain area of the J aggregates was dependent on the amount of the organic cosolvent, while the domain thickness was dependent on the type of the cosolvent. In pure aqueous solution, the J aggregates at the mica/water interface had a three-dimensional structure with the height of approximately 3 nm (multilayer structure). In mixed solvents of PrOH/water or Dox/water (5 or 10 vol%), the interfacial aggregates became a bilayer or monolayer structure, respectively, assuming that PIC molecules are adsorbed on their molecular plane perpendicular to the mica surface. Meanwhile, optical properties (band width and peak position) of the J band were invariant upon addition of the organic cosolvents, suggesting that molecular packing in the J aggregates is essentially unchanged. These results revealed that spectroscopic properties of the interfacial PIC J aggregates were determined only by the lateral (two-dimensional) interaction within the adsorbed monolayer of PIC molecules on mica, and interlayer interaction in the multilayered J aggregate was consequently small.     

Weak first-order tilting transition in monolayers of mono- and bipolar docosanol derivatives

A systematic analysis of pressure-area isotherms and grazing incidence X-ray diffraction (GIXD) data of 22-methoxydocosan-1-ol (H3C-O-(CH2)22-OH, MDO), docosan-1-ol (H3C-(CH2)21-OH, DO), and docosyl methyl ether (H3C-(CH2)21-O-CH3, DME) monolayers on pure water between 10 and 35 degrees C is presented. All monolayers form fully condensed phases in the investigated temperature region. The GIXD data reveal that the monolayers exhibit the phase sequence -S at lower temperature and -LS at higher temperature. Phase diagrams have been established. Inserting a second hydrophilic group at the opposite end of the molecule (bipolar MDO) shifts the S/LS boundary to higher temperatures. All monolayers exhibit herringbone (HB) packing at lower temperatures. The "kink" in the isotherms observed at lower temperatures is replaced by a very small plateau region at higher temperatures. The entropy changes connected with this weak first-order tilting transition are much smaller compared with the first-order transition from liquid-expanded (LE) to condensed (LC). Additionally, this transition is endothermic in contrast to the LE/LC transition. The reason for the endothermic transition is the weaker positional correlation in the nontilted state compared with the tilted one. The appearance of the weak first-order endothermic transition can be connected with the changed phase sequence. X-ray photoelectron spectroscopy (XPS) measurements provide information about the polar group orientation. Considerations based on GIXD and XPS data as well as adhesion energy of the different terminal end groups lead to the conclusion that the hydroxyl group of the bipolar MDO is attached to the water surface while the methoxy group is in contact with air. The presented results show that the second hydrophilic group influences the monolayer properties in a mild way.     

INDUSTRIAL NEWS

Interactions between monolayers of trilaurylamine (TLA) and various water-soluble corrosion, hydrate and scale inhibitors, as well as acetic acid and FeCl₂, have been studied by means of the Langmuir technique, The water-soluble compounds were present in the subphase before TLA was spread. Their influence on the monolayer properties were measured by means of surface pressure-area ( k-A) isotherms and constant surface pressure-area relaxation measurements. All the added water soluble species gave rise to monolayer expansion. The monolayer expansion observed for TLA at pH 1, increased when FeCl₂ was added to the subphase at the same pH. Polyethyleneglycols with molecular weight higher than 200 destabilized totally the TLA-monolayer, probably due to bending and folding into multilayers. Monolayer expansion and film instability increased with increased acetic acid concentrations. The destabilization mechanism in this case was most likely due to a combination of nucleation and dissolution of film components into the subphase. The corrosion inhibitor Dyno KI 384 had a higher surface activity than TLA. Hence the monolayer     

Air-water interfacial behavior of linear-dendritic block copolymers containing PEG and azobenzene chromophores

Fabrication of Langmuir films at the air-water interface of four linear-dendritic block copolymers (LDBCs) is described. The LDBCs are composed of a linear hydrophilic chain of poly(ethylene glycol) (PEG) and the first four generations of hydrophobic aliphatic polyester dendrons functionalized at the periphery with cyanoazobenzene chromophores. Langmuir films of the LDBCs, coded as PEG-AZOn (n indicates the number of cyanoazobenzene units at the periphery of the dendritic block), have been characterized by a combination of surface pressure versus area per molecule isotherms, UV-vis reflection spectroscopy and Brewster angle microscopy. The observed PEG-AZOn Langmuir film behavior depends strongly on the hydrophilic/hydrophobic ratio. A typical transition, related to PEG chains desorption from the air-water interface into the water subphase is observed for all the LDBCs, except for PEG-AZO16. In addition, PEG-AZO2 and PEG-AZO4 show a second transition whose nature has been studied in detail. Azobenzene chromophore interactions have been shown to be relevant in the organization of PEG-AZOn (n=4, 8 and 16) Langmuir films. Moreover, for PEG-AZO16 the orientation of the azobenzene units has been determined, revealing the formation of a well organized structure of azobenzene moieties at the air-water interface.     

Langmuir monolayer properties of perfluorinated double long-chain salts with divalent counterions of separate electric charge at the air-water interface

The novel perfluorinated double long-chain salts with divalent counterions of separate electric charge, 1,1-(1,omega-alkanediyl)-bispyridinium perfluorotetradecane- carboxylate [CnBP(FC14)2 : n = 2, 6, 10, 14], were newly synthesized and their interfacial behavior was investigated by Langmuir monolayer methods. Surface properties [surface pressure (pi)-, surface potential (DeltaV)-, dipole moment (micro perpendicular)-area (A) isotherms] and morphological images of CnBP(FC14)2 monolayers on a subphase of water and on various NaCl concentrations were measured by employing the Wilhelmy method, the ionizing electrode method, fluorescence microscopy (FM), and Brewster angle microscopy (BAM). CnBP(FC14)2 formed a stable monolayer on water at 298.2 K, where these pi-A isotherms shifted to a larger molecular area with increasing charge separation and had no transition point from a disordered phase to an ordered one. On the contrary, the pi-A isotherms on NaCl solutions moved to the smaller areas, showed the transition and higher collapse pressures compared to the pi-A isotherms on water. These results suggested that a sodium chloride subphase induced the condensation of CnBP(FC14)2 molecules upon compression. In addition, it is quite noticeable that a dissociation of CnBP counterion from CnBP(FC14)2 occurs on NaCl solutions, depending on the extent of charge separation. This phenomenon was supported by the changes of the limiting area, transition pressure, collapse pressure, repeated compression-expansion cycle curve, and DeltaV behavior of perfluorotetradecanoic acid (FC14). Furthermore, temperature dependence of these monolayers was investigated, and an apparent molar quantity change on the phase transition was evaluated on 0.15 M NaCl. The morphological behavior of CnBP(FC14)2 and FC14 monolayers was also confirmed by FM and BAM images.     

Phase transition in Langmuir monolayers

The main features of several theoretical models which describe the main phase transition and correspondingly the non-horizontal shape of the Π-A isotherms of the Langmuir monolayers, are discussed. New equations of state are based on the generalised Volmer's equation and consider the coexistence monomers and large clusters as bimodal distribution. A further generalisation for the case of quasi-bimodal distribution allows the consideration of monomers with small aggregates on one side of the spectrum and large clusters on the other side. The new theoretical model is corroborated by the Π-A isotherms of various amphiphilic monolayers the condensed phases of which have various gradual differences in the crystalinity and packing density data. The large variety of shapes of the Π-A isotherms in the region A<A_c can be determined by a single general equation whose parameters, except A_c, correspond to the isotropic fluid-like monolayer region. The application of the generalised equation of state on the experimental Π-A isotherms indicates the formation of small aggregates in the region A>A_c.