Optical waveguiding and nonlinear optical characterization of 2-docosylamino-5-nitropyridine Langmuir-Blodgett films

Well ordered Y-type Langmuir-Blodgett (LB) multilayers of 2-docosylamino-5-nitropyridine (DCANP) have been characterized with linear and nonlinear optical techniques. The nonlinear optical susceptibilities have been determined for the wavelengths λ = 1064 nm (d₃₃ = (7.8±1) pm/V, d₃₁ = (2.0 ± 0.5) pm/V) and λ = 1318 nm (d₃₃ = (5.6 ± 1) pm/V). Waveguiding experiments with grating couplers allowed the excitation of TE_o and TM_o modes propagating over more than 20 mm. Guided wave attenuation coefficients as low as 12 dB/cm (at the wavelength λ = 632.8 nm) are reported. Coupling experiments allowed the determination of the dispersion of the refractive index n₃ (n₃^{632.8 nm} = 1.598). The refractive index data could be used to calculate phase-matching conditions for frequency doubling with TE modes in DCANP waveguides.     

Relationships between equilibrium spreading pressure and phase equilibria of phospholipid bilayers and monolayers at the air-water interface

The intricate interplay between the bilayer and monolayer properties of phosphatidylcholine (PC), phosphatidylglycerol (PG), and phosphatidylethanolamine (PE) phospholipids, in relation to their polar headgroup properties, and the effects of chain permutations on those polar headgroup properties have been demonstrated for the first time with a set of time-independent bilayer-monolayer equilibria studies. Bilayer and monolayer phase behavior for PE is quite different than that observed for PC and PG. This difference is attributed to the characteristic biophysical PE polar headgroup property of favorable intermolecular hydrogen-bonding and electrostatic interactions in both the bilayer and monolayer states. This characteristic hydrogen-bonding ability of the PE polar headgroup is reflected in the condensed nature of PE monolayers and a decrease in equilibrium monolayer collapse pressure at temperatures below the monolayer critical temperature, T(c) (whether above or below the monolayer triple point temperature, T(t)). This interesting phenomena is compared to equilibrated PC and PG monolayers which collapse to form bilayers at 45 mN/m at temperatures both above and below monolayer T(c). Additionally, it has been demonstrated by measurements of the equilibrium spreading pressure, pie, that at temperatures above the bilayer main gel-to-liquid-crystalline phase-transition temperature, T(m), all liquid-crystalline phospholipid bilayers spread to form monolayers with pie around 45 mN/m, and spread liquid-expanded equilibrated monolayers collapse at 45 mN/m to form their respective thermodynamically stable liquid-crystalline bilayers. At temperatures below bilayer T(m), PC and PG gel bilayers exhibit a drop in bilayer pi(e) values < or =0.2 mN/m forming gaseous monolayers, whereas the value of pic of spread monolayers remains around 45 mN/m. This suggests that spread equilibrated PC and PG monolayers collapse to a metastable liquid-crystalline bilayer structure at temperatures below bilayer T(m) (where the thermodynamically stable bilayer liquid-crystalline phase does not exist) and with a surface pressure of 45 mN/m, a surface chemical property characteristically observed at temperatures above bilayer T(m) (monolayer T(c)). In contrast, PE gel bilayers, which exist at temperatures below bilayer T(m) but above bilayer T(s) (bilayer crystal-to-gel phase-transition temperature), exhibit gel bilayer spreading to form equilibrated monolayers with intermediate pie values in the range of 30-40 mN/m; however, bilayer pie and monolayer pic values remain equal in value to one another. Contrastingly, at temperatures below bilayer T(s), PE crystalline bilayers exhibit bilayer pie values < or =0.2 mN/m forming equilibrated gaseous monolayers, whereas spread monolayers collapse at a value of pic remaining around 30 mN/m, indicative of metastable gel bilayer formation.     

Degradation behaviors of polyester monolayers at the air/water interface: Alkaline and enzymatic degradations

The degradation kinetics of Langmuir monolayer films of a series of biodegradable polyesters has been studied to investigate the effect of degradation medium, alkalinity and enzymes. The degradation behavior of polyester monolayers strongly depended on both degradation medium and surface pressure. As the surface pressure was increased, the degradation rates of poly(l-lactide) (PLLA) and poly[(R)-3-hydroxybutyrate] (P(3HB)) increased in both degradation media. When monolayers were exposed to an alkaline subphase, the degradation of PLLA monolayers occurred at relatively low surface pressures; the PLLA monolayers were hydrolyzed at pH 10.5 regardless of surface pressure, while the alkaline degradation of P(3HB) monolayer occurred over a constant surface pressure of 7 mN/m at pH 11.8. These results have been explained by the difference in hydrophilic/hydrophobic balance of the polymers; PLLA is more hydrophilic than P(3HB). In contrast, the enzymatic degradations of both polymer monolayers occurred at higher constant surface pressures than those of the alkaline treatment; 7 mN/m for PLLA and 10 mN/m for P(3HB). This behavior was attributed to the enzymes being much larger than the alkaline ions: the enzymes need a larger contact area with the submerged monolayers to be activated.     

Sum frequency generation spectroscopic studies on phase transitions of phospholipid monolayers containing poly(ethylene oxide) lipids at the air-water interface

Vibrational sum frequency generation (SFG) spectroscopy was applied to study the phase transitions of the mixed monolayers of l-alpha-distearoyl phosphatidylethanolamine (DSPE) and DSPE covalently coupled with poly(ethylene oxide) at the amino head group (DSPE-EO(45), DSPE with 45 ethylene oxide monomers) at the air-water interface. The SFG spectra were measured for the mixed monolayers with the mole fractions of DSPE-EO(45) of 0, 1.3, 4.5, 9.0, 12.5, and 16.7% at the surface pressures of 5, 15, and 35 mN/m. The monolayer compression isotherms indicated that the mixed monolayers at 5, 15, are 35 mN/m are mainly in the so-called "pancake", "mushroom", and "brush" states, respectively. The SFG spectra in the OH stretching vibration region give rise to SFG bands near 3200 and 3400 cm(-1). The mean molecular amplitude of the former band due to the OH stretching of the "icelike" water molecules associated mainly with the hydrophilic poly(ethylene oxide) (PEO) chains, exhibits appreciable decrease on compression of the mixed monolayers from 5 to 15 mN/m. The result corroborates the model for the pancake-mushroom transition, which presumes the dissolution of the PEO chains from the air-water interface to the water subphase. Further compression of the mixed monolayers to 35 mN/m causes a slight decrease of the line amplitude, which can be explained by considering a squeezing out of water molecules from the hydrophilic groups of DSPE-EO(45) in the brush state, where the PEO chains strongly interact with each other to form a tight binding state of the hydrophilic groups. The relative intensities of the SFG bands due to the CH3 asymmetric and symmetric vibrations were used to estimate the tilt angles of the terminal methyl group of DSPE, indicating that the angle increases with increasing the mole fraction of DSPE-EO(45). The angles almost saturate at the mole fraction larger than 10%, the saturation angle being nearly 90 degrees at 5 mN/m, ca. 60 degrees at 15 mN/m, and ca. 47 degrees at 35 mN/ m. Then, the introduction of the hydrophilic PEO head group causes a large tilting of the alkyl groups of DEPE in the mixed monolayers.     

Effects of the surface pressure on the formation of Langmuir-Blodgett monolayer of nanoparticles

The effects of the surface pressure on the particle arrangement of Langmuir-Blodgett (LB) monolayers of alkanethiol-capped gold nanoparticles were studied. The LB monolayers were prepared from a highly concentrated particle solution, which increases film fabrication efficiency but readily causes small particle voids in the particle array. Overcompressing the LB monolayer to a high surface pressure restructured the particles and eliminated the voids. When the gold particles capped by dodecanethiol were 8.5 nm in diameter, the particle arrangement was vastly improved and a wafer-scale LB monolayer was transferred onto a substrate at the surface pressure of 20 mN/m.     

Comparison of the structure and stability of monolayers prepared with 12-phenyldodecyl mercaptan and 11-phenoxyundecyl mercaptan

Monolayers are prepared by self-assembly of 12-phenyldodecyl mercaptan or 11-phenoxyundecyl mercaptan on gold substrates. Reflection-absorption infrared spectra of these monolayers show that the 12-phenyldodecyl mercaptan monolayer forms a well-organized interface while 11-phenoxyundecyl mercaptan monolayers do not. Infrared spectra also suggest that the C1-C4 axis of the phenyl ring is largely parallel to the Au-substrate for 12-phenyldodecyl mercaptan layers, but has a more perpendicular orientation relative to the substrate for 11-phenoxyundecyl mercaptan layers. Despite these structural differences, Zisman analysis of contact angle data shows that these monolayers have similar surface energies, 27.1 (+/-6.7) mN/m for 12-phenyldodecyl mercaptan and 26.1 (+/-5.3) mN/m for 11-phenoxyundecyl mercaptan. Fowkes analysis of the contact angle data suggest that dispersive interactions account for all of the measured surface energy for both modified interfaces. The structural differences are, however, reflected in the lateral stabilization energy measurements where monolayers of 12-phenyldodecyl mercaptan are found to be 24 (+/-5) kJ/mol more stable than those of 11-phenoxyundecyl mercaptan.     

Surface behavior and peptide-lipid interactions of the cyclic neuropeptide melanin concentrating hormone

The kinetics and the thermodynamics of melanin concentrating hormone (MCH) adsorption, penetration, and mixing with membrane components are reported. MCH behaved as a surface active peptide, forming stable monolayers at a lipid-free air-water interface, with an equilibrium spreading pressure, a collapse pressure, and a minimal molecular area of 11 mN/m, 13 mN/m, and 140 A (2), respectively. Additional peptide interfacial stabilization was achieved in the presence of lipids, as evidenced by the expansion observed at pi > pi sp in monolayers containing premixtures of MCH with zwitterionic or charged lipids. The MCH-monolayer association and dissociation rate constants were 9.52 x 10 (-4) microM (-1) min (-1) and 8.83 x 10 (-4) min (-1), respectively. The binding of MCH to the dpPC-water interface had a K d = 930 nM at 10 mN/m. MCH penetration in lipid monolayers occurred even up to pi cutoff = 29-32 mN/m. The interaction stability, binding orientation, and miscibility of MCH in monolayers depended on the lipid type, the MCH molar fraction in the mixture, and the molecular packing of the monolayer. This predicted its heterogeneous distribution between different self-separated membrane domains. Our results demonstrated the ability of MCH to incorporate itself into biomembranes and supports the possibility that MCH affects the activity of mechanosensitive membrane proteins through mechanisms unrelated with binding to specific receptors.     

Protein-directed assembly of binary monolayers at the interface and surface patterns of protein on the monolayers

Ferritin-directed assembly of binary monolayers of zwitterionic dipalmitoylphosphatidylcholine and cationic dioctadecyldimethylammonium bromide (DOMA) at the interface and surface patterns of ferritin on the monolayers have been investigated using a combination of infrared reflection absorption spectroscopy, surface plasmon resonance, and atomic force microscopy. Ferritin binding to the binary monolayers at the air-water interface at the surface pressure 30 mN/m, primarily driven by the electrostatic interaction, gives rise to a change in tilt angle of hydrocarbon chains from 15 degrees +/- 1 degrees to 10 degrees +/- 1 degrees with respect to the normal of the monolayer at the mole fraction of DOMA (XDOMA) of 0.1. The chains at XDOMA = 0.3 are oriented vertical to the water surface before and after protein binding. A new mechanism for protein binding to the binary monolayers is proposed. The secondary structures of the adsorbed ferritin are prevented from changing to some extent due to the existence of the monolayers. The amounts of the bound protein on the monolayers at the air-water interface are increased in comparison with those on the pre-immobilized monolayers at low XDOMA. The increased amounts and different patterns of the adsorbed protein at the monolayers are mostly attributed to the formation of multiple binding sites available for ferritin, which is due to the lateral reorganization of the lipid components in the monolayers induced by the protein in the subphase. The created multiple binding sites on the monolayer surfaces through the protein-directed assembly can be preserved for subsequent protein binding.     

Hydrolytic behavior of enantiomeric poly(lactide) mixed monolayer films at the air/water interface: stereocomplexation effects

The Langmuir film balance technique was used to determine the hydrolytic kinetics of monolayers of the stereocomplex formed from mixtures of enantiomeric polylactides, poly(L-lactide) (L-PLA) and poly(D-lactide) (D-PLA), spread at the air-water interface. The present study investigated parameters such as degradation medium, mixture composition, and time on the relative degradation rate. The pi-A isotherms of monolayers of the mixtures provide clear evidence for the presence of a stereocomplex; the isotherms of monolayers of individual polyenantiomer show a transition at about 8.5 mN/m, whereas the transition of monolayers containing a stereocomplex formed from the equimolar mixture shifted to higher surface pressure, about 11 mN/ m. The rate of hydrolysis was recorded by a change in occupied area when the monolayer is maintained at a constant surface pressure. The hydrolysis of the mixture monolayers under basic conditions was slower than that of individual polyenantiomer monolayers, depending on the composition or the degree of complexation. In the presence of proteinase K, the enzymatic hydrolysis rate of mixture monolayers with >50 mol % l-PLA was much slower than that of the single-component L-PLA monolayer. The monolayers formed from mixtures with < or =50 mol % L-PLA did not show any change of occupied areas. This result is explained by the inactivity of D-PLA and stereocomplexed chains to the enzyme. From both results, it can be concluded that the retardation of the hydrolysis of mixture monolayers is mainly due to a strong interaction between D- and L-lactide unit sequences, which prevents the penetration of water or enzyme into the bulk.     

二维纳米金单层膜的构建及其生物电化学应用

利用Langmuir-Blodgett (LB)技术在氧化铟锡(ITO)电极上制备了二维纳米金(nano-Au)单层膜,采用扫描电子显微镜表征了二维纳米金单层膜.实验结果表明:表面压为28 mN/m时,可获得分散性好、形状规则且分布均匀的二维球形纳米金单层膜.利用LB技术制备了肌红蛋白(Mb)薄膜,并将其固定在二维纳米金单层膜修饰的电极表面,研究了肌红蛋白LB膜的直接电化学行为.结果表明:纳米金粒子能够有效地加速肌红蛋白的电子转移,其电子转移速率为1.415 s-1.     

A straightforward way to form close-packed TiO2 particle monolayers at an air/water interface

The aim of this study was to analyze if and how monolayers of TiO(2) particles could be directly formed at the air/water interface and if these monolayers could be transferred to a solid surface. TiO(2) particles with diameters of 300 nm, 500 nm, 1 μm, 5 μm, 10 μm, and 20 μm formed stable monolayers at pH 2. At low surface pressures, the particles formed small two-dimensional aggregates. Particles up to a radius of 5 μm displayed close packing at increased surface pressures. Particles of 10 μm radius formed a loose network, which is attributed to the strong adhesion caused by the weight-induced lateral capillary attraction. Every monolayer of particles could be transformed to a solid surface by the Langmuir-Blodgett deposition. At pH 6 or 11, the particles did not form stable monolayers at the air/water interface. They were instead dispersed in the aqueous phase and eventually sank to the bottom of the trough. At pH 11 the monolayer could, however, be stabilized by the addition of salt (0.5 M NaCl). The results are interpreted based on a changed wettability of the particles depending on pH and salt concentration.     

Monolayer behavior of binary systems of betulinic acid and cardiolipin: thermodynamic analyses of Langmuir monolayers and AFM study of Langmuir-Blodgett monolayers

Betulinic acid (BA, a natural pentacyclic triterpene) can induce mitochondrial membrane damage and trigger the mitochondrial pathway of apoptosis in tumor cells. The monolayer behavior of binary systems of BA and cardiolipin (CL, a unique phospholipid found only in mitochondria membrane in animals) was studied by surface pressure-area (π-A) measurements and analyses and Atomic force microscopy (AFM) observation. The miscibility analysis presents that in mixed monolayers BA takes both tilted and nearly perpendicular orientations at surface pressure below 30 mN/m but only nearly perpendicular orientation at 30 mN/m. The thermodynamic stability analysis indicates that phase separation and repulsion occur in mixed BA/CL monolayers. The compressibility analysis shows that at 30 mN/m, 20% addition of BA does markedly translate the liquid-condensed CL monolayer to mixed BA/CL monolayer with the coexistence of liquid-condensed and liquid-expanded phases. The AFM images of supported monolayers give direct evidence of the conclusions obtained from the analyses of π-A isotherms. These results confirm that at high surface pressure near to real biologic situations, BA orients nearly perpendicularly with hydroxyl group toward water, causes phase separation and changes the permeability of CL film, which correlates with the mitochondrial membrane damage induced by BA.     

Effect of molecular surface packing on the enzymatic activity modulation of an anchored protein on phospholipid Langmuir monolayers

The catalytic activity of a glycosylphosphatidylinositol (GPI)-anchored alkaline phosphatase has been studied in Langmuir phospholipid monolayers at different surface pressures. The enzyme substrate, p-nitrophenyl phosphate, was injected into the subphase of mixed enzyme/lipid Langmuir monolayers. Its hydrolysis product was followed by monitoring the absorbance at 410 nm in situ in the monolayer subphase of the Langmuir trough. Several surface pressures, corresponding to different molecular surface densities, were attained by lateral compression of the monolayers. The morphology of the monolayers, observed by fluorescence microscopy, showed three different types of domains owing to the heterogeneous partition of the enzyme within the mixed enzyme/lipid film. The catalytic activity was modulated by the enzyme surface density, and it increased until a pressure of 18 mN/m was reached, but it decreased significantly when the equilibrium in-plane elasticity (surface compressional modulus) increased more noticeably, resulting in alterations in the interface morphology. A model for the modulation of the enzyme orientation and catalytic activity by lipid/enzyme surface morphology and enzyme surface packing at the air/liquid interface is proposed. The results might have an important impact on the comprehension of the enzymatic activity regulation of GPI-anchored proteins in biomembranes.     

Mixing behavior of a poly(ethylene glycol)-grafted phospholipid in monolayers at the air/water interface

Mixed phospholipid monolayers hosting a poly(ethylene glycol) (PEG)-grafted distearoylphosphatidylethanolamine with a PEG molecular weight of 5000 (DSPE-PEG5000) spread at the air/water interface were used as model systems to study the effect of PEG-phospholipids on the lateral structure of PEG-grafted membrane-mimetic surfaces. DSPE-PEG5000 has been found to mix readily with distearoylphosphoethanolamine-succinyl (DSPE-succynil), a phospholipid whose structure resembles closely that of the phospholipid part of the DSPE-PEG5000 molecule. However, properties of mixed monolayers such as morphology and stability varied significantly with DSPE-PEG5000 content. In particular, our surface pressure, epifluorescence microscopy (EFM), and Brewster angle microscopy (BAM) studies have shown that mixtures containing 1-9 mol % of DSPE-PEG5000 form stable condensed monolayers with no sign of microscopic phase separation at surface pressures above approximately 25 mN/m. Yet, at 1 mol % of DSPE-PEG5000 in mixed monolayers, the two components have been found to behave nearly immiscibly at surface pressures below approximately 25 mN/m. For monolayers containing 18-75 mol % of DSPE-PEG5000, a high-pressure transition has been observed in the low-compressibility region of their isotherms, which has been identified on the basis of continuous BAM imaging of monolayer morphology, as reminiscent of the collapse nucleation in a pure DSPE-PEG5000 monolayer. Thus, the comparative analysis of our surface pressure, EFM, and BAM data has revealed that there exists a rather narrow range of mixture compositions with DSPE-PEG5000 content between 3 and 9 mol %, where somewhat homogeneous distribution of DSPE-PEG5000 molecules and high pressure stability can be achieved. This finding can be useful to "navigating" through possible mixture compositions while developing guidelines to the rational design of membrane-mimetic surfaces with highly controlled bio-nonfouling properties.     

Influence of alpha-hydroxylation of glycolipids on domain formation in lipid monolayers

By means of fluorescence and scanning force microscopy (SFM), we investigated the phase behavior of lipid monolayers composed of a mixture of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine, sphingomyelin, and cholesterol (5/2/3) with either alpha-hydroxylated or nonhydroxylated galactocerebroside. Fluorescence images of lipid monolayers at the air-water interface demonstrate that, independent of the lipid mixture, phase separation occurs at low surface pressure up to 4-6 mN m(-1), while an almost homogeneous phase is observed at larger surface pressures. However, by means of SFM of lipid monolayers transferred by the Langmuir-Blodgett technique at around 30 mN m(-1), nanometer-sized domains became discernible in those lipid mixtures that contained galactocerebroside, while, in that without a glycolipid, no such domain formation was visible. Moreover, the alpha-hydroxy group of the galactocerebroside alters the size and the total area of the domains significantly.     

Ellipsometry and Infrared Reflection Absorption Spectroscopy of Adsorbed Layers of Soluble Surfactants at the Air-Water Interface

Optical techniques play an increasingly important role in the characterization of microstructure and surface densities of thin films at various interfaces. In this study, ellipsometry and infrared reflection absorption spectroscopy (IRRAS) were used for determining the surface densities of adsorbed layers of cationic surfactants in situ at the air-water interface. The surfactants were N(alpha)-lauroyl-arginine methyl ester (LAM) and N(alpha), N(omega)-bis(N(alpha)-lauroyl-arginine)-alpha,omega-alkylidenediamide (C(6)(LA)(2)). In ellipsometry, the ellipsometric phase angle Delta was obtained at various surfactant concentrations and was referenced to that of the solvent. Three algorithms were used for analyzing the data. The surface densities are 3.3+/-0.3x10(-6) mol/m(2) at 1 mM for LAM and 1.5+/-0.3x10(-6) mol/m(2) at 0.1 mM for C(6)(LA)(2) by using an algorithm for which the monolayer thickness was estimated from molecular modeling. The corresponding surface densities from literature surface tension data and the Gibbs adsorption isotherm procedure are 2.2+/-0.4x10(-6) mol/m(2) and 1.2+/-0.2x10(-6) mol/m(2), respectively. In addition, IRRAS spectra were obtained from monolayers of LAM and C(6)(LA)(2) at the air-water interface. The frequencies of the methylene stretching vibration bands indicate that the monolayers are liquid-like. The surface densities were determined from the reflectance-absorbance data by using the model of either an isotropic film or an anisotropic film on the aqueous subphase. The IRRAS-based surface densities from either model, by using DPPC monolayers for calibration, are 2.4+/-0.7x10(-6) mol/m(2) at 1 mM for LAM and 1.5+/-0.6x10(-6) mol/m(2) at 0.1 mM for C(6)(LA)(2), which are in fair agreement with the ellipsometry- and the surface-tension-based surface densities. Copyright 2001 Academic Press.     

Thermodynamic characteristics and Langmuir-Blodgett deposition behavior of mixed DPPA/DPPC monolayers at air/liquid interfaces

The role of dipalmitoylphosphatic acid (DPPA) as a transfer promoter to enhance the Langmuir-Blodgett (LB) deposition of a dipalmitoylphosphatidylcholine (DPPC) monolayer at air/liquid interfaces was investigated, and the effects of Ca2+ ions in the subphase were discussed. The miscibility of the two components at air/liquid interfaces was evaluated by surface pressure-area per molecule isotherms, thermodynamic analysis, and by the direct observation of Brewster angle microscopy (BAM). Multilayer LB deposition behavior of the mixed DPPA/DPPC monolayers was then studied by transferring the monolayers onto hydrophilic glass plates at a surface pressure of 30 mN/m. The results showed that the two components, DPPA and DPPC, were miscible in a monolayer on both subphases of pure water and 0.2 mM CaCl2 solution. However, an exception occurs between X(DPPA)=0.2 and 0.5 at air/CaCl2-solution interface, where a partially miscible monolayer with phase separation may occur. Negative deviations in the excess area analysis were found for the mixed monolayer system, indicating the existence of attractive interactions between DPPA and DPPC molecules in the monolayers. The monolayers were stable at the surface pressure of 30 mN/m for the following LB deposition as evaluated from the area relaxation behavior. It was found that the presence of Ca2+ ions had a stabilization effect for DPPA-rich monolayers, probably due to the association of negatively charged DPPA molecules with Ca2+ ions. Moreover, the Ca2+ ions may enhance the adhesion of DPPA polar groups to a glass surface and the interactions between DPPA polar groups in the multilayer LB film structure. As a result, Y-type multilayer LB films containing DPPC could be fabricated from the mixed DPPA/DPPC monolayers with the presence of Ca2+ ions.     

Mode of interaction of hydrophobic amphiphilic alpha-helical peptide/dipalmitoylphosphatidylcholine with phosphatidylglycerol or palmitic acid at the air-water interface

Surface pressure (pi)-, surface potential (DeltaV)-, dipole moment (mu( perpendicular))-area (A) isotherms and morphological behavior at the air-water interface were obtained for multicomponent monolayers of two different systems for dipalmitoylphosphatidylcholine (DPPC)/egg-phosphatidylglycerol (PG) (= 68:22, by weight)/Hel 13-5 and DPPC/palmitic acid (PA) (= 90:9, by weight)/Hel 13-5 (Hel 13-5 is a newly designed 18-mer amphiphilic alpha-helical peptide with 13 hydrophobic and 5 hydrophilic amino acid residues). The phase behavior of these model systems was investigated on a subsolution of 0.02 M tris(hydroxymethyl)aminomethane (Tris) buffer (pH 8.4) with 0.13 M NaCl at 298.2 K by employing the Wilhelmy method, the ionizing electrode method, and fluorescence microscopy. Especially, the present study focuses on the interfacial effect of the addition of Hel 13-5 on two binary systems, DPPC/egg-PG and DPPC/PA monolayers, as the substitute for pulmonary surfactant proteins, and on the respective roles of PG and PA for the monolayers in the three-component systems. Constant kink points ( approximately 42 mN m(-1)) clearly appear on the pi-A isotherms, independent of the compositions in the ternary systems, which corresponds to the Hel 13-5 collapse pressure similar to that of SP-B and SP-C as functions in multicomponent monolayers. This implies that Hel 13-5 is squeezed out of ternary monolayers above approximately 42 mN m(-1), resulting in two- to three-dimensional phase transformation. Furthermore, Langmuir isotherms clearly show that Hel 13-5 with egg-PG is squeezed out of the DPPC/egg-PG/Hel 13-5 system, whereas only Hel 13-5 is squeezed out of the DPPC/PA/Hel 13-5 system. Cyclic compression and expansion isotherms of these systems were carried out to confirm the spreading and respreading capacities. In addition, the interfacial behavior of the ternary mixtures has been analyzed by the additivity rule. Morphological examinations and comparisons have verified the interactions of Hel 13-5 with the representative miscible mixture (DPPC/PA system) by fluorescence microscopy. Consequently, distinct morphological variations corresponding to the squeeze-out behavior are observed as a fluorescent contrast recovery. Herein, a new mechanism of the refluorescent phenomenon is proposed by varying the surface composition of Hel 13-5.     

Monolayers of a novel ionoselective butadienyl dye

The novel amphiphilic benzodithia-18-crown-6 butadienyl dye (1) forms relatively stable insoluble monolayers on distilled water (collapse pressure of 41 mN/m) and on aqueous subphases containing alkali metal or heavy metal salts (collapse pressures in the range of 27-38 mN/m, respectively). The dye 1 monolayer organization depends on chromophore association and interactions (especially complex formation) with heavy and alkali metal ions as deduced from surface pressure-area and surface potential-area isotherms as well as reflection spectra and Brewster angle microscopy observations. Dye 1 undergoes specific interactions with Hg(2+) and Ag(+), respectively (formation of different complexes). Nonspecific interactions have been observed with other salts, such as KClO(4) or Pb(ClO(4))(2). Further, dye 1 monolayers on 1 mM Hg(ClO(4))(2) solution undergo reversible photoisomerization, in contrast to monolayers on water and other aqueous salt subphases.