X-ray reflectivity study of cyclic peptide monolayers at the air-water interface

The dynamic and living characteristics of monolayers at the air-water interface of a cyclohexapeptide ( C6G ) and a cyclooctapeptide ( C8G ), both composed of glutamic acid and 3-aminobenzoic acid subunits in an alternating sequence, were investigated using the Langmuir balance technique, Brewster angle microscopy (BAM), and X-ray reflectivity (XR). An alanine-containing cyclohexapeptide ( C6A ) was included in this study for comparison. All three cyclopeptides preferentially adopt an orientation parallel to the subphase at low surface pressure. Continuous compression then causes the molecules to flip to a perpendicular state, thus minimizing their molecular area. In contrast to C8G and C6A , a pronounced hysteresis observed during the compression-expansion cycle of C6G indicates that strong intermolecular interactions between the cyclopeptide rings occur in the monolayers of this peptide. This result is supported by BAM measurements that show the formation of crystallite structures for C6G at high surface pressures, whereas no structures were observed for C8G and C6A . These results indicate that C6G is able to self-assemble upon surface compression, an ability that is obviously critically dependent on the correct ring size and composition of the peptide. The presence of hydrogen bond acceptors in the side chains of C6G suggests that the structural stabilization of the monolayer is due to H-bonding, possibly between ring NH groups and side chain CO groups. Our in situ study thus provides a detailed understanding of the molecular dynamics and uninterrupted interfacial behavior of the three peptides in a real-time frame.     

Surface viscosities of phospholipids alone and with cholesterol in monolayers at the air-water interface

Surface viscosities in lipid monolayers at the air-water interface were measured by the oscillating pendulum method. The logarithms of successive oscillations decreased linearly with time. Surface viscosity is reported here in terms of the rate constant, k, for decay of oscillation. Viscosities were measured as a function of surface pressure at 22±2 C. Lipids investigated included cholesterol, 1-palmitoyl-2-arachidonoyl phosphatidylcholine (PC), 4 other unsaturated PC, 1-palmitoyl-2-stearoyl PC, 1,2-distearoyl PC, 1-palmitoyl-lysophosphatidylcholine, 1-palmitoyl-lysophosphatidylserine, tetrapalmitoyl bisphosphatidic acid, and the dipalmitoyl species of PC, phosphatidylethanolamine (PE), phosphatidyldimethylethanolamine, phosphatidylmonomethylethanolamine, phosphatidylglycerol and phosphatidic acid. Pressure-area curves are presented for the saturated phospholipids. Surface viscosities of most of the phospholipids were high and increased with increasing surface pressure. However, surface viscosiries in monolayers of cholesterol, 1-palmitoyl-2-arachidonoyl PC, lysophosphatidylcholine or lysophosphatidylserine were very low and barely detectable under our experimental conditions. One mol % of cholesterol in monolayers of dipalmitoyl PC greatly reduced the surface viscosity of the film and, in mixed films containing 10% or more of cholesterol, surface viscosity was too low to measure. Cholesterol also reduced surface viscosities in monolayers of the other dipalmitoyl phospholipids. It is suggested that cholesterol functions in lung surfactant by reducing the surface viscosity of its highly saturated phospholipid components.     

Effect of ethanol on monoglyceride monolayers at the air-aqueous phase interface

We have studied the behavior of monopalmitin and monoolein monolayers, spread at the air-aqueous phase interface, as a function of temperature and surface pressure. The subphases were aqueous ethanol solutions at 0.5 and 1 mol/L. The structural characteristics of these films at interface were deduced from the π-A isotherms, as measured with an automated Langmuir-type film balance. The monolayer structure and stability were functions of hydrocarbon chain length and the presence of a double bond. Generally, the factors that decreased monolayer stability produced transformations toward configurations with more expanded structures. Changes in the subphase composition had a direct influence on the monolayer molecular structure. This study showed the existence of interactions between film and ethanol molecules at the interface. As a consequence of these interactions, a contraction in the monolayer structure was observed. The magnitude of interactions between monoglyceride and ethanol molecules at the interface depends on the surface pressure, temperature, and surface composition. Stronger film-substrate interactions produced changes in monolayer stability. Relationships between film elasticity and structural characteristics are also discussed.     

Neutron reflectometry from stereotactic isomers of poly(methyl methacrylate) monolayers spread at the air-water interface

Surface pressure-area isotherms on poly(methyl methacrylate) ‘monolayers’ spread at the air-water interface show that the isotactic polymer is in good solvent conditions, whereas the atactic and syndiotactic isomers are in thermodynamic environments that are worse than ‘theta’ conditions. Neutron reflectometry data have been analysed to give monolayer thickness and composition. At low surface concentrations, the layer thickness of the syndiotactic polymer increases to an eventually constant value although the surface pressure and concentration increase still further. For the isotactic polymer, the layer thickness is essentially constant over the range of surface concentrations investigated. Initially, the monolayers contain a considerable volume fraction of air, which decreases as the surface concentration increases. The monolayer thicknesses obtained are considerably larger than recent estimates from ellipsometry measurements, and possible sources for this discrepancy are discussed.     

碱性蛋白酶的空气/水界面行为研究

通过粒径和Zeta电位随时间和质量分数的变化考察了碱性蛋白酶在水溶液中的聚集过程,研究了碱性蛋白酶在空气/水界面的动态表面张力及其扩张流变性质,并测定了碱性蛋白酶酶活力随时间的变化,探讨了碱性蛋白酶酶活力与界面性质和溶液聚集行为之间的关系。研究结果显示:碱性蛋白酶在水中形成的聚集体粒径随时间和质量分数的增大而增大,而Zeta电位则降低,最终导致碱性蛋白酶与水发生相分离;碱性蛋白酶聚集体粒径增大,还将导致碱性蛋白酶扩散到界面上的动力学过程发生改变,由此影响到表面张力和界面膜黏弹性;随着碱性蛋白酶在溶液中的聚集,其酶活力下降。     

Influence of side chain structure on sorption of xenon and carbon dioxide gas by polypeptide membranes

The influence of side chain structure on sorption of xenon and carbon dioxide gas by polypeptides is examined in two ways: chemical structure of side chain and higher order structure of the side chain region. Sorption of xenon gas increases progressively with increase in side chain length of poly(n-alkyl L -glutamates). The sorption isotherms of a polymer at various temperatures are reduced to one curve when the data are arranged in the form of sorption per residue versus fugacity ratio (as activity). The sorption of carbon dioxide gas shows a minimum at an intermediate length of side chains, corresponding to poly(n-propyl or n-butyl glutamate). The behavior is considered the result of overlapping influences of looseness of the side chain region and density of polar groups. The Langmuir constants and the enthalpy and entropy of sorption explain this quantitatively. Sorption of carbon dioxide gas is influenced by the higher-order structure of poly(γ-benzyl L -glutamate). In this polymer, the benzyl ester part is the main sorption site, and it was confirmed that the stacking between side-chain benzene rings significantly reduces the extent of sorption.     

The energetics of surfactant adsorption at the air-water interface

The standard free energy, enthalpy, and entropy changes (ΔG, ΔH, and ΔS, respectively) for the adsorption at the air-water interface of a commercial ethylene oxide (EO) adduct of straight chain nonylphenol from monomer solution at the critical micelle concentration (CMC) have been calculated from surface tension-concentration data at 21C–45C using the Gibbs equation, the standard free energy change equation ΔG=−RT (In interfacial monomer concentration/CMC), and the Gibbs-Helmholtz equation which gave ΔH directly from the slope of the (ΔG/T) vs (1/T) function. The CMC and surface tension at the CMC (γCMC) decreased, and −ΔG and molecular area increased slightly, with increasing temperature. The ΔH and ΔS values were positive, and appear explainable by the postulations applied to micellization. At ambient temperature (28C) an increase in the (EO) mol ratio of straight chain nonylphenol and C₁₃ secondary alcohol ethoxylates resulted in increases of CMC, γCMC, and molecular area, and slight decreases in −ΔG. A comparison of the 9 (EO) mol ratio adducts of C₁₃ straight chain primary and secondary alcohols showed that the CMC and molecular area of the secondary alcohol ethoxylate were larger, and the γCMC and −ΔG smaller, than the corresponding values of the primary alcohol ethoxylate.     

Effect of pre-imidization on the aggregation structure and properties of polyimide films

In this paper, series of polyimide films with different aggregation structures were prepared through a partially pre-imidization process, and the properties were investigated in detail. The pre-imidization degree (pre-ID) was successfully controlled by adjusting the amount of dehydrating reagents, which was confirmed by FT-IR spectra measurement. PI films exhibited a quite different degree of crystallinity, which increased with pre-ID increasing. DSC curves showed obvious melt endothermic peaks at about 450 °C. The tensile strength, tensile moduli and elongation at break of the PI films increased obviously with pre-ID increasing. The corresponding values of PI-100 increased by 37.9%, 11.7% and 336%, respectively, compared with those of PI-0. TMA measurement showed that the coefficient of thermal expansion of PI films decreased with pre-ID increasing, indicating that the PI molecular chains were restricted by the crystalline structure. Meanwhile, those PI films obtained by the pre-imidization process still possess excellent thermal stability. The present work provided the theoretic indicators for controllable preparing of high-performance polyimide films.     

一类含柔性侧链结构质子交换膜的制备与性能

通过利用含侧甲基结构聚芳醚砜的溴化和接枝磺化反应，制备得到一系列结构单元中含有4个柔性侧链结构的磺化聚芳醚砜质子交换膜（4SPAES-x）。通过1HNMR表征其化学结构，并利用原子力显微镜对膜材料的相分离形态结构进行分析，证实所制膜材料具有良好的亲水/疏水相分离形态结构。4SPAES-x膜的离子交换容量在1.12~1.74 mmol/g，30 ℃时的吸水率、溶胀率和质子传导率分别在11%~32%、7%~22%和21~86 mS/cm，均随磺化比例的增大而增大。4SPAES-25膜组装的钒流单电池在40 mA/cm2电流密度下最高能量效率为83.3%，高于Nafion 115的81.5%。此外，该单电池的效率还具有良好的循环稳定性。     

Effect of chemical structure and preparation process on the aggregation structure and properties of polyimide film

In this article, polyimide (PI) films were fabricated via the three‐step method including the reactions of condensation polymerization, chemical imidization, and thermal imidization. In comparison with the conventional two‐step method to produce PI films, there was an additional step in the present method, i.e., chemical imidization. The aim of chemical imidization was to get PI intermediates with different pre‐imidization degree (pre‐ID). And PI component in PI intermediates acted as in‐situ rigid‐rod segments and induced orientation in the films of PI intermediates. Then the orientations of molecular chains were preserved in the following thermal imidization, and caused the difference in aggregation structure and property of the final PI films. The test results indicated that the orderly degree of molecular chains and mechanical properties of PI films increased with pre‐ID increasing. Furthermore, this tendency was much more obvious for more rigid backbone structure. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Effect of side chain structure on the liquid crystalline properties of polymers bearing cholesterol, dihydrocholesterol and bile acid pendant groups

Four methacrylate derivatives of steroid molecules, namely cholic acid methyl ester, lithocholic acid methyl ester, cholesterol and dihydrocholesterol, were synthesized. A 10-carbon spacer separates the rigid core and the polymerizable methacrylate group. These monomers and their corresponding polymers were characterized for their liquid crystalline properties by differential scanning calorimetry, polarizing optical microscopy and X-ray diffraction. It was found that only the compounds bearing the planar cholesterol-based moieties, and not those bearing the esterified bile acid groups, possess liquid crystalline phases. A detailed X-ray study showed that the dihydrocholesterol polymer possesses the same two mesophases as the cholesterol polymer, but with reduced thermal stability.     

Mechanical tuning of molecular machines for nucleotide recognition at the air-water interface

Molecular machines embedded in a Langmuir monolayer at the air-water interface can be operated by application of lateral pressure. As part of the challenge associated with versatile sensing of biologically important substances, we here demonstrate discrimination of nucleotides by applying a cholesterol-armed-triazacyclononane host molecule. This molecular machine can discriminate ribonucleotides based on a twofold to tenfold difference in binding constants under optimized conditions including accompanying ions in the subphase and lateral surface pressures of its Langmuir monolayer. The concept of mechanical tuning of the host structure for optimization of molecular recognition should become a novel methodology in bio-related nanotechnology as an alternative to traditional strategies based on increasingly complex and inconvenient molecular design strategies.     

Effect of plasticizers on structure and mechanical properties of Thiokol sealant

The compatibility of the Thiokol sealant U-30M with plasticizers of different types was estimated. The rheological properties of the plasticized compositions and the effect of the nature and concentration of plasticizer on the properties of vulcanizates were studied. It is shown that the addition of chlorinated paraffins (KhP-52 and KhP-470), a flotation agent, and PL-105 as plasticizers considerably improves the dielectric properties of the sealant. The formulations of coatings designed for the protection of concrete structures from corrosive media and for the manufacture of roofing and waterproofing materials were developed.     

Effect of pH on structure and mechanical properties of electrodeposited iron

Relationships between the mechanical properties of electrodeposited iron and the pH of the bulk of the solution were obtained for a chloride/fluoborate plating bath. Both ultimate tensile strength and percentage elongation at fracture passed through a maximum over a narrow range of pH. Structural studies using optical and electron microscopy supported the results of the experiments on mechanical properties. A relationship between the average surface-roughness height of the deposit and the bulk pH was also obtained. The results are explained in terms of hydrogen evolution and hydroxide precipitation.     

Effect of heat treatment on the structure and mechanical properties of polyacrylonitrile fibers

Strucure and mechanical properties of untreated and thermally treated (at 110°C and 150°C) drawn polyacrylonitrile (PAN) fiber have been studied through IR spectroscopy, wide angle X-ray diffraction, and tensile and dynamic mechanical properties measurements. The results are interpreted in terms of the change in the degree of bound nitrile group pairs on heat treatment. The heat treatment at both the temperatures resulted in higher degree of bound nitrile groups (through their dipolar interactions) in the heat-treated PAN than that of the untreated PAN, as evidenced by IR spectroscopy, X-ray diffraction, and the mechancal properties. The difference between the results for the two heat-treatment temperatures are not sufficiently large, although the changes produced by heat treatment at 110°C seems somewhat more prominent than those produced by heat treatment at 150°C.     

Self-assembly of conjugated oligomers and polymers at the interface: structure and properties

In this review, we give a brief account on the recent scanning tunneling microscopy investigation of interfacial structures and properties of π-conjugated semiconducting oligomers and polymers, either at the solid-air (including solid-vacuum) or at the solid-liquid interface. The structural aspects of the self-assembly of both oligomers and polymers are highlighted. Conjugated oligomers can form well ordered supramolecular assemblies either at the air-solid or liquid-solid interface, thanks to the relatively high mobility and structural uniformity in comparison with polymers. The backbone structure, substitution of side chains and functional groups can affect the assembling behavior significantly, which offers the opportunity to tune the supramolecular structure of these conjugated oligomers at the interface. For conjugated polymers, the large molecular weight limits the mobility on the surface and the distribution in size also prevents the formation of long range ordered supramolecular assembly. The submolecular resolution obtained on the assembling monolayers enables a detailed investigation of the chain folding at the interface, both the structural details and the effect on electronic properties. Besides the ability in studying the assembling structures at the interfaces, STM also provides a reasonable way to evaluate the distribution of the molecular weight of conjugated polymers by statistic of the contour length of the adsorbed polymer chains. Both conjugated oligomers and polymers can form composite assemblies with other materials. The ordered assembly of oligomers can act as a template to controllably disperse other molecules such as coronene or fullerene. These investigations open a new avenue to fine tune the assembling structure at the interface and in turn the properties of the composite materials. To summarize scanning tunneling microscopy has demonstrated its surprising ability in the investigation of the assembling structures and properties of conjugated oligomers and polymers. The information obtained could benefit the understanding of the elements affecting the film morphology and helps the optimization of device performance.     

Effect of synthetic phospholipids on platelet aggregation and serotonin release

1-O-Hexadecyl-2-O-acetyl-sn-glycerol-3-phosphocholine (platelet-activating factor, PAF) is known to stimulate platelet aggregation and serotonin release in concentrations ranging from 10^?10–10^?5 M. Since a variety of synthetic PAF analogues are potent antineoplastic agents in vitro and in vivo, it was the aim of this study to examine the PAF-like activity of 15 analogues, including 1-O-actadecyl-2-O-methyl-rac-glycero-3-phosphocholine (ET-18-OCH₃) and a thioether analogue. In platelet-rich plasma from human blood, platelet aggregation and serotonin release were studied to compare the effects on PAF and the analogues. Platelet function was controlled by testing their response to adenosine diphosphate, arachidonic acid, collagen and epinephrine. Our results show that only PAF was able to induce platelet aggregation and serotonin release in concentrations from 10^?9 to 10^?5 M, whereas all the tested analogues up to a concentration of 10^?3 M failed to induce these effects.     

Dependence of structure of polymer side chain on water structure in hydrogels

To investigate the effects of structure of polymer side chains on structure of water in hydrogels, Raman spectra of polyacrylamide (PAA) and poly-N,N,-dimethylacrylamide (PDMAA) hydrogels were measured. PAA and PDMAA have similar chemical structures, except for the side chain structure. The result shows that the frequency of the O–H stretching mode for the PAA hydrogel decreases as the water content decreases, while that for the PDMAA hydrogel increases. Furthermore, the analysis of the relative intensities of the O–H stretching modes shows that the number density of tetragonal water structures differs between the hydrogels. We conclude that most of the bound water molecules in the PAA hydrogel form four strong hydrogen bonds with the hydrophilic groups in the side chain of PAA, whereas those in the PDMAA hydrogel form weak hydrogen bonds with surrounding water molecules. The water structure is an important factor governing the physical and chemical properties of gel materials.     

Polyurethane microcellular elastomers: 2. Effect of chain extender on the mechanical properties

The influence of ethylene glycol, triethanol amine and a rigid polyol on the tensile strength, elongation at break and tear resistance of a microcellular elastomer (used in RIM technology), have been studied using multiple linear regression analysis. Using the statistical Student-t test, the importance of the above parameters has been established. It has been found that the moulded density has the biggest influence on the tensile strength; so that the tensile strength is increased by the ethylene glycol and rigid polyol content, while triethanol amine reduces it. All the chain extenders studied have a negative influence on the elongation at break. Similarly the moulded density and the ethylene glycol content have a beneficial effect on the tear resistance, while triethanol amine and the rigid polyol reduce this property.