A nonlinear uniaxial stress-strain constitutive model for viscoelastic membrane materials

Membrane materials with the excellent thermal, optical, electrical and chemical properties have attracted significant attention in numerous research fields recently. However, while being used to construct the membrane structures, the mechanical behaviors of membrane materials are more foundational than the other properties in evaluating the structure safety. This paper thus proposes a nonlinear stress-strain constitutive model for revealing the viscoelastic behaviors of membrane materials under uniaxial tensile loading. To this end, the constitutive equations for expressing the uniaxial tensile stress-strain relationships of viscoelastic materials are established gradually from the kinematic equations of the generalized Maxwell model that includes several basic Maxwell models and one basic spring element. Meanwhile, the uniaxial tensile tests of two typical viscoelastic membrane materials were carried out in order to examine the proposed constitutive model. The constitutive model parameters of the stress-strain properties of both membrane materials are accurately identified using the least square method. By comparing the true stress-strain curves between experimental results and constitutive models, good agreements with the maximum differences of 4.67% and 3.41% are acquired for the two employed viscoelastic membrane materials, respectively. These observations are able to validate the accuracy and efficiency of this proposed constitutive model in predicting the uniaxial stress-strain behaviors of viscoelastic membrane materials, which are significant in the nonlinear structural analysis of membrane structures.     

Counterion-mediated pattern formation in membranes containing anionic lipids

Most lipid components of cell membranes are either neutral, like cholesterol, or zwitterionic, like phosphatidylcholine and sphingomyelin. Very few lipids, such as sphingosine, are cationic at physiological pH. These generally interact only transiently with the lipid bilayer, and their synthetic analogs are often designed to destabilize the membrane for drug or DNA delivery. However, anionic lipids are common in both eukaryotic and prokaryotic cell membranes. The net charge per anionic phospholipid ranges from − 1 for the most abundant anionic lipids such as phosphatidylserine, to near − 7 for phosphatidylinositol 3,4,5 trisphosphate, although the effective charge depends on many environmental factors. Anionic phospholipids and other negatively charged lipids such as lipopolysaccharides are not randomly distributed in the lipid bilayer, but are highly restricted to specific leaflets of the bilayer and to regions near transmembrane proteins or other organized structures within the plane of the membrane. This review highlights some recent evidence that counterions, in the form of monovalent or divalent metal ions, polyamines, or cationic protein domains, have a large influence on the lateral distribution of anionic lipids within the membrane, and that lateral demixing of anionic lipids has effects on membrane curvature and protein function that are important for biological control.     

Helfrich's concept of intrinsic force and its molecular origin in bilayers and monolayers

Bilayers and monolayers are excellent models of biological membranes. The constituents of the biological membranes such as lipids, cholesterols and proteins are chiral. Chiral molecules are abundant in nature (protein, nucleic acid and lipid). It is obvious that relationship between chirality and morphology (as well as function) of biological membrane is of interest for its fundamental importance and has technological implication regarding various membrane functions. The recent years have witnessed that a number of experimental studies in biomimetic systems have shown fascinating morphologies where chirality of the constituent molecule has decisive influence. Significant progress is made towards the understanding of these systems from the theoretical and computational studies. Helfrich's concept of intrinsic force arising from chirality is a milestone in understanding the biomimetic system such as bilayer and the related concepts, further progresses in molecular understanding made in recent years and experimental studies revealing the influence of chirality on morphology are the focus of the present review. Helfrich's concept of intrinsic force arising due to chirality is useful in understanding two-dimensional bilayers and one-dimensional monolayers and related mimetic systems. Various experimental techniques are used, which can probe the molecular architecture of these mimetic systems at different length scales and both macroscopic (thermodynamic) as well as microscopic (molecular) theories are developed. These studies are aimed to understand the role of chirality in the molecular interaction when the corresponding molecule is present in an aggregate. When one looks into the variety of morphologies exhibited by three-dimensional bilayer and two-dimensional monolayer, the later types of systems are more exotic in the sense that they show more diversity and interesting chiral discrimination. Helfrich's concept of intrinsic force may be considered useful in both cases. The intrinsic force due to chirality is the decisive factor in determining morphology which is explained by molecular approaches. Finally, biological and technological implications of such morphological variations are briefly mentioned.     

二氧化钛结合超滤膜富集和分离肿瘤患者唾液中的磷酸化肽和唾液酸化糖肽

利用TiO2富集、超滤膜滤过实现了选择性富集和分离磷酸化肽和含唾液酸的N-链接糖肽.首先选取截留分子量为1×104的超滤膜分离糖肽和磷酸化肽,然后利用酪蛋白和牛血清白蛋白的酶解物验证所建立的方法,该方法的最低检出限是0.16 pmol.将上述方法应用于肺癌患者的唾液检测,成功检测并鉴定到8个磷酸化肽和5个糖肽.本方法可有效提高磷酸化肽和糖肽检测的选择性和灵敏度,为疾病生物标志物的检测提供了新方法.     

Fabrication of Hydrophilic and Sponge-like PVDF/Brush-like Copolymer Blend Membranes Using Triethylphosphate as Solvent简

Porous PVDF blend membranes with good hydrophilicity and a symmetric structure were prepared by the phase inversion method using amphiphilic brush-like copolymers, P（MMA-r-PEGMA）, as hydrophilic additive and triethylphosphate （TEP） as solvent. P（MMA-r-PEGMA） was synthesized by radical polymerization in TEP. Then the obtained amphiphilic copolymer solution was mixed with PVDF and TEP to prepare the dope solution. The effects of P（MMA-r-PEGMA） content and coagulation composition on membrane morphologies were investigated using scanning electron microscopy （SEM）. The results demonstrated that, even blended with amphiphilic copolymers, a symmetric structure can be formed. Hollow fiber membranes with a mainly symmetric structure were also fabricated. The dry hollow fiber membranes showed good hydrophilicity, high flux and good rejection performance because of their hydrophilic surface and pores wall.     

Nano-electromembrane extraction

The present work has for the first time described nano-electromembrane extraction (nano-EME). In nano-EME, five basic drugs substances were extracted as model analytes from 200 μL acidified sample solution, through a supported liquid membrane (SLM) of 2-nitrophenyl octyl ether (NPOE), and into approximately 8 nL phosphate buffer (pH 2.7) as acceptor phase. The driving force for the extraction was an electrical potential sustained over the SLM. The acceptor phase was located inside a fused silica capillary, and this capillary was also used for the final analysis of the acceptor phase by capillary electrophoresis (CE). In that way the sample preparation performed by nano-EME was coupled directly with a CE separation. Separation performance of 42,000–193,000 theoretical plates could easily be obtained by this direct sample preparation and injection technique that both provided enrichment as well as extraction selectivity. Compared with conventional EME, the acceptor phase volume in nano-EME was down-scaled by a factor of more than 1000. This resulted in a very high enrichment capacity. With loperamide as an example, an enrichment factor exceeding 500 was obtained in only 5 min of extraction. This corresponded to 100-times enrichment per minute of nano-EME. Nano-EME was found to be a very soft extraction technique, and about 99.2–99.9% of the analytes remained in the sample volume of 200 μL. The SLM could be reused for more than 200 nano-EME extractions, and memory effects in the membrane were avoided by effective electro-assisted cleaning, where the electrical potential was actively used to clean the membrane.     

Regio‐ and Stereospecificity in the Oxygenation of Arachidonic Acid Catalyzed by Leu597 Mutants of Rabbit 15‐Lipoxygenase: A QM/MM Study

We combined quantum mechanics/molecular mechanics calculations with molecular dynamics simulations to study the addition of O₂ to the pentadienyl radical of arachidonic acid (AA) catalyzed by the Leu597Val and Leu597Ala mutants of rabbit 15‐lipoxygenase (15‐rLO). In the Leu597Val mutant, the addition of O₂ to C15 of AA is the predominant path, although it reduces the C15/C11 product ratio by almost ten times with respect to the wildtype enzyme. The S stereochemistry is kept. Mutation to Ala causes just the opposite effect: regiospecificity favoring addition to C15 is somewhat sharper than that in the wildtype, but the stereochemistry is R. This is because the extra space created by the mutation to Ala is big enough for AA to move so that it can adopt an alternative binding mode, and this opens new feasible paths for the attack of O₂. So, we showed that the Leu597Ala mutant of 15r‐LO works as an aspirin‐acetylated cyclooxygenase‐2, which makes 15‐(R)‐ hydroperoxyeicosatetraenoic acid.     

Preparation of two flavonoid glycosides with unique structures from barley seedlings by membrane separation technology and preparative high‐performance liquid chromatography

Barley seedlings are rich in flavones that can have positive effects on people with antihypoxia and antifatigue. Lutonarin and saponarin are two major flavonoid glycosides that have unique structures in barley seedlings. This study presents a new approach for the preparation of lutonarin and saponarin from barely seedlings by membrane separation technology and preparative high‐performance liquid chromatography. Preparative conditions of these two flavonoid glycosides by membrane separation technology were studied using response surface methodology. Under the optimized conditions, the total contents of these two flavonoid glycosides amounts to 17.0%.     

Influence of Charging Current and Potential Drop on the Propagation of the Change in the Membrane Potential

Propagation of a change in a potential difference between two aqueous phases (W1 and W2) across a membrane was examined by using three membrane cells (A, B and C). At first, the cell A was electrically connected with the cell B by controlling the ionic compositions. By changing the connection with the cell A from the cell B to the cell C indicating the different membrane potential, the change of the membrane potential was propagated. The delay and decrement of the propagation was observed by setting capacitors or resistors in the electric circuit.