小角X射线散射法研究CH2Cl2,CHCl3和CCl4对PE液晶结构的影响

采用小角X射线散射法研究了CH_2Cl_2、CHCl_3和CCl_4对磷脂酰乙醇胺(PE)液晶结构影响的机理.CH_3Cl_2、CHCl_3和CCl_4对PE液晶结构影响的差别主要是其空间旋转电子云密度分布形状不同所致.空间旋转电子云密度分布呈椭球状的物质有使PE液晶形成六角形H_1相的趋向;呈圆锥状的物质有诱发PE液晶形成立方六角相的趋向;呈球状的物质有使PE液晶形成片层立方相的趋向.     

Coexistent Fluid-Phase Equilibria in Biomembranes with Bending Elasticity

The theory of fluid surfaces with elastic resistance to bending is applied to coexistent phase equilibria in biomembranes composed of lipid bilayers. A simplified version of the model is used to simulate the necking and budding of closed vesicles.      

Theory of lateral and flip-flop phase separations in bilayer biomembranes and surfactants

In this work, we consider bilayer biomembranes or surfactants made of two amphiphiles A and B. Under a variation of a suitable parameter, such as temperature or difference of lengths of hydrophobic chains, these systems undergo a phase separation from a homogeneous liquid-phase to two distinct liquid-phases. Two physical situations can be distinguished: (1) The amphiphiles A and B prefer to jump from a monolayer to the other (flip-flop transition), (2) the mixture phase separates on each monolayer, and there is no jump from one sheet towards the second one (lateral transition). To investigate the associated critical phase behavior, we first introduce a field theory, constructed with two order parameters (or fields) φ and ψ, which are nothing else but the composition fluctuations relative to the monolayers. Beside the usual terms proportional to φ², ψ², φ⁴ and ψ⁴, the free energy contains an extra one, −Cφψ, which describes the lowest order coupling between the two monolayers. The coupling constant C is positive for the lateral phase separation, and negative for the vertical one. We show that its sign results from a competition between the chemical segregation of amphiphiles and the curvature asymmetry. With the help of this free energy, we first identify the liquid-phases, and show the existence of a critical point, T_c, of which the location depends naturally on the value of the coupling constant C. In particular, for those bilayer biomembranes or surfactants made of amphiphiles of the same chemical nature but with different lengths, and at fixed temperature, we show the existence of a critical line in the -plane, along which the bilayer undergoes a phase separation. Here, and account for the curvature gap and the length difference, respectively. Second, we determine the behavior of the composition fluctuations, φ and ψ, and the total one, Φ=φ+ψ, upon temperature, T, and chemical potential difference, , in the critical region. Third, we determine the critical behavior of the partial compressibilities, κ_φφ, κ_ψψ and κ_φψ, and the overall one, . Finally, we remark that the flip-flop phase separation shows some analogy with the classical para-ferrimagnetic transition of coupled paramagnetic materials of Curie-Weiss type.     

Thin-film voltammetry and its analytical applications: A review

Electrochemical reactions at the interfaces of immiscible electrolyte solutions (ITIES) are of fundamental importance in the fields of chemical, biological and pharmaceutical sciences. Four-electrode cell setup, scanning electrochemical microscopy (SECM) and thin-film voltammetry are the three most frequently used methods for studying the electrochemical processes at these interfaces. The principle, experimental design, advantages and challenges of the three methods are described and compared. The thin-film voltammetry is highlighted for its simplicity in experimental operation and kinetic data analysis. Its versatile analytical applications are discussed in detail, including the study of redox properties of hydrophobic compounds, evaluation of interfacial electron transfer kinetics, synthesis of nanoparticles/nanostructures, and illustration of cross-membrane ion transport phenomena.     

Synthesis of a photoactivatable phospholipidic probe containing tetrafluorophenylazide

In order to study lipid-lipid and lipid-protein interactions using a photolabeling approach, we synthesized and characterized a phospholipidic probe in which the photoactivatable tetrafluorophenylazido group is incorporated into the fatty acid chain. This probe is stable in the dark and becomes highly reactive upon being exposed to photoirradiation. It shows fast, clear-cut photochemical reactions and holds promise for further use to study lipid-lipid and protein-lipid interactions.     

Evaluation of antitubercular drug insertion into preformed dipalmitoylphosphatidylcholine monolayers

Insertion profiles of antitubercular drugs isoniazid (INH), rifampicin (RFM) and ethambutol (ETH) into dipalmitoylphosphatidylcholine (DPPC) membrane models were evaluated by Langmuir monolayer technique. Maximum drug insertion into DPPC monolayer was observed with rifampicin with a surface pressure increase (Δπ_max) in the range of 21–33 mN/m depending upon rifampicin concentration. Isoniazid had minimal insertion resulting in a lower Δπ_max of about 2–3 mN/m, suggestive of minimal interactions between INH and DPPC. Ethambutol surface pressure increment on insertion resulted in an intermediate rise in the Δπ_max (6–10 mN/m). Antitubercular drug combination in the ratio of 2 mM:0.7 mM:4.5 mM for INH:RFM:ETH, attained Δπ_max between 25 and 33 mN/m. Insertion profiles similar to rifampicin were exhibited by the antitubercular drug mixture suggestive of predominant rifampicin insertion into the DPPC monolayer. The extent of drug insertion into the DPPC monolayer is suggestive of the drug penetration potential into biological membranes in vivo. Higher RFM Δπ_max is suggestive of excellent cell membrane penetration, which explains broad reach of the drug to all the organs including the cerebrospinal fluid while lower Δπ_max of INH suggests poor membrane penetration restricting the entry of the drug in different biological membranes. DPPC membrane destabilization was observed at higher antitubercular drug concentrations indicated by the negative slopes of the surface pressure–time curves. This may correlate with the dose related toxic effects observed in tuberculosis affected patients. Drug insertion studies offer a potential tool in understanding the pharmacotoxicological behavior of the various pharmacological agents.     

Immobilization of cationic rifampicin-loaded liposomes on polystyrene for drug-delivery applications

Polymer-associated infections are a major problem in implanted or intravascular devices. Among others, microorganisms of the staphylococcal family have been identified as the most important culprit. Prevention of bacterial adhesion and colonization of polymeric surfaces by release of antimicrobial agents incorporated into the polymers itself are currently under study. We have developed a novel method for the functionalization of a polymeric surface which is based on the deposition of covalently coupled lipid structures from antibiotic loaded vesicles. We have found that such process significantly reduces the bacterial growth on polystyrene material. In this work, lipid coverage obtained from multilamellar (MLVs) and extruded unilamellar (LUVs) vesicles were analyzed with respect to their adhesion efficiency on three types of polystyrene (PS) well-plates. Two methods of lipid deposition were characterized and compared in terms of surface lipid density and time stability: deposition of cationic vesicles on negatively charged surfaces and formation of covalent linkages between functionalized lipids and amines enriched surfaces. In order to study the antibiotic encapsulation efficiency we measured how the rifampicin (RIF) loading was affected by changes of liposome charge upon introduction of various amounts of stearylamine (SA), distearoyl-trimethylammonium propane (DSTAP) or dioleoyloxypropyl-trimethylammonium chloride (DOTAP) into the liposomal formulation. RIF-coated polymeric surfaces were also tested against a Staphylococcus epidermidis strain to evaluate their efficacy in vitro, showing that only approximately 2% of such bacteria inoculated on MLV-treated PS substrate were able to proliferate. Covalently immobilized lipid films showed about a tenfold increase in time stability compared to electrostatically bonded lipid films. Furthermore, substrates covalently modified with RIF-loaded MLVs retained an antibacterial activity for up to 12 days when aged in buffer at 37 degrees C. Such antimicrobial polymer coatings show promise for their use as antibacterial barrier for the prevention of catheter-related infections.     

疏水纳米颗粒对仿生膜流动性的影响: 尺寸效应

金属纳米颗粒与仿生膜的组装材料具有广阔的应用前景, 但纳米颗粒载入对仿生膜的物理性质的影响却存在争议. 在本文中, 我们合成了大于与小于膜厚度的疏水金纳米颗粒 (LPNs与SNPs), 系统地研究了粒子载入对膜流动性的影响. 通过荧光漂白恢复实验发现, LPNs与SNPs分别降低与提高磷脂的侧向扩散率. 基于荧光光谱实验与理论分析, 得到LPNs与SNPs分别通过空间位阻、改变磷脂有序度两种机制影响磷脂的侧向运动. 研究结果对构建功能性纳米颗粒磷脂组装体, 通过纳米材料调控仿生膜功能方面提供了参考.     

(1)H-(13)C INEPT MAS NMR correlation experiments with (1)H-(1)H mediated magnetization exchange to probe organization in lipid biomembranes

Two-dimensional (1)H-(13)C INEPT MAS NMR experiments utilizing a (1)H-(1)H magnetization exchange mixing period are presented for characterization of lipid systems. The introduction of the exchange period allows for structural information to be obtained via (1)H-(1)H dipolar couplings but with (13)C chemical shift resolution. It is shown that utilizing a RFDR recoupling sequence with short mixing times in place of the more standard NOE cross-relaxation for magnetization exchange during the mixing period allowed for the identification and separation of close (1)H-(1)H dipolar contacts versus longer-range inter-molecular (1)H-(1)H dipolar cross-relaxation. These 2D INEPT experiments were used to address both intra- and inter-molecular contacts in lipid and lipid/cholesterol mixtures.     

Derivation of a new free energy for biological membranes

A new free energy for thin biomembranes depending on chemical composition, degree of order and membranal-bending deformations is derived in this paper. This is a result of constitutive and geometric assumptions at the three-dimensional level. The enforcement of a new symmetry group introduced in (Deseri et al., in preparation) and a 3D--2D dimension reduction procedure are among the ingredients of our methodology. Finally, the identification of the lower order term of the energy (i.e. the membranal contribution) on the basis of a bottom-up approach is performed; this relies upon standard statistical mechanics calculations. The main result is an expression of the biomembrane free energy density, whose local and non-local counterparts are weighted by different powers of the bilayer thickness. The resulting energy exhibits three striking aspects: