全氟化合物热力学性质的密度泛函理论研究

用密度泛函理论B3LYP方法,在6-31G基组水平上,对12个全氟化合物分子进行了全优化计算,得到其分子零点振动能EZPV、热能校正值Eth、恒容热容CVΦ、标准熵SΦ以及配分函数lgQ等热力学参数,并计算了这些分子的电性拓扑状态指数Em.通过最佳变量子集回归建立了电性拓扑状态指数与热力学参数之间的QSPR模型,模型的相关系数R2分别为1.000,1.000,1.000,0.999和1.000,采用逐一剔除法得到的交叉验证相关系数R2cv分别为0.999,1.000,1.000,0.999和1.000,利用建构的数学模型得到热力学性质的相对平均误差分别为0.43%,0.41%,0.46%,0.41%和0.71%.从方程可以看出,F原子取代基数量是影响全氟化合物分子热力学参数大小的主要因素,检验证明所建模型具有良好的稳定性和预测能力.     

Influence of pyrene-labeling on fluid lipid membranes

We elucidate the influence of pyrene-labeled phospholipids on the structural properties of a fluid dipalmitoylphosphatidylcholine lipid membrane. To this end, we employ extensive atomic-scale molecular dynamics simulations with varying concentrations of pyrene-linked lipids. We find pyrene labeling to perturb the membrane structure significantly in the vicinity of the probe, the correlation length in the bilayer plane being about 1.0-1.5 nm. The local perturbations lead to enhanced ordering and packing of lipid acyl chains located in the vicinity of the probe. Surprisingly, this holds true not only for lipids that reside in the same leaflet as the pyrene-labeled probe but also for lipids in the opposite monolayer. The latter is due to substantial interdigitation of the pyrene moiety into the opposite leaflet, suggesting that occasional excimer formation may take place for probes in different leaflets. As a related issue, we also discuss the location and conformational orientation of the pyrene moieties. In particular, the orientational distribution of pyrene turns out to be more broad and diverse than the distribution of the corresponding acyl tails of nonlabeled lipids.     

Polarization modulation infrared reflection-absorption spectroscopy studies of the influence of perfluorinated compounds on the properties of a model biological membrane

A combination of the Langmuir-Blodgett and Langmuir-Schaefer techniques has been used to build a 1,2-dimyristoyl- sn-glycero-3-phosphocholine (DMPC) bilayer at a Au(111) electrode surface with hydrogen-substituted acyl chains in the top leaflet (solution side) and deuterium-substituted acyl chains in the bottom leaflet (gold side). Polarization modulation infrared reflection-absorption spectroscopy was used to determine changes in the conformation and orientation of the acyl chains of DMPC caused by the incorporation of two selected perfluorinated compounds, perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS), into the top leaflet of the bilayer. The incorporation of perfluorinated compounds into the DMPC bilayer caused a broadening of the methylene peaks and a shift in the methylene band positions toward higher frequencies. In addition, the tilt angle of the acyl chains decreased in comparison to the tilt angle of a pure DMPC bilayer. The reported tilt angles were smaller upon insertion of PFOS ( approximately 24 degrees ) than in the presence of PFOA ( approximately 30 degrees ). Overall, the results show that the incorporation of the perfluorinated acids has an effect on the bilayer similar to that of cholesterol by increasing the membrane fluidity and thickness due to a decrease in the tilt angle of the acyl chains.     

Influence of organotin compounds on phosphatidylserine membranes

Organotin compounds are widely distributed toxicants. They are membrane‐active molecules with broad biological toxicity. We have studied the interaction of tributyltin and triphenyltin with phosphatidylserine model membranes using differential scanning calorimetry, infrared spectroscopy and X‐ray diffraction techniques. Organotin compounds produced a broadening of the gel to the liquid‐crystalline phase transition of the phospholipid and a shifting of the phase transition temperature to lower values. Infrared spectroscopy experiments showed that tributyltin exerted a fluidizing effect on the apolar part of the bilayer, and that both tributyl‐ and triphenyltin interact with the interfacial region of the bilayer, making the carbonyl groups less accessible to water. As seen by X‐ray diffraction experiments, organotin compounds were unable to change the bilayer macroscopic organization of the phospholipid, but they were able to reduce the long‐range order of the multibilayer system and to disorder the packing of the phospholipid molecules. The observed interaction between organotin compounds and phosphatidylserine membranes promotes physical perturbations that could affect membrane function and may mediate some of their toxic effects. Copyright © 2004 John Wiley & Sons, Ltd.     

Electroanalytical properties of membranes based on metal-polyethoxylate-tetraphenylborate compounds

Cationic functions of selective electrodes based on tetraphenylborate salts of polyethoxylate complexes of Pb(II), Ba(II), Ca(II), Cd(II), and Zn(II) (M-PEO-TPB) are studied in solutions of these metal cations and polyethoxylates. It is shown that the linearity range of the electrode response, detection limit, and the slope of the electrode function are governed by the stoichiometry of reaction, stability, and solubility of complex [M-PEO]²⁺ cations in water. Lead-selective electrodes based on a Pb(II)-PEO-TPB compound exhibit the best parameters. The behavior of these electrodes in polyethoxylate solutions is studied and their performance characteristics (response time, potential drift, service life) are determined.     

Relationship between ionic conductivity of perfluorinated ionomeric membranes and nonaqueous solvent properties

Ionic conductivity and swelling data are measured for Nafion^® perfluorinated ionomeric membranes in nonaqueous solvents and solvent mixtures and correlated with solvent physical properties. The dependence of ionic conductivity on solvent uptake and cation type is examined for Nafion^® 117 membranes with a nominal equivalent weight of 1100 g/eq. The most important factors determining ionic conductivity in membranes swollen with polar nonaqueous solvents are the solvent viscosity, molar volume, donor properties, and the solvent uptake by the membrane. Ionic conductivity is generally limited by dissociation of the cation from the fixed anion site indicating that the ionomer fixed anion site basicity is the critical membrane property. Means for increasing membrane ionic conductivity are discussed.     

Atomic force microscopy of model lipid membranes

Supported lipid bilayers (SLBs) are biomimetic model systems that are now widely used to address the biophysical and biochemical properties of biological membranes. Two main methods are usually employed to form SLBs: the transfer of two successive monolayers by Langmuir–Blodgett or Langmuir–Schaefer techniques, and the fusion of preformed lipid vesicles. The transfer of lipid films on flat solid substrates offers the possibility to apply a wide range of surface analytical techniques that are very sensitive. Among them, atomic force microscopy (AFM) has opened new opportunities for determining the nanoscale organization of SLBs under physiological conditions. In this review, we first focus on the different protocols generally employed to prepare SLBs. Then, we describe AFM studies on the nanoscale lateral organization and mechanical properties of SLBs. Lastly, we survey recent developments in the AFM monitoring of bilayer alteration, remodeling, or digestion, by incubation with exogenous agents such as drugs, proteins, peptides, and nanoparticles.

Influence of polymyxins on the structural dynamics of Escherichia coli lipid membranes

Polymyxins are a family of nonribosomic cationic peptide antibiotics highly effective against Gram-negative bacteria. Two members of this family, Polymyxins B and E (PxB, PxE), form molecular vesicle-vesicle contacts and promote a selective exchange of phospholipids at very low concentrations in the membrane, a biophysical phenomenon that can be the basis of their antibiotic mode of action. To get more insight into the interaction of these antibiotics with the lipid membrane, their effect on the structural dynamics of bilayers prepared with lipids extracted from the membrane of Escherichia coli was determined using fluorescently labeled phopholipids. Steady-state anisotropy measurements with probes that localize at different positions in the membrane give information on the effects of polymyxins on the mobility of the phospholipids. Results with PxB, PxE, colymycin M and polymyxin B nonapeptide (PxB-NP), a deacylated derivative with no antibiotic properties, are compared. At low peptide concentrations (<2 mol%) PxB and PxE bind to the membranes superficially, affecting very slightly the ordering of the lipids at the outermost part of the bilayer. Above this concentration, PxB and PxE insert more deeply in the bilayer, increasing lipid order both in the gel and liquid-crystal states and modifying phase transitions. Fluorescence experiments with pyrene labeled phospholipids indicate that the increase in lipid packing is accompanied by an enrichment of phospholipids in the bilayers. In contrast, colymycin M and PxB-NP did not modify lipid packing or phase transition, nor did they induce microdomain formation. The possible significance of these results in the antibiotic mode of action of PxB and PxE is discussed. The combination of spectroscopic techniques described here can be useful as part of a general method of screening for new antibiotics that act on the membrane by the same mechanism as polymyxins.     

Interactions between model inclusions on closed lipid bilayer membranes

Protein inclusions in the membranes of living cells interact via the deformations they impose on that membrane. Such membrane-mediated interactions lead to sorting and self-assembly of the inclusions, as well as to membrane remodelling, crucial for many biological processes. For the past decades, theory, numerical calculations and experiments have been using simplified models for proteins to gain quantitative insights into their behaviour. Despite challenges arising from nonlinearities in the equations, the multiple length scales involved and the nonadditive nature of the interactions, recent progress now enables for the first time a direct comparison between theoretical and numerical predictions and experiments. We review the current knowledge on the biologically most relevant case, inclusions on lipid membranes with a closed surface and discuss challenges and opportunities for further progress.     

Influence of solvents on the properties of chemical compounds

Based on Sinanolu's solvent theory in connection with a method for the calculation of the molecular volume given by Beveridge a modified continuum model is suggested. The model is tested by the determination of the conformational structure of the title compounds. The predictions obtained by this model are in good agreement with the experimentally determined data.     

Influence of Pb(II) on the radical properties of humic substances and model compounds

The influence of Pb(II) ions on the properties of the free radicals formed in humic acids and fulvic acids was investigated by electron paramagnetic resonance spectroscopy. It is shown that, in both humic acid and fulvic acid, Pb(II) ions shift the radical formation equilibrium by increasing the concentration of stable radicals. Moreover, in both humic acid and fulvic acid, Pb(II) ions cause a characteristic lowering of the stable radicals' g-values to g = 2.0010, which is below the free electron g-value. This effect is unique for Pb ions and is not observed with other dications. Gallic acid (3,4,5-trihydroxybenzoic acid) and tannic acid are shown to be appropriate models for the free radical properties, i.e., g-values, Pb effect, pH dependence, of humic and fulvic acid, respectively. On the basis of density functional theory calculations for the model system (gallic acid-Pb), the observed characteristic g-value reduction upon Pb binding is attributed to the delocalization of the unpaired spin density onto the Pb atom. The present data reveal a novel environmental role of Pb(II) ions on the formation and stabilization of free radicals in natural organic matter.     

The usefulness of optically active perfluorinated compounds

The usefulness of optically active (+),(?)-perfluoropropoxy- and perfluoroisopropoxypropionic acids as derivatives for the analysis of chiral amines and alcohols is mentioned. Lanthanide chelates of (+),(?)-di(perfluoropropoxypropionyl)methane behaved as a useful shift reagent for the pmr analysis of optically active compounds.     

Theoretical studies on the structure of hydration in perfluorinated lithium salt membranes

Using an ab initio molecular orbital (MO) method, the normal frequencies are calculated for perfluorinated lithium sulfonate and carboxylate membranes by construction of a cluster model, which severs the ion core from the polymer chain, and then analysis of the experimentally observed infrared (IR) spectra is carried out. During the process of dehydration, small sharp peaks at about 3650 and 3700 cm(-1) appeared on the shoulder of the broad band at about 3500 cm(-1). These sharp peaks are identified as the symmetric and asymmetric stretching modes of the free water molecule. Furthermore, by estimation of the evaporation ratio based on thermochemical analysis, it can be assumed that the first hydration shells are naked in some part of the ion core, thereby allowing evaporation to take place within the external hydration shell during the dehydration process.     

Stability of model membranes in the presence of organotin compounds

The influence of tri‐ and di‐alkyltins (TATs and DATs) as well as di‐ and triphenyltin compounds (DPhTs and TPhTs) on haemolysis of red blood cells (RBCs) and stability of planar lipid membranes (PLMs) has been studied. The results obtained show that the efficiency of TATs (trimethyl‐, triethyl‐, tri‐n‐propyl‐ and tributyl‐tin chlorides) in destroying PLMs did not differ greatly when the compounds were studied in solutions of physiological pH (phosphate buffer, pH 7.4). A decrease in pH to 5.0 caused small changes in the efficiency of the three largest TAT molecules and a significant decrease in the efficiency of trimethyltin chloride. Both haemolytic and PLM experiments showed that the most active TAT was tri‐n‐propyltin chloride. The destructive action of DAT (dimethyl‐ and dibutyltin) and DPhT dichlorides was somewhat more differentiated. Dimethyltin dichloride (DMT) interaction with model membranes was a little weaker than that of DPhT and dibutyltin dichlorides and all these compounds influenced the model membranes to a lesser extent than TATs or TPhT. To bring about comparable haemolysis effects the dichlorides had to be used at much greater concentrations than the chlorides. The haemolytic properties of the dichlorides, especially of that of DMT, significantly increased in solution at pH 5.0. TPhT chloride interacted with model membranes similarly to TAT chlorides. Also, no great difference in efficiency of this compound was found for the two buffer solutions used. Copyright © 2000 John Wiley & Sons, Ltd.     

Wetting properties of model biological membranes

Various aspects of native and model biological membrane wettability are discussed. Among others hydration of mono-, bi-, and multi-layers of lipids as well as wettability of macroscopic surfaces of solid supported lipid films was investigated via apparent contact angle measurements and calculation of the apparent surface free energy of the films. The effects of relative humidity on the layer hydration and contact angle changes are also discussed. Finally, the effect of liposomes and enzymes (due to the hydrolysis reactions) on the hydrophobic/hydrophilic character of the film surfaces is overviewed.     

Influence of adsorbed molecules on the permeation properties of silicalite membranes

Gas permeation through zeolite membranes can be extremely sensitive to organics or moisture, adsorbed from feed impurities or from the atmosphere during storage, and thus, thermal pretreatment is necessary to obtain reliable permeation rates. Altered adsorption properties of the external surface and selective pore blockage might have caused the observed trends.     

A monolayer study on interactions of docetaxel with model lipid membranes

Docetaxel (DCT) is an antineoplastic drug for the treatment of a wide spectrum of cancers. DCT surface properties as well as miscibility studies with l-alpha-dipalmitoyl phosphatidylcholine (DPPC), which constitutes the main component of biological membranes, are comprehensively described in this contribution. Penetration studies have revealed that when DCT is injected under DPPC monolayers compressed to different surface pressures, it penetrates into the lipid monolayer promoting an increase in the surface pressure. DCT is a surface active molecule able to decrease the surface tension of water and to form insoluble films when spread on aqueous subphases. The maximum surface pressure reached after compression of a DCT Langmuir film was 13 mN/m. Miscibility of DPPC and DCT in Langmuir films has been studied by means of thermodynamic properties as well as by Brewster angle microscopy (BAM) analysis of the mixed films at the air-water interface, concluding that DPPC and DCT are miscible and they form non-ideally mixed monolayers at the air-water interface. Helmholtz energies of mixing revealed that no phase separation occurs. In addition, Helmholtz energies of mixing become more negative with decreasing areas per molecule, which suggests that the stability of the mixed monolayers increases as the monolayers become more condensed. Compressibility values together with BAM images indicate that DCT has a fluidizing effect on DPPC monolayers.     

Study of the selective retention of fluorinated compounds on perfluorinated stationary phases

Summary The basis of the selective retention of perfluorinated compounds on perfluorinated bonded phases is examined. It is shown that the selective retention increases dramatically by increasing chain length and strand multiplicity.