Effect of Polyacrylic Acid on Phase State of Lipids and Diffusion in Lipid-Water System

Lipid vesicles interacting with polyanions are promising for controlled drug delivery. However, different aspects of the interaction of these polymers with lipids are far from complete understanding. In this work we studied the influence of polyacrylic acid (PAA) with small concentrations (1–4 mol%) on the change of the phase state, lateral diffusion of these lipids in lamellar phase and transmembrane water diffusion in macroscopically oriented bilayers of lipid-water systems formed by dimiristoylphosphatidylcholine (DMPC) and dioleoylphosphatidylcholine. Measurements were performed by ³¹P nuclear magnetic resonance (NMR) spectroscopy and the ¹H NMR technique with a pulsed field gradient. It was found that the presence of PAA does not change the lamellar structure of the system. However, a part of bilayers changes their originally flat geometry and forms vesicles with a higher surface curvature. Macroscopic orientation of bilayers disappeared. For DMPC the presence of PAA leads to a shift of the gel-to-liquid crystalline phase-transition temperature to higher temperatures. An increase of PAA concentration leads to a monotonous decrease in the lateral diffusion coefficient of lipids that is caused, probably, by the ordering of lipids in bilayers. The transbilayer diffusion coefficient of water increases in the presence of PAA, but it depends slightly on the PAA concentration. An increase of pH leads to a change of the lipid lateral and transbilayer diffusion coefficients to the values typical for a pure bilayer. Authors' address: Andrey Filippov, Kazan State University, Kremlevskaya ulitsa 18, Kazan 420008, Russian Federation     

Nano-control of self-assembled biomolecular structures

We have controlled the structure of self-assembled systems by introducing charges (charge effect) and polymeric tails (steric effects) on a spherical–cylindrical shape of nonionic surfactant micelles. In detail, we studied the effects of a phospholipid (DL-α-phosphatidycholine dimyristol: DMPC) on the shape of nonionic surfactant micelles (penta-ethyleneglycol mono-n-dodecyl ether: C₁₂E₅), which has been studied in terms of an aggregation number, critical micellization concentration (CMC), second virial coefficient (A₂), and hydrodynamic diameter (D_H) by laser light scattering. DMPC, DOPC (DL-α-phosphatidylcholine dioleoyl), and DMPE (DL-α-phosphoethanolamin dimyristol) molecules added in C₁₂E₅ micelle solutions decrease the spontaneous curvatures, leading to an increase of the end-cap energy E_c that favors micellar growth. Based on the CMC values, the total free energy per micelle of C₁₂E₅/DMPC mixtures is estimated. The free energy per micelle of C₁₂E₅/DMPC mixtures decreases as DMPC is added. This is consistent with the decrease of A₂ and the strong hydrophobicity of DMPC compared with C₁₂E₅. The average contour length, the diffusion coefficient, and the end-cap energy of mixed micelles are estimated based on the CMC and molecular specific volumes of the moiety. The end-cap energy of the mixed micelles and the average contour length increase as DMPC is added, which is also reasonable considering the molecular structure of DMPC. Furthermore, the diffusion coefficients obtained from dynamic laser light scattering are in excellent agreement with the estimated diffusion coefficients obtained from a one-dimensional growth model based on static light scattering measurements.Charged lipid (1,2-dioleoyl-3-trimethylammonium-propane) and polymer lipid (1,2-dimyristoyl-sn-glycero-3-phosphoethanolamine-[poly(ethylene glycol)]) increase the spontaneous curvatures, resulting in breaking micelles into small size. When the lipids are added, the hydrodynamic diameter of the micelles of C₁₂E₅/lipids is nearly temperature independent due to the strong charge or steric repulsion.     

Fractal aggregates induced by liposome-liposome interaction in the presence of Ca2+

We present a study of the fractal dimension of clusters of large unilamellar vesicles (LUVs) formed by egg yolk phosphatidylcholine (EYPC), dimyristoylphosphocholine (DMPC) and dipalmitoylphosphocholine (DPPC) induced by Ca²⁺ . Fractal dimensions were calculated by application of two methods, measuring the angular dependency of the light scattered by the clusters and following the evolution of the cluster size. In all cases, the fractal dimensions fell in the range from 2.1 to 1.8, corresponding to two regimes: diffusion-limited cluster aggregation (DLCA) and reaction-limited cluster aggregation (RLCA). Whereas DMPC clusters showed a typical transition from the RLCA to the DLCA aggregation, EYPC exhibited an unusual behaviour, since the aggregation was limited for a higher concentration than the critical aggregation concentration. The behaviour of DPPC was intermediate, with a transition from the RLCA to the DLCA regimes with cluster sizes depending on Ca²⁺ concentration. Studies on the reversibility of the aggregates show that EYPC and DPPC clusters can be re-dispersed by dilution with water. DMPC does not present reversibility. Reversibility is evidence of the existence of secondary minima in the DLVO potential between two liposomes. To predict these secondary minima, a correction of the DLVO model was necessary taking into account a repulsive force of hydration.     

Deposition of molecular probes in heavy ion tracks

By using polarized fluorescence techniques the physical properties of heavy ion tracks such as the dielectric number, molecular alignment and track radius can be traced by molecular fluorescence probes. Foils of poly(ethylene terephthalate) (PET) were used as a matrix for the ion tracks wherein fluorescence probes such as aminostyryl-derivatives can be incorporated using a suitable solvent, e.g. N,N′-dimethylformamide (DMF) as transport medium. The high sensitivity of fluorescence methods allowed the comparison of the probe properties in ion tracks with the virgin material. From the fluorescence Stokes shift the dielectric constants could be calculated, describing the dielectric surroundings of the molecular probes. The lower dielectric constant in the tracks gives clear evidence that there is no higher accomodation of the highly polar solvent DMF in the tracks compared with the virgin material. Otherwise the dielectric constant in the tracks should be higher than in the virgin material. The orientation of the molecular probes was examined by polarized fluorescence spectroscopy. It is shown that deposited molecular probes witha high aspect ratio have a preferential orientation parallel to the ion track axis.     

A method for quantitative interpretation of fluorescence detection of poly(ethylene glycol)-mediated 1-palmitoyl-2-[[[2-[4-(phenyl-trans-1,3,5-hexatrienyl) phenyl]ethyl]oxyl]carbonyl]3-sn-phosphatidylcholine (DPHpPC) transfer and fusion between phospholipid vesicles in the dehydrated state

A method has been developed for calculating the expected fluorescence lifetime of the DPH _p PC probe distributed between different membrane environments. We show how this method can be used to distinguish between lipid transfer and fusion between large unilamellar vesicles occurring in the presence of poly(ethylene glycol) (PEG). This application of the calculation took into consideration the heterogeneity of microenvironments experienced by the probe in a sample containing vesicle aggregates of different sizes. Assuming that the aggregate size distribution was a delta function of the aggregate size, comparison of the calculated and observed lifetimes yielded an estimate of the vesicle aggregate size. For vesicles of varying compositions in the presence of dehydrating concentrations of PEG, this method suggested that only small aggreggates formed. For vesicles that could be demonstrated by other means not to have fused, the data were consistent with lipid transfer occurring only between the outer leaflets of two to four vesicles, even at high PEG concentrations. For vesicles that could be demonstrated to fuse by contents mixing and size changes, the fluorescence lifetime data were consistent with lipid transfer between both the inner and the outer leaflets of two to four fused vesicles. At very high PEG concentrations, where extensive rupture and large, multilamellar products were previously observed, the lifetime data were consistent with much more extensive lipid transfer within larger aggregates. The agreement of predictions made on the basis of lifetime measurements with other observations attests to the validity of the fluorescence lifetime method. In addition, the model and data presented here provide evidence that fusion occurs between small numbers of PEG-aggregated vesicles before the removal of PEG.     

Tunability of the photoluminescence in porous silicon due to different polymer dielectric environments

In this report we demonstrate control over porous silicon (PSi) emission properties by changing the dielectric environment surrounding the silicon crystallites, as well as provide information on the effects of pore infiltration of PSi. This is achieved by making PSi–polymer nanocomposites by diffusing or polymerizing a range of varying dielectric constant polymers into the pores. The degree of modification in photoluminescence (PL) depends on the dielectric constant of the polymers. By increasing the dielectric constant of the environment surrounding the crystallites, a blue shift in PL as high as 222 meV has been observed. The blue shift is attributed to the high dielectric constant of the polymers relative to PSi, which causes a partial screening of the excitons allowing the excitonic levels to shift closer to the bandgap. The shift in excitonic levels increases when the dielectric constant of the polymer increases. PSi–polymer nanocomposites also exhibit an increase in PL intensity, which suggest that the inert infiltrated polymers are able to passivate existing nonradiative channels.     

Determination of the rate of rapid lipid transfer induced by poly(ethylene glycol) using the SLM Fourier transform phase and modulation spectrofluorometer

Rate constants were determined for the transfer of the fluorescent lipid probe 1-palmitoyl-2-[[2-[4-(6-phenyl-trans-1,3,5-hexatrienyl)phenyl]ethyl] oxy]carbonyl]-3-sn-phosphatidylcholine (DPHpPC) between large, unilamellar extrusion vesicles composed either of dipalmitoyl phosphatidylcholine (DPPC) or of DPPC mixed with a small amount (0.5 mol%) of lyso phosphatidylcholine (Lyso PC). Transfer of the lipid probe in the presence of varying concentrations of poly(ethylene glycol) (PEG) was monitored using the SLM 48000-MHF Multi-Harmonic Fourier Transform phase and modulation spectrofluorometer to collect multifrequency phase and modulation fluorescence data sets on a subsecond time scale. The unique ability of this instrument to yield accurate fluorescence lifetime data on this time scale allowed transfer to be detected in terms of a time-dependent change in the fluorescent lifetime distribution associated with the lipid-like DPHpPC probe. This probe demonstrates two short fluoresence decay times (ca. 1.1–1.4 and 4.3–4.8 ns) in a probe-rich environment but a single long lifetime (ca. 7 ns) in a probe-poor environment. A simple two-state model for initial lipid transfer was used to analyze the multifrequency data sets collected over a 4-s time frame to obtain the time rate of change of the concentrations of donor and acceptor probe populations following rapid mixing of vesicles with PEG. The ability to measure fluorescence lifetimes on this time scale has allowed us to show that the of rate of lipid transfer increased dramatically at 35% PEG in both fusing and nonfusing vesicle systems. These results are interpreted in terms of a distinct interbilayer structure associated with intimate bilayer contact induced by high and potentially fusogenic concentrations of PEG.     

Heat treatment effects on dielectric and physico-chemical properties of an epoxy polymer

Infrared (IR) spectroscopy, dielectric spectroscopy (DS), and thermally stimulated depolarization current (TSDC) have been used to study heat treatment effects on an epoxy-based polymer. Variations in physico-chemical and dielectric properties were examined for annealing temperatures between 55 and 170 °C. IR results have shown that heating causes both chain scission and thermal oxidation of the polymer, increasing thus the amount of trapped charges. The complex dielectric permittivity and the dielectric modulus have been analyzed, by means of DS, to highlight and separate charge relaxation phenomena from conduction contributions. Results indicate structural rearrangements, leading to a decrease of dipolar relaxation frequency (from 16 to 13.5 kHz) and an increase of the relaxation strength (around 20%). TSDC measurements have shown a current peak shift towards higher temperatures, and a significant intensity decrease, which is proportional to the quantity of released charges.     

Phosphatidylcholine Membrane Fusion Induced by Dimethyl Sulfoxide and Diethyl Sulfoxide

The kinetics of unilamellar vesicle fusion induced by the addition of dimethyl sulfoxide (DMSO) and diethyl sulfoxide (DESO) with mole fractions of 0.1 and 0.2 is studied in the liquid-crystal phase using small-angle neutron scattering. Multilamellar vesicles formed due to the partial fusion of unilamellar vesicles of 1,2-dimyristoyl-sn-glycero-3-phosphadylcholine (DMPC) with the addition of DMSO (Х_DMSO = 0.1, 0.2) and DESO (Х_DESO = 0.2) are stable for a long time. The cooling–heating process does not affect the stability of the formed systems. The presence of DMSO and DESO with a mole fraction of 0.2 leads to disappearance of the ripple phase. The addition of DESO to the unilamellar vesicles of DMPC in D₂O with a mole fraction of 0.1 does not affect the structure of unilamellar vesicles for 5–15 minutes after adding the sulfoxide in the liquid-crystal phase. Three hours later, a stable system consisting of unilamellar vesicles with a lipid bilayer thickness of 27.3(2) Å and multilamellar vesicles with a repeat distance of d = 43.6(2) Å is formed. During cooling, multilamellar vesicles are destroyed in the region of the main phase transition (T'_m = 24.8(9)°C for the investigated system) and unilamellar vesicles are formed.     

Influence of ceramide on the internal structure and hydration of the phospholipid bilayer studied by neutron and X-ray scattering

Small angle neutron scattering (SANS), neutron diffraction and X-ray powder diffraction were used to investigate influence of N-stearoyl phytosphingosine (CER[NP]) and α-hydroxy-N-stearoyl phytosphingosine (CER[AP]) on the internal structure and hydration of DMPC membrane in fully and partly hydrated states at T = 30 °C. Application of Fourier analysis for diffraction data and model calculations for the SANS data evidence that addition of both CER[NP] and CER[AP] in small concentrations promotes significant changes in the organization of DMPC bilayers, such as the increase of the hydrophobic core region. SANS data evidence a decrease in the average radius and polydispersity of the vesicles that can be ascribed to hydrogen bonds interactions that favor tight lipid packing with a compact, more rigid character.     

Determination of the dynamics of microcharacteristics of solutions by time laser spectroscopy

The development of simple methods used to analyze the available data and separate the contributions from the main physicochemical processes responsible for the kinetics of instantaneous emission spectra is of considerable interest due to the intensive investigation of the time-resolved characteristics of fluorescence of various quantum objects that are used to examine the most important photophysical and photochemical reactions in excited states. The relation between the functions of the shift in the instantaneous spectra of spontaneous emission of molecules (obtained using kinetic spectroscopy methods) and the time variations in the configurational structure of solvates, as well as the charge transfer in the excited singlet state of molecules of the luminescent probe, has been shown using simple analytical expressions of solvatofluorochromism. The fluorescence of solutions of one of the most important molecular probes, namely, 1-(phenylamino)naphthalene in glycerol, has been investigated with a picosecond time resolution. This molecular probe is known because its electric dipole moment is formed in the excited singlet state, which almost instantaneously initiates processes of intermolecular rearrangement of solvent molecules. It has been demonstrated that it is possible to determine the time dependence of the function of the dielectric response of the polar solvent to the dipole of this probe and, hence, the time evolution of the microcharacteristics of the solvent that determine the above function.     

Thermodynamics of membrane elasticity – A molecular level approach to one- and two-component fluid amphiphilic membranes,Part II: Applications

The theoretical framework developed in the accompanying publication is applied to a number of experimentally relevant amphiphilic systems. These include the influence of thermodynamic conditions and non-ideal mixing on bending elasticity, ellipsoidal modes of microemulsions and vesicles, hydrocarbon chain coupling in bilayers and the effect of osmotic and hydrostatic pressure on inverse hexagonal (H II) phases. It is found that the bending moduli at constant surface tension and constant chemical potentials are markedly different only for two-component membranes and non-ideal mixing with a tendency towards phase separation. The results indicate that non-ideal mixing is the main reason behind the experimentally observed strong compositional dependence of membrane elasticity. It is generally recommended to prefer the bending elastic moduli at constant chemical potentials to those at constant surface tension. A comparison between the area-difference-elasticity (ADE) model and explicit free energy calculations using a molecular model shows a good qualitative agreement for the sphere-to-ellipsoid transition of vesicles. Results for different free energy models of the hydrocarbon chains of amphiphilic molecules suggest that monolayer-monolayer chain coupling is responsible for the relatively higher bending stiffness of bilayers compared to single monolayers. For H II-phases an instability at negative pressure differences is predicted.     

静电力显微镜研究二相材料及其界面介电特性

利用静电力显微镜(EFM)研究了二相材料不同区域的介电特性. 制备了高定向石墨/聚乙烯、云母/聚乙烯等层叠状二相材料复合物, 在EFM相位检测模式下观测二相材料过渡界面处, 可以发现二相材料中介电常数较大的材料会引起较大的相位滞后角Δθ该相位滞后角正切值tan(Δθ) 与探针电压V_EFM存在二次函数关系, 且函数二次项系数与样品的介电常数存在增函数关系, 进而可在微纳米尺度下区分不同微区域内材料的介电常数差异. 研究表明EFM 可用于对材料介电特性的微纳米尺度测量, 这对分析复合材料二相界面区域特性有积极意义.     

Light scattering and dielectric manifestations of secondary relaxations in molecular glassformers

Photon correlation data of the molecular glass-forming materials 2-picoline, dimethylphthalate (DMP) and salol are compared with their dielectric loss spectra in the time-frequency range where the dielectric data reveal secondary relaxations. Slow secondary relaxation processes in molecular liquids are commonly studied by dielectric spectroscopy (DS) and, based on such studies, believed to be characteristics of the deeply super-cooled liquid state. However, there has been no direct experimental evidence that they are similarly detected by other experimental techniques. In the present study, we experimentally address this question for the anisotropic (depolarized) light scattering (LS). In the first approximation, DS and LS probe the same molecular reorientation dynamics, and therefore are expected to provide qualitatively similar spectra. We find however that this is not the case, namely i) the magnitude of the slow secondary relaxations is much less in LS than in DS data, which is the opposite to expectations; ii) the shape of the relaxation spectrum is qualitatively different, concerning both the main and secondary processes. We discuss possible sources of these differences in the context of related data from the literature.Received: 21 July 2003, Published online: 23 December 2003PACS: 64.70.Pf Glass transitions - 77.22.Gm Dielectric loss and relaxation - 78.35. + c Brillouin and Rayleigh scattering other light scatteringJ. Mattsson: Presently at: DEAS and Physics Department, Harvard University, Cambridge, MA 02138, USA     

Spectroscopic Studies of a New Multi-Element Sensitive Fluorescent Probe Derived from 2-(2-pyridyl)benzimidazole: Selective Discrimination of Zn2+ from Its Congeners

ABSTRACT

A new multielement sensitive fluorescent probe, 1-(2-(phenylthio)ethyl)-2-(pyridin-2-yl)-1H-benzo[d]imidazole (L), has been synthesized by the reaction between 2-(pyridyl) benzimidazole and 2-chloroethyl phenyl sulfide. Excitation and emission wavelength of L are at 330 and 371 nm, respectively. Among various transition and nontransition metal ions, it can selectively read Zn²⁺ ion as the emission wavelength of L undergoes a red shift by 31 nm upon binding with Zn²⁺ in methanol. In the presence of Cd²⁺, Hg²⁺, and other common cations, the emission wavelength of L remains unchanged, and thus allows us to discriminate Zn²⁺ from its congeners. Both L and its Zn²⁺ complex are well characterized by different spectroscopic techniques like ¹H-NMR, ESI-TOF (+) mass, FT-IR, and elemental analysis data. The binding constant value of the complexation reaction between L and Zn²⁺ is found as 724.6 M^?1 in methanol. Density functional theoretical (DFT) studies nicely demonstrate the red shift in the emission wavelength of L upon binding with Zn^2+.     

Viscoelastic and Dielectric Behavior of Poly(1‐Butene)/Multiwalled Carbon Nanotube Nanocomposites

Linear viscoelastic properties and dielectric behavior of poly(1‐butene)/multiwalled carbon nanotube (MWCNT) nanocomposites were investigated. Dynamic mechanical analysis showed significant increase in storage modulus in the rubbery regime. The tan δ peak temperature remained constant; however, the peak intensity was lowered for the nanocomposites. In melt rheological studies the nanocomposites showed a shift in crossover frequency to the lower side, suggesting delayed relaxation of the molecular chains in the presence of MWCNT and this shift was found to depend on the content of MWCNT. The dielectric constant increased from 2.2 to 70 for the nanocomposite with 7 wt. % MWCNT. The electrical conductivity increased significantly, from 10^?15 to 10^?3 S/cm. The results of rheology and dielectric studies indicate that a percolation network is formed that is responsible for the observed changes.     

Dielektrische Wellenleiter als Plasmasonden

A microwave interferometer is described, by which the electron density in a small volume inside of a plasma can be measured. By dielectric rods acting as waveguides the microwave is led to this volume. For a given electron density the phase shift is calculated for this volume and for the dielectric rods. By a suitable choice of the radius and the dielectric constant of the rods the phase shift along the rods can be surpressed. The influence of plasma sheaths is discussed. In a special application to the plasma of the negative glow these interferometric measurements are compared with Langmuir probe measurements.     

Laser-induced synthesis of silver nanoparticles and their conjugation with protein

Partially oxidized spherical silver nanoparticles (AgNPs) of different size are prepared by pulsed laser ablation in water and directly conjugated to protein S-ovalbumin for the first time and characterized by various optical techniques. UV–Visible spectrum of AgNPs showed localized surface plasmon resonance (LSPR) peak at 396 nm which red shift after protein addition. Further the increased concentration of AgNPs resulted a decrease in intensity and broadening of S-ovalbumin peak (278 nm), which can be related to the formation of protein NPs complex caused by the partial adsorption of S-ovalbumin on the surface of AgNPs. The red shift in LSPR peak of AgNPs after mixing with S-ovalbumin and decrease in protein-characteristic peak with increased silver loading confirmed the formation of protein–AgNPs bioconjugates. The effect of laser fluence on the size of AgNPs and nanoparticle–protein conjugation in the size range 5–38 nm is systematically studied. Raman spectra reveal broken disulphide bonds in the conjugated protein and formation of Ag–S bonds on the nanoparticle surface. Fluorescence spectroscopy showed quenching in fluorescence emission intensity of tryptophan residue of S-ovalbumin due to energy transfer from tryptophan moieties of albumin to AgNPs. Besides this, small blue shift in emission peak is also noticed in presence of AgNPs, which might be due to complex formation between protein and nanoparticles. The binding constant (K) and the number of binding sites (n) between AgNPs and S-ovalbumin have been found to be 0.006 M^?1 and 7.11, respectively.     

Polymer formation from aqueous solutions of α-D-glucose by ultrasound and λ-rays

The production of polymeric material from aqueous glucose solutions treated by ultrasound or λ-rays was investigated by size-exclusion chromatography (SEC). The polymers show strong electrostatic interactions with the SEC-columns and the non-ideal elution behaviour indicates that charged molecules are produced during irradiation. By optimization of the size-exclusion separation applying salt solutions in the mobile phase it was possible to determine the molecular weight (MW) as a function of irradiation dose. For both types of irradiation in the presence of oxygen, products in the molecular weight range up to 4000 dalton were formed. For sonolysis in the absence of oxygen, the same effect was observed due to the fact that small amounts of oxygen are formed by ultrasound irradiation of water. In the case of λ-irradiation in the absence of oxygen there is no inhibition of the polymerization resulting in higher molecular weights of many thousand dalton. The resulting molecular weights depend on dose and dose rate.     

Temperature dependence of the dielectric constant of acrylic dielectric elastomer

The dielectric constant is an essential electrical parameter to the achievable voltage-induced deformation of the dielectric elastomer. This paper primarily focuses on the temperature dependence of the dielectric constant (within the range of 173 K to 373 K) for the most widely used acrylic dielectric elastomer (VHB 4910). First the dielectric constant was investigated experimentally with the broadband dielectric spectrometer (BDS). Results showed that the dielectric constant first increased with temperature up to a peak value and then dropped to a relative small value. Then by analyzing the fitted curves, the Cole–Cole dispersion equation was found better to characterize the rising process before the peak values than the Debye dispersion equation, while the decrease process afterward can be well described by the simple Debye model. Finally, a mathematical model of dielectric constant of VHB 4910 was obtained from the fitted results which can be used to further probe the electromechanical stability of the dielectric elastomers.