Iolanda Porcar

Macromolecules in ordered media: 1. Interfacial interactions between a cationic polymer and oppositely charged liposomes

Abstract The incorporation of a cationic polymer, such as poly(4-vinylpyridine), to the outer leaflet of negatively charged phospholipid vesicles of dimyristoylphosphatidic acid has been investigated by viscometry and intrinsic fluorescence. Viscosity changes of dilute solutions follow the size of the polymer and the polymer-liposome complex. Variations in the emission spectrum of poly(4-vinylpyridine) upon the addition of lipid vesicles under different conditions give the amount of polymer bound per mole of accessible lipid. These experimental results have been interpreted by means of the Gouy-Chapman formalism, which calculates the effective number of charges per polymer chain at the interface. The size of the adsorbed polyion has been computed from a discrete charge virial expansion that takes into account the mobility of charged groups at the interface.

Solution properties of polyelectrolytes. VIII: A comparative study of the elution behaviour on two organic-based packings

Abstract Aqueous size-exclusion chromatography was used to analyse the elution behaviour of several standard ionic polymers, including poly( l -glutamic acid), sodium poly(styrene sulphonate) and poly(acrylic acid), different in nature and chain flexibility, as a function of the pH and ionic strength of the eluent. Two organic-based hydrophilic packings, Spherogel TSK PW4000 and Ultrahydrogel 250, were tested in order to select the optimal conditions of macromolecular separation, and the results obtained for each column were compared. A set of calibration graphs for the above polyions as a function of eluent pH and ionic strength were obtained and compared with those obtained for uncharged standards (dextran and polyethylene oxide, PEO) under the same experimental conditions. The divergence between both charged and uncharged plots served to interpret the separation mechanisms for the polyions, other than pure size exclusion. Deviations from ideal elution behaviour have been attributed to ion-exclusion and hydr ophobic effects, as a consequence of the repulsive or attractive interactions between the ionizable groups of the polyelectrolyte and the residual surface charge of the support.

Macromolecules in ordered media: 4. Poly(2-vinyl pyridine)-liposome association induced by electrostatic interactions

Abstract We have investigated the interaction between vesicles based on dimyristoyl phosphatidic acid (DMPA) and a polymer, such as poly(2-vinyl pyridine) (P2VPy), on the basis of both partition and binding models. The resulting association curves are non monotonous being three regions clearly evidenced. The electrostatic and steric origin of the attractive or repulsive interactions are analyzed as a function of pH, ionic strength and temperature using the fluorescence spectroscopy technique. We emphasize the importance of differentiating the ideal definition of the binding constant from the theoretically evaluated including the activity coefficient, γ, to take into account shifts from the ideality of the polymer in both aqueous and lipidic domains. Furthermore, we propose an equation relating both models that predicts fairly well the experimental data.

Solution properties of polyelectrolytes: X. Influence of ionic strength on the electrostatic secondary effects in aqueous size-exclusion chromatography☆

Abstract The retention behaviour of polyelectrolytes in aqueous size-exclusion chromatography (SEC), where electrostatic repulsion is the main secondary effect affecting to the separation mechanism, was investigated. A theoretical treatment was developed in order to establish the influence of the mobile phase ionic strength on calibration graphs, often used for the characterization of polyions by SEC. A master equation, derived in closed form, involving two terms related to the net charge of polyion and the residual surface charge on the gel packing, was derived. The formalism provides the basis for a more detailed analysis of chromatographic retention data in electrostatic interaction systems. Moreover, the introduction of some approximations in the original equation served to obtain an equivalent expression that is easier to use, and in which the functionality with respect to the ionic strength, I, remains unaltered. Reported data on the elution of sodium polystyrene sulphonate and poly( l -glutamic acid) from both organic and silica-based packings were used to test the goodness of the predictions carried out with the above-mentioned equations.

Macromolecules in ordered media: 7. Influence of ionic strength and bilayer composition on the association of polyelectrolytes to mixed liposomes

The effect of both the phospholipid composition of the bilayer and the ionic strength of the medium on the association between poly(2-vinyl pyridine) and mixed liposomes based on dimyristoyl phosphatidic acid and dimyristoyl phosphatidil choline was investigated using fluorescence spectroscopy. Intensity changes upon addition of vesicles have allowed us to estimate the extent of the association and data have been processed as association isotherms. We found that the association is enhanced by increasing both the ionic strength and the anionic phospholipid fraction. However, whereas the negative net charge of the bilayer strongly enhances the interaction, the presence of more and more electrolyte in the aqueous media induces a slight increase of the binding. Moreover, surfaces with at least 50% of charged phospholipid are necessary. Results were consistent with those obtained in preceding reports.

Solution properties of polyelectrolytes: IX. Quantitative dependence on eluent ionic strength of elution volumes in aqueous size-exclusion chromatography☆

Abstract Quantitative evaluation of elution volumes (Ve) of polyelectrolytes in salt-containing eluents was performed, taking into account electric double-layer effects and the effective radius of polyions, and assuming that polyelectrolytes behave as rigid hydrodynamic spheres and that the geometry of gel pores is cylindrical. A polynomial Ve dependence on ionic strength (I) was obtained, namely a second order one in I− 1 2 . The semi-empirical polynomial coefficients depend on column characteristics, namely V0 (interstitial volume), Vp (total pore volume) and a (mean pore radius), as well as on effective coil radius (Rη) and on k (a parameter related to the electric effects). Fair correlations between predicted and measured elution volumes for uncharged polymers hold, at least for the polyelectrolyte—gel systems tested here: polyelectrolytes poly( l -glutamic acid), sodium poly(styrene sulphonate) and poly(acrylic acid); gels Spherogel TSK PW4000 and Ultrahydrogel 250.

Preferential solvation of a dicyanate ester monomer and poly(sulfone) in different organic solvents by size-exclusion chromatography☆

Preferential solvation parameters A in the ternary systems solvent (1) -monomer (2) -polymer (3) were determined as a tool to measure the compatibility between the cyanate ester monomer Arocy B10 and poly(sulfone), PSF, in the presence of three organic solvents: tetrahydrofuran, dimethylformamide and dicloromethane. The A parameter was measured by size-exclusion chromatography at different monomer-to-polymer ratios. The quantitative evaluation was rigorously made at polymer-diluted conditions. PSF was found to be preferentially solvated by the monomer. Concerning the solvent used, systems containing tetrahydrofuran showed the strongest solvation, the lowest A values being those obtained in dicloromethane. These results were in accordance with the intrinsic viscosity values of the PSF-solvent systems. The variation of A values with the Arocy B10 concentration is strongly dependent upon the nature of the solvent.

Interaction of quinine with negatively charged lipid vesicles studied by fluorescence spectroscopy Influence of the pH

Abstract The interaction of quinine with dimyristoylphosphatidic acid (DMPA) and dimyristoylphosphatidyl glycerol (DMPG) small unilamellar vesicles in the gel phase was studied by steady-state fluorescence spectroscopy at pHs 7, 6, 5 and 4 and 20°C. In aqueous solution, with excitation at 335 nm, the emission fluorescence spectrum of quinine varied with pH reflecting the occurrence of different charged species of the drug. In all cases, the emission maximum centered at 383 or 443 nm shifted to lower wavelength in the presence of vesicles. This indicates that the membrane-bound state quinine is in an environment of low polarity. Drug monocationic species were deeply buried in DMPG relative to DMPA bilayers whereas no significant differences were observed for dicationic species, the fluorophore being located in this case in a more aqueous-like environment. Experimental association isotherms generated from fluorescence intensity changes were quantitatively analyzed in terms of the binding equilibrium model. Although the binding affinity of quinine to anionic membranes was always higher for DMPG over DMPA, dicationic species showed a reduced ability to bind the negatively charged membrane. In addition, the binding model has been related with the partition model leading to a good agreement between the theoretical (calculated from the binding model) and the experimental (from the initial slope of the experimental isotherms) partition coefficient derived in each case.

Solution Properties of Poly-Electrolytes. XI. Adsorption Effects in Aqueous Size-Exclusion Chromatog-Raphy of Polyanions

Abstract Elution profiles of a series of polymer standards such as sodium poly(styrene sulphonate), poly(acrylic acid) and poly(L-glutamic acid) have been obtained from size-exclusion chromatography experiments using separately two types of hydrophilic supports. A variety of mobile phase compositions have been performed to enhance adsorption effects in order to study how this phenomenon can affect to the chromatographic separation mechanism of polyanions. Distribution coefficient values, in general greater than unity, have served to quantify the adsorption effect, as well as to analyze their dependence on eluent ionic strength, on the ionic groups of the support and on the chemical nature and molar mass of the polyion. The physical basis of the weak polymer-gel attractive interaction have been attributed to hydrogen-bonding and to hydrophobic effects. We present basic equations derived from the Flory-Huggins theory of polymer solutions to explain the adsorption process in terms of preferential interaction...

Solution properties of polyelectrolytes XII. Semi-quantitative approach to mixed electrostatic and hydrophobic polymer-gel interactions

Abstract Aqueous size-exclusion chromatography of polyanions, where secondary effects affect the total separation mechanism, was investigated. For elution of polyelectrolytes on inorganic silica-based supports, the electrostatic polymer-gel repulsive interactions were evaluated through the values for a hypothetical repulsion layer, X e , according to the model developed by other workers. Using a similar procedure, the existence of an effective barrier defined as X e − X h is proposed for those systems in which electrostatic repulsion and hydrophobic interactions take place simultaneously as secondary mechanisms. X h can be viewed as a measure of the “enlargement” of the geometrical pore radius due to reversible adsorption of polyanions in organic polymeric packings. In the light of the values of this effective barrier on the pore surface, the intensity and contribution of each type of solute-matrix interaction to the overall chromatographic process can be analysed as a function of polymer size and mobile phase composition, pH and ionic strength.