gerardo prieto - Academia.edu (original) (raw)

Papers by gerardo prieto

Research paper thumbnail of Thermodynamics of Micellization of N-Octyltrimethylammonium Bromide in Different Media

Langmuir, 1995

The critical micelle concentrations (cmc) of n-octyltrimethylammonium bromide in pH 3.2,7.0, and ... more The critical micelle concentrations (cmc) of n-octyltrimethylammonium bromide in pH 3.2,7.0, and 10.0 media, at the temperatures 15, 20,25, 30 and 35 "C have been determined by the electrical conductivity method. Cmcvalues decrease to a greater value ofpH. From the conductivity data, the degrees of counterion binding of the micelles have been calculated. Thermodynamic parameters of micellization have been obtained by application of the model of Evans and Ninham in terms ofhydrophobic and surface contributions, which are pH dependent.

Research paper thumbnail of Denaturation of lysozyme by n-alkyltrimethylammonium bromides in alkaline solution

Journal of The Chemical Society, Faraday Transactions, 1995

Research paper thumbnail of Conformational Changes in Human Serum Albumin Induced by Sodium Perfluorooctanoate in Aqueous Solutions

Journal of Physical Chemistry B, 2005

Conformational changes in the bulk solution and at the air-aqueous interface of human serum album... more Conformational changes in the bulk solution and at the air-aqueous interface of human serum albumin (HSA) induced by changes in concentration of sodium perfluorooctanoate (C 7 F 15 COO -Na + ) were studied by difference spectroscopy, -potential data, and axisymmetric drop shape analysis. -potential was used to monitor the formation of the HSA-C 7 F 15 COO -Na + complex and the surface charge of the complex. The conformational transition of HSA in the bulk solution was followed as a function of denaturant concentration by absorbance measurements at 280 nm. The data were analyzed to obtain values for the Gibbs energies of the transition in water (∆G 0 W ) and in a hydrophobic environment (∆G 0 hc ) pertaining to saturated protein-surfactant complexes. The conformational changes that surfactants induce in HSA molecules alter its absorption behavior at the air-water interface. Dynamic surface measurements were used to evaluate this behavior. At low [C 7 F 15 COO -Na + ], proteins present three adsorption regimes: induction time, monolayer saturation, and interfacial gelation. When surfactant concentration increases and conformational changes in the bulk solution occur, the adsorption regimes disappear. HSA molecules in an intermediate conformational state migrate to the air-water interface and form a unique monolayer. At high [C 7 F 15 COO -Na + ], the adsorption of denatured molecules exhibits a behavior analogous to that of dilute solutions.

Research paper thumbnail of A Comparative Study of the Determination of the Critical Micelle Concentration by Conductivity and Dielectric Constant Measurements

Langmuir, 1998

Conductivities and dielectric constant measurements in water at 25°C have been made on the amphip... more Conductivities and dielectric constant measurements in water at 25°C have been made on the amphiphilics sodium n-dodecyl sulfate, n-dodecyltrimethylammonium bromide, and chlorpromazine hydrochloride. By using the conductivity/concentration data, critical micelle concentrations (cmc) have been determined by applying the Williams definition and two forms of the Phillips method. This first Phillips form consists of an approximation to Gaussians of the second derivative of the conductivity/concentration data followed by two consecutive integrations. The second form, which is proposed here, consists of the application of a combination of the Runge-Kutta numerical integrations method and the Levenberg-Marquardt leastsquares fitting algorithm. The proposed method permits the determination of the cmc in systems with low aggregation numbers and with slow variations of physical property/concentration curves allowing the determination of the so-called second cmc. A comparative study with results obtained by dielectric constant measurements has been carried out. With this new technique, the cmc's (first and second) are directly obtained as singular points in the dielectric constant/concentration curves, and thus, this technique is an alternative to the determination of cmc's from conductivities.

Research paper thumbnail of Size and stability of liposomes: A possible role of hydration and osmotic forces

European Physical Journal E, 2006

Dynamic light scattering and electrophoretic mobility measurements have been used to characterize... more Dynamic light scattering and electrophoretic mobility measurements have been used to characterize the size, size distribution and zeta potentials (ζ-potentials) of egg yolk phosphatidylcholine (EYPC) liposomes in the presence of monovalent ions ( Na+ and K+). To study the stability of liposomes the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory has been extended by introducing the hydrated radius of the adsorbed ions onto the liposome surfaces. The decrease of liposome size is explained on the basis of the membrane impermeability to some ions which generate osmotic forces, which leads to evacuate water from liposome inside.

Research paper thumbnail of A Comparative Study of the Interaction between Nafcillin and Catalase by Equilibrium Dialysis and ζ-Potential Measurements

Journal of Physical Chemistry B, 2001

The characterization of the interactions of the penicillin nafcillin with the protein catalase in... more The characterization of the interactions of the penicillin nafcillin with the protein catalase in aqueous solution at pH 3.2, 7.4, and 10.0 at 25°C has been determined using a combination of equilibrium dialysis and -potential measurements. At pH 3.2, the -potential of catalase tends to reverse charge from positive to negative values as the negatively charged penicillin bind to it. At pH 7.4 and 10.0, this -potential becomes more negative as the nafcillin concentration increases. The number of drug molecules bound to catalase per protein molecule (υ) was obtained from equilibrium dialysis and compared with the -potential data. Nafcillin binds largely to catalase to various extents ranging from ∼750 (pH 3.2) to ∼10 000 (pH 7.4 and 10.0) drug molecules per catalase molecule as the free concentration approaches the critical micelle concentration (cmc) of the free drug. Gibbs energies per drug molecule bound (∆G υ j ) were obtained as a function of υ j from equilibrium dialysis using the Wyman potential (J. Mol. Biol. 1965, 11, 631). ∆G υ j is large and negative at low values of penicillin concentration and becomes less negative as the drug concentration increases. ∆G υ j against drug concentration plots obtained from the interpretation of the variation of -potential measurements were of the form and magnitude similar to those obtained from direct binding measurements using the equilibrium dialysis technique.

Research paper thumbnail of Study of the interactions between lysozyme and a fully-fluorinated surfactant in aqueous solution at different surfactant–protein ratios

International Journal of Biological Macromolecules, 2003

The interactions of a fluorinated surfactant, sodium perfluorooctanoate, with lysozyme, have been... more The interactions of a fluorinated surfactant, sodium perfluorooctanoate, with lysozyme, have been investigated by a combination of UV absorbance, electrical conductivity and dynamic light scattering to detect and to characterize the conformational transitions of lysozyme. By using difference spectroscopy, the transition was followed as a function of surfactant concentration, and the data were analyzed to obtain the Gibbs energy of the transition in water ( G • w ) and in a hydrophobic environment ( G • h ) for saturated protein-surfactant complexes. Electrical conductivity was used to determine the critical micelle concentration of the surfactant in the presence of different lysozyme concentration. From these results, the average number of surfactant monomer per protein molecule was calculated. Finally, dynamic light scattering show that only changes in the secondary structure of the protein can be observed.

Research paper thumbnail of On the Effect of Ca 2+ and La 3+ on the Colloidal Stability of Liposomes

Langmuir, 2005

This work deals with the effect of Ca 2+ and La 3+ on the colloidal stability of phosphatidylchol... more This work deals with the effect of Ca 2+ and La 3+ on the colloidal stability of phosphatidylcholine (PC) liposomes in aqueous media. As physical techniques, nephelometry, photon correlation spectroscopy, electrophoretic mobility, and surface tension were used. The theoretical predictions of the colloidal stability of liposomes were followed using the Derjaguin-Landau-Verwey-Overbeek theory. Changes in the size of liposomes and high polydispersity values were observed as La 3+ concentration increases, suggesting that this cation induces the aggregation of liposomes. However, changes in polydispersity were not observed with Ca 2+ , suggesting a coalescence mechanism or fusion of liposomes. The stability factor (W), calculated from the nephelometry measurements indicated that aggregation/fusion occurs at a critical concentration (c.c.) of 0.3 and 0.7 M for La 3+ and Ca 2+ , respectively. To gain a better insight into the interaction mechanism between the liposomes and the studied ions, the interaction between PC monolayers and Ca 2+ and La 3+ was studied. Changes in the surface area per lipid molecule (A0) in the monolayer at the c.c. values were found for both ions, with a more pronounced effect in the case of Ca 2+ . This corresponds with a larger reduction of the steric repulsive interaction between the headgroups at the phospholipid membrane (πhead). The experimental result validates the hypothesis made on the liposome fusion in the presence of Ca 2+ and liposome aggregation in the presence of La 3+ . These aggregation mechanisms have also been confirmed by transmission electron microscopy.

Research paper thumbnail of A study of the interaction between proteins and fully-fluorinated and fully-hydrogenated surfactants by ζ-potential measurements

Colloids and Surfaces A-physicochemical and Engineering Aspects, 2004

An experimental investigation on the adsorption of two different surfactants, hydrogenated (sodiu... more An experimental investigation on the adsorption of two different surfactants, hydrogenated (sodium octanoate (SO)) and fluorinated (sodium perfluorooctanoate (SPFO)) on three proteins (lysozyme, hemoglobin and catalase) by measurements of the zeta-potential (ζ-potential) has been performed in water at the temperature of 298.15 K. The electrophoretic mobility of the complexes has shown hydrophobic adsorption of the surfactants on the protein surface, leading to surface saturation. The number of adsorption sites on the proteins was determined from the observed increases of the ζ-potential as a function of surfactant concentration in the regions of the negative ζ-potential, where the adsorption was a consequence of the hydrophobic effect. The ζ-potentials become more negative as the cmc of surfactant is approached, except in the interaction catalase-sodium octanoate where the ζ-potential become less negative. In the interactions of proteins with the fluorinated surfactant the ζ-potentials are more negative than in the interaction with the hydrogenated surfactant. In the case of catalase the shape of the curve ζ-potential-surfactant concentration suggests induced conformational change on catalase.

Research paper thumbnail of A Study of the Aggregation Behavior of Hexyltrimethylammonium Bromide in Aqueous Solution

Journal of Colloid and Interface Science, 1998

The self-association of n-hexyltrimethylammonium bromide (C 6 TAB) in aqueous solution has been e... more The self-association of n-hexyltrimethylammonium bromide (C 6 TAB) in aqueous solution has been examined as a function of temperature and electrolyte concentration. The critical micelle concentration (CMC) and the degree of counterion binding (␤) were determined by conductivity measurement at temperatures over the range 288.15-318.15 K. Ultrasound velocity measurements were used to obtain the CMC in water and in a range of concentrations of electrolyte (0.1 to 0.6 mol kg ؊1 NaBr) and static light scattering to obtain the aggregation number and the degree of counterion binding in water at 298.15 K. The enthalpy change on micellization in water was measured by microcalorimetry. Apparent adiabatic compressibilities were calculated from a combination of density and ultrasound velocity measurements. Changes in the thermodynamic properties on micellization were determined by applying the mass action model; good agreement was found between experimental and theoretical enthalpy changes. From comparison with the properties of other n-alkyltrimethylammonium bromides it has been shown that the CMC of C 6 TAB in water is lower than that predicted from the linear relationships between CMC and the number of carbon atoms in the alkyl chain. Similarly, the standard Gibbs energy of micellization is less negative than predicted, and the degree of counterion binding is much lower than for other C n TABs. It is suggested that the anomalous behavior of C 6 TAB is a consequence of the more highly organized core of the aggregates of very low aggregation number (3-4) and the high degree of exposure of the micellar components to the aqueous environment.

Research paper thumbnail of Thermodynamics of Micellization of N-Octyltrimethylammonium Bromide in Different Media

Langmuir, 1995

The critical micelle concentrations (cmc) of n-octyltrimethylammonium bromide in pH 3.2,7.0, and ... more The critical micelle concentrations (cmc) of n-octyltrimethylammonium bromide in pH 3.2,7.0, and 10.0 media, at the temperatures 15, 20,25, 30 and 35 "C have been determined by the electrical conductivity method. Cmcvalues decrease to a greater value ofpH. From the conductivity data, the degrees of counterion binding of the micelles have been calculated. Thermodynamic parameters of micellization have been obtained by application of the model of Evans and Ninham in terms ofhydrophobic and surface contributions, which are pH dependent.

Research paper thumbnail of Denaturation of lysozyme by n-alkyltrimethylammonium bromides in alkaline solution

Journal of The Chemical Society, Faraday Transactions, 1995

Research paper thumbnail of Conformational Changes in Human Serum Albumin Induced by Sodium Perfluorooctanoate in Aqueous Solutions

Journal of Physical Chemistry B, 2005

Conformational changes in the bulk solution and at the air-aqueous interface of human serum album... more Conformational changes in the bulk solution and at the air-aqueous interface of human serum albumin (HSA) induced by changes in concentration of sodium perfluorooctanoate (C 7 F 15 COO -Na + ) were studied by difference spectroscopy, -potential data, and axisymmetric drop shape analysis. -potential was used to monitor the formation of the HSA-C 7 F 15 COO -Na + complex and the surface charge of the complex. The conformational transition of HSA in the bulk solution was followed as a function of denaturant concentration by absorbance measurements at 280 nm. The data were analyzed to obtain values for the Gibbs energies of the transition in water (∆G 0 W ) and in a hydrophobic environment (∆G 0 hc ) pertaining to saturated protein-surfactant complexes. The conformational changes that surfactants induce in HSA molecules alter its absorption behavior at the air-water interface. Dynamic surface measurements were used to evaluate this behavior. At low [C 7 F 15 COO -Na + ], proteins present three adsorption regimes: induction time, monolayer saturation, and interfacial gelation. When surfactant concentration increases and conformational changes in the bulk solution occur, the adsorption regimes disappear. HSA molecules in an intermediate conformational state migrate to the air-water interface and form a unique monolayer. At high [C 7 F 15 COO -Na + ], the adsorption of denatured molecules exhibits a behavior analogous to that of dilute solutions.

Research paper thumbnail of A Comparative Study of the Determination of the Critical Micelle Concentration by Conductivity and Dielectric Constant Measurements

Langmuir, 1998

Conductivities and dielectric constant measurements in water at 25°C have been made on the amphip... more Conductivities and dielectric constant measurements in water at 25°C have been made on the amphiphilics sodium n-dodecyl sulfate, n-dodecyltrimethylammonium bromide, and chlorpromazine hydrochloride. By using the conductivity/concentration data, critical micelle concentrations (cmc) have been determined by applying the Williams definition and two forms of the Phillips method. This first Phillips form consists of an approximation to Gaussians of the second derivative of the conductivity/concentration data followed by two consecutive integrations. The second form, which is proposed here, consists of the application of a combination of the Runge-Kutta numerical integrations method and the Levenberg-Marquardt leastsquares fitting algorithm. The proposed method permits the determination of the cmc in systems with low aggregation numbers and with slow variations of physical property/concentration curves allowing the determination of the so-called second cmc. A comparative study with results obtained by dielectric constant measurements has been carried out. With this new technique, the cmc's (first and second) are directly obtained as singular points in the dielectric constant/concentration curves, and thus, this technique is an alternative to the determination of cmc's from conductivities.

Research paper thumbnail of Size and stability of liposomes: A possible role of hydration and osmotic forces

European Physical Journal E, 2006

Dynamic light scattering and electrophoretic mobility measurements have been used to characterize... more Dynamic light scattering and electrophoretic mobility measurements have been used to characterize the size, size distribution and zeta potentials (ζ-potentials) of egg yolk phosphatidylcholine (EYPC) liposomes in the presence of monovalent ions ( Na+ and K+). To study the stability of liposomes the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory has been extended by introducing the hydrated radius of the adsorbed ions onto the liposome surfaces. The decrease of liposome size is explained on the basis of the membrane impermeability to some ions which generate osmotic forces, which leads to evacuate water from liposome inside.

Research paper thumbnail of A Comparative Study of the Interaction between Nafcillin and Catalase by Equilibrium Dialysis and ζ-Potential Measurements

Journal of Physical Chemistry B, 2001

The characterization of the interactions of the penicillin nafcillin with the protein catalase in... more The characterization of the interactions of the penicillin nafcillin with the protein catalase in aqueous solution at pH 3.2, 7.4, and 10.0 at 25°C has been determined using a combination of equilibrium dialysis and -potential measurements. At pH 3.2, the -potential of catalase tends to reverse charge from positive to negative values as the negatively charged penicillin bind to it. At pH 7.4 and 10.0, this -potential becomes more negative as the nafcillin concentration increases. The number of drug molecules bound to catalase per protein molecule (υ) was obtained from equilibrium dialysis and compared with the -potential data. Nafcillin binds largely to catalase to various extents ranging from ∼750 (pH 3.2) to ∼10 000 (pH 7.4 and 10.0) drug molecules per catalase molecule as the free concentration approaches the critical micelle concentration (cmc) of the free drug. Gibbs energies per drug molecule bound (∆G υ j ) were obtained as a function of υ j from equilibrium dialysis using the Wyman potential (J. Mol. Biol. 1965, 11, 631). ∆G υ j is large and negative at low values of penicillin concentration and becomes less negative as the drug concentration increases. ∆G υ j against drug concentration plots obtained from the interpretation of the variation of -potential measurements were of the form and magnitude similar to those obtained from direct binding measurements using the equilibrium dialysis technique.

Research paper thumbnail of Study of the interactions between lysozyme and a fully-fluorinated surfactant in aqueous solution at different surfactant–protein ratios

International Journal of Biological Macromolecules, 2003

The interactions of a fluorinated surfactant, sodium perfluorooctanoate, with lysozyme, have been... more The interactions of a fluorinated surfactant, sodium perfluorooctanoate, with lysozyme, have been investigated by a combination of UV absorbance, electrical conductivity and dynamic light scattering to detect and to characterize the conformational transitions of lysozyme. By using difference spectroscopy, the transition was followed as a function of surfactant concentration, and the data were analyzed to obtain the Gibbs energy of the transition in water ( G • w ) and in a hydrophobic environment ( G • h ) for saturated protein-surfactant complexes. Electrical conductivity was used to determine the critical micelle concentration of the surfactant in the presence of different lysozyme concentration. From these results, the average number of surfactant monomer per protein molecule was calculated. Finally, dynamic light scattering show that only changes in the secondary structure of the protein can be observed.

Research paper thumbnail of On the Effect of Ca 2+ and La 3+ on the Colloidal Stability of Liposomes

Langmuir, 2005

This work deals with the effect of Ca 2+ and La 3+ on the colloidal stability of phosphatidylchol... more This work deals with the effect of Ca 2+ and La 3+ on the colloidal stability of phosphatidylcholine (PC) liposomes in aqueous media. As physical techniques, nephelometry, photon correlation spectroscopy, electrophoretic mobility, and surface tension were used. The theoretical predictions of the colloidal stability of liposomes were followed using the Derjaguin-Landau-Verwey-Overbeek theory. Changes in the size of liposomes and high polydispersity values were observed as La 3+ concentration increases, suggesting that this cation induces the aggregation of liposomes. However, changes in polydispersity were not observed with Ca 2+ , suggesting a coalescence mechanism or fusion of liposomes. The stability factor (W), calculated from the nephelometry measurements indicated that aggregation/fusion occurs at a critical concentration (c.c.) of 0.3 and 0.7 M for La 3+ and Ca 2+ , respectively. To gain a better insight into the interaction mechanism between the liposomes and the studied ions, the interaction between PC monolayers and Ca 2+ and La 3+ was studied. Changes in the surface area per lipid molecule (A0) in the monolayer at the c.c. values were found for both ions, with a more pronounced effect in the case of Ca 2+ . This corresponds with a larger reduction of the steric repulsive interaction between the headgroups at the phospholipid membrane (πhead). The experimental result validates the hypothesis made on the liposome fusion in the presence of Ca 2+ and liposome aggregation in the presence of La 3+ . These aggregation mechanisms have also been confirmed by transmission electron microscopy.

Research paper thumbnail of A study of the interaction between proteins and fully-fluorinated and fully-hydrogenated surfactants by ζ-potential measurements

Colloids and Surfaces A-physicochemical and Engineering Aspects, 2004

An experimental investigation on the adsorption of two different surfactants, hydrogenated (sodiu... more An experimental investigation on the adsorption of two different surfactants, hydrogenated (sodium octanoate (SO)) and fluorinated (sodium perfluorooctanoate (SPFO)) on three proteins (lysozyme, hemoglobin and catalase) by measurements of the zeta-potential (ζ-potential) has been performed in water at the temperature of 298.15 K. The electrophoretic mobility of the complexes has shown hydrophobic adsorption of the surfactants on the protein surface, leading to surface saturation. The number of adsorption sites on the proteins was determined from the observed increases of the ζ-potential as a function of surfactant concentration in the regions of the negative ζ-potential, where the adsorption was a consequence of the hydrophobic effect. The ζ-potentials become more negative as the cmc of surfactant is approached, except in the interaction catalase-sodium octanoate where the ζ-potential become less negative. In the interactions of proteins with the fluorinated surfactant the ζ-potentials are more negative than in the interaction with the hydrogenated surfactant. In the case of catalase the shape of the curve ζ-potential-surfactant concentration suggests induced conformational change on catalase.

Research paper thumbnail of A Study of the Aggregation Behavior of Hexyltrimethylammonium Bromide in Aqueous Solution

Journal of Colloid and Interface Science, 1998

The self-association of n-hexyltrimethylammonium bromide (C 6 TAB) in aqueous solution has been e... more The self-association of n-hexyltrimethylammonium bromide (C 6 TAB) in aqueous solution has been examined as a function of temperature and electrolyte concentration. The critical micelle concentration (CMC) and the degree of counterion binding (␤) were determined by conductivity measurement at temperatures over the range 288.15-318.15 K. Ultrasound velocity measurements were used to obtain the CMC in water and in a range of concentrations of electrolyte (0.1 to 0.6 mol kg ؊1 NaBr) and static light scattering to obtain the aggregation number and the degree of counterion binding in water at 298.15 K. The enthalpy change on micellization in water was measured by microcalorimetry. Apparent adiabatic compressibilities were calculated from a combination of density and ultrasound velocity measurements. Changes in the thermodynamic properties on micellization were determined by applying the mass action model; good agreement was found between experimental and theoretical enthalpy changes. From comparison with the properties of other n-alkyltrimethylammonium bromides it has been shown that the CMC of C 6 TAB in water is lower than that predicted from the linear relationships between CMC and the number of carbon atoms in the alkyl chain. Similarly, the standard Gibbs energy of micellization is less negative than predicted, and the degree of counterion binding is much lower than for other C n TABs. It is suggested that the anomalous behavior of C 6 TAB is a consequence of the more highly organized core of the aggregates of very low aggregation number (3-4) and the high degree of exposure of the micellar components to the aqueous environment.