Félix Sarmiento

PSEUDOLATTICE THEORY OF STRONG ELECTROLYTE SOLUTIONS

The quadratic concentration dependence of the logarithmic activity coefficient and relative partial molar enthalpies of concentrated strong electrolyte solutions can be explained by assuming the solution to have a pseudolattice structure and taking short-range interactions into account through a hard-core potential with an attractive 1/r6 tail. The pseudolattice model is shown to reproduce the experimental data of activity coefficients and relative partial molar enthalpies with a great degree of accuracy over a large range of concentration. The validity of the short-range potential is explicitly tested with experimental data for a number of substances.

Study of the interactions between lysozyme and a fully-fluorinated surfactant in aqueous solution at different surfactant-protein ratios.

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 (DeltaGw(o)) and in a hydrophobic environment (DeltaGh(o)) 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.

Interactions between DMPC liposomes and the serum blood proteins HSA and IgG.

The interaction between two serum blood proteins, namely human serum albumin (HSA) and human immunoglobulin G (IgG), with 1,2-dimyristoyl-sn-glycero-3-phosphatidylcholine (DMPC) liposomes has been studied in detail using dynamic light scattering, flow cytometry, enzyme-linked immunosorbent assay (ELISA), electrophoretic mobility, differential scanning calorimetry (DSC), and surface tension measurements. HSA and IgG interact with liposomes forming molecular aggregates that remain stable at protein concentrations beyond those of total liposome coverage. Both HSA and IgG penetrate into the liposome bilayer. An ELISA assay indicates that the Fc region of IgG is the one that is immersed in the DMPC membrane. The liposome-protein interaction is mainly of electrostatic nature, but an important hydrophobic contribution is also present.

Excess enthalpies of (secondary amine + alcohol) at 298.15 K

The excess enthalpies HmE of N-butyl methyl amine + 1-butanol, + 1-pentanol, and + 1-hexanol and of dibutylamine + 1-propanol, + 1-butanol, + 1-pentanol, + 1-hexanol, and + 1-heptanol were measured at 298.15 K, and normal atmospheric pressure, using a Calvet microcalorimeter. All mixtures give HmE < 0; the values do not vary significantly when the chain length of the alcohol increases.

Excess enthalpies of five examples of (2-hexanone + an n-alkane) and five of (2-hexanone + an n-alkanol) at 298.15 K

Abstract Excess molar enthalpies H m E have been measured as a function of the mole fraction x at 298.15 K and atmospheric pressure for the 10 mixtures: {x CH 3 COC 4 H 9 + (1−x) C n H 2n+2 }, (n = 6, 7, 8, 9, and 10) ; {x CH 3 COC 4 H 9 + (1−x) C n H 2n+1 OH }, (n = 6, 7, 8, 9, and 10) . Measurements were carried out using a Calvet microcalorimeter, and variable-degree polynomials have been fitted to the results.

STATIC STRUCTURE OF ELECTROLYTE SYSTEMS AND THE LINEAR RESPONSE FUNCTION ON THE BASIS OF A DRESSED-ION THEORY

The static structure of a bulk electrolyte solution or colloid system is investigated in the framework of a dressed-ion theory (DIT). The number–number, charge–number, and charge–charge static structure factors are calculated and are seen to depend only on the linear response function of the DIT α(k), the α function therefore determining the charge structure of the fluid in what is an expression of the fluctuation–dissipation theorem. The expression of the static structure factors for one-component charged spheres (OCCS) is evaluated in the random-phase approximation and in a modified version of the mean-spherical approximation (MSA), using the hard-sphere fluid as a reference system, and an explicit expression for the linear response function and dielectric function is obtained. The effective screening length (κ−1) and the transition from monotonic exponential to oscillatory behavior obtained from the modified MSA expression of the α function are seen to improve the ones derived from the second moment c...

Denaturation of lysozyme by n-alkyltrimethylammonium bromides in alkaline solution

The interactions of a homologous series of n-alkyltrimethylammonium bromides (C8–C14 TABs) with hen egg lysozyme have been investigated using microcalorimetry. The C8 TAB does not interact with lysozyme whereas the C10–C14 TABs interact endothermically and deactivate the enzyme. The endothermicity of the TAB–lysozyme interaction is in marked contrast to the exothermic interactions between n-alkyl sulfates and lysozyme which have been attributed to specific binding between the anionic sulfate head groups and cationic amino acid residues. The enthalpies of interaction between the cationic surfactants and lysozyme follow sigmoidal curves characteristic of an interaction dominated by the endothermic unfolding of the native structure. The enthalpy data have been used to obtain the Gibbs energy and entropy for surfactant-induced denaturation. At pH 10 the Gibbs energy, enthalpy and entropy of denaturation are 17.9 ± 4.2 kJ mol–1, 148 ± 15 kJ mol–1 and 436 J mol–1 K–1 at 25 °C.

Interaction between sodium n-undecyl sulfate and insulin

The binding of sodium n-undecyl sulfate with bovine insulin was studied at pH 3.2 and 10 by equilibrium dialysis at 25 degrees C. The binding data have been used to determine the Gibbs energies of interaction using the theoretical model of the Wyman binding potential. The curves of Gibbs energies as a function of the number of bound ligands (v) tend to limiting values of around -14 kJ mol-1 at high values of v. The enthalpies of interaction have been measured directly by microcalorimetry showing an increase of exothermicity at lower pH. The results have been compared with similar data for the interaction of anionic surfactants with insulin.

Double Charge Inversion in Polyethylenimine-Decorated Liposomes

The study of the interaction of a cationic polymer as PEI with phospholipids membranes is of special relevance for gene therapy because the PEI is a potential nonviral vector to transfer DNA in living cells. We used light scattering, zeta potential, and electron transmission microscopy to characterize the interaction between DMPG and DOPC liposomes with PEI as a function of the charge molar ratio, pH, temperature, initial size of the liposomes, and headgroup of the lipids. Unexpectedly, a double charge inversion and two different ranges of PEI-liposome concentrations where an aggregation occurs were found, when the proper pH and initial size of the liposomes were chosen. The interaction is analyzed in terms of the interaction potential proposed by Velegol and Thwar for colloidal particles with a nonuniform surface charge distribution. Results show a remarkable dependence of the stability on pH and the initial size of the liposomes, which explains the low reproducibility of the experiments if no special care is taken in preparing the samples. Comparatively small changes in the pH or in the liposomes size lead to a completely different stability behavior.

Thermodynamic studies on the interaction of n-alkyl sulfates with insulin in aqueous solution

Binding isotherms for the interaction of a homologous series of sodium n-alkyl sulfates (chain lengths 8–10 and 12) on interaction with bovine insulin at 25 °C, at pH 3.2 and 10.0, have been determined. The isotherms show two unusual features, first the binding ranges up to 60–70 surfactant molecules per insulin molecule and secondly it is only slightly dependent on the n-alkyl chain length. The enthalpy changes on surfactant–insulin interaction have been measured by microcalorimetry and have been discussed in terms of exothermic contributions from sulfate head-group interactions with cation amino acid residues in the protein and an endothermic unfolding contribution. The exothermicities of the head-group interactions increase with decreasing pH but are in excess of those predicted from model studies on surfactant–polypeptide interactions. The Gibbs energies of binding per surfactant molecule decrease with increasing binding to a limiting value of ca.– 15 kJ (mol surfactant)–1 independent of pH. The high binding levels coupled with the slight chain length dependence suggest a micellar-based structure for the insulin–surfactant complexes.