Alfredo González-Pérez

Biomimetic Triblock Copolymer Membrane Arrays: A Stable Template for Functional Membrane Proteins

It is demonstrated that biomimetic stable triblock copolymer membrane arrays can be prepared using a scaffold containing 64 apertures of 300 microm diameter each. The membranes were made from a stock solution of block copolymers with decane as a solvent using a new deposition method. By using decane, we avoid low molecular weight solvents such as chloroform and toluene, which are strong protein denaturants. The membranes show a low ionic conductance and a long lifetime at room temperature. Contrast phase microscopy shows the presence of a polymer region delimited by a Plateau-Gibbs border similar to what is observed in black lipid membranes. The ion-channel gramicidin A was successfully incorporated into the membrane in a functional form.

Micellization of dodecyldimethylethyl- ammonium bromide in aqueous solution

Specific conductivity of aqueous solutions of dodecyldimethylethylammonium bromide has been determined in the temperature range of 15-40°C. The critical micelle concentration (cmc) and ionization degree of the micelles, b, were determined from the data. Thermodynamic functions, such as standard Gibbs free energy, ΔGm°, enthalpy, ΔGm°, and entropy, ΔGm°, of micellization, were estimated by assuming that the system conforms to the mass action model. The change in heat capacity upon micellization, ΔGm°, was estimated from the temperature dependence of ΔGm°. An enthalpy-entropy compensation phenomenom for the studied system has been found.

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.

Cyclodextrin−Surfactant Complex: A New Route in DNA Decompaction

In the present work, we show a new approach for decompaction of DNA-cationic surfactant complexes, e.g., lipoplexes, by using beta-cyclodextrin (beta-CD). The DNA decompaction was achieved by dissolving the surfactant aggregates in the complex by making use of the high affinity between the beta-CD and the free surfactant in solution. The results from fluorescence microscopy and adiabatic compressibility measurements indicate that coils and globules do not coexist. The reported procedure using beta-CD is an efficient way to decompact DNA surfactant complexes because the association constant of surfactants with beta-CD is large. The surfactants interaction with beta-CD is specific and the nonspecific interaction between beta-CD and biological interfaces is small.

Release of DNA from surfactant complexes induced by 2-hydroxypropyl-beta-cyclodextrin.

Decompaction of DNA-CTA self-assembled complexes by 2-hydroxypropyl-beta-cyclodextrin (2-HP-beta-CD) was studied and the results were compared with beta-CD. Different degrees of 2-HP substitution (0.6, 0.8 and 1.0, respectively) were used and the decompaction was successful with all degrees of substitution. Fluorescence microscopy, steady state fluorescence spectroscopy, density and sound velocity measurements, thermal melting and circular dichroism were used. Compared to previous work using alpha- and beta-CD, the fluorescence spectroscopy results showed that the 2-HP-substituted CDs more efficiently released DNA into solution. Furthermore, dissociation of macroscopically phase separated DNA-CTA complexes was achieved upon addition of 2-HP-beta-CD and the results gave strong indications on the non-equilibrium nature of the system. The globule-to-coil transition was not found to proceed through a coexistence region, which seems to be a general phenomenon in DNA decompaction using CDs.

Penetration of Action Potentials During Collision in the Median and Lateral Giant Axons of Invertebrates

The collisions of two simultaneously generated impulses in the giant axons of both earthworms and lobsters propagating in orthodromic and antidromic direction are investigated. The experiments have been performed on the extracted ventral cords of Lumbricus terrestris and the abdominal ventral cord of a lobster, Homarus americanus, by using external stimulation and recording. The collision of two nerve impulses of orthodromic and antidromic propagation did not result in the annihilation of the two signals, contrary to the common notion that is based on the existence of a refractory period in the well-known Hodgkin-Huxley theory. However, the results are in agreement with the electromechanical soliton theory for nerve-pulse propagation, as suggested by Heimburg and Jackson [On Soliton Propagation in Biomembranes and Nerves, Proc. Natl. Acad. Sci. U.S.A. 102, 9790 (2005).].

Characterization of fluorinated catansomes: a promising vector in drug-delivery.

Catansomes, which are vesicles prepared from mixtures of oppositely charged surfactants, have been suggested as effective alternatives to phospholipid vesicles, i.e., liposomes, in applications such as drug-delivery. This is mainly due to their enhanced chemical and physical stability as well as to their relatively easy preparation, which is an advantage for large-scale productions. In this study we have investigated catansomes prepared from a perfluorinated anionic surfactant (sodium perfluorooctanoate) premixed with a hydrogenated cationic surfactant (dodecyltrimethylammonium bromide or 1-dodecylpyridinium chloride). The aim was to gain insights into the physicochemical properties of these systems, such as size, stability, surface charge, and membrane morphology, which are essential for their use in drug-delivery applications. The catansomes were mostly unilamellar and 100-200 nm in size, and were stable for more than five months at room temperature. After loading the catansomes with the fluorescent marker calcein, they were found to exhibit an appreciable encapsulation efficiency and a low calcein leakage over time. The addition of fatty acids to calcein-loaded catansomes considerably promoted the release of calcein, and the rate and efficiency of calcein release were found to be proportional to the fatty acid concentration and chain length. Our results prove the feasibility of utilizing catansomes as drug-delivery vehicles as well as provide a means to efficiently release the encapsulated load.

Biomimetic triblock copolymer membranes: from aqueous solutions to solid supports

In the present paper, we studied the preparation of biomimetic triblock copolymer (ABA) membranes in aqueous solution and their deposition into solid supports. The self-assembly structures of the ABA in aqueous solution was investigated by using optical microscopy, dynamic light scattering, electron microscopy (EM) and SAXS. Spherical and tubular polymersomes were found at the highest concentrations investigated. The mechanism of deposition on solid supports (mica and glass) was elucidated by using atomic force microscopy (AFM). The deposition results in the formation of a uniform defect-free membrane at suitable polymer concentrations.

Thermodynamics of self-assembly of sodium octanoate: comparison with a fully fluorinated counterpart

The isotherms of conductivity of sodium octanoate were measured and the critical micelle concentration (cmc) and degree of ionization of the micelles, β, determined in a range of temperatures (273–343 K) above the Krafft point. The thermodynamic parameters, Gibbs free energy ΔG m 0, enthalpy ΔH m 0, and entropy ΔS m 0 of micelle formation, were determined from polynomial adjustments of the temperature dependence of cmc and from a proposed thermodynamic model based on the works of Muller [1993, Langmuir, 9, 96] and Rodríguez et al. [2002, J. Colloid Interface Sci., 250, 438]. The increase in heat capacity upon micellization, ΔC pm 0, was estimated from the parameters of the model and the enthalpy—entropy compensation phenomena discussed. Finally, for information on their structural differences, hence to understand their different behaviours, thermodynamic parameters are discussed, comparing the corresponding fluorocarbon compound. A remarkable shift in minimum temperature in the U-shaped curve of cmc versus temperature was found when hydrogen was substituted by fluorine in the hydrophobic chain of the surfactant. This behaviour is a consequence of the special characteristics of the fluorine substituent in the hydrophobic tail and was reflected in the thermodynamic parameters and in the enthalpy—entropy compensation parameters, presenting different intercepts at the same compensation temperature.

A spectroscopic study of the interaction catalase–cationic surfactant (n-decyltrimethylammonium bromide) in aqueous solutions at different pH and temperatures

A surfactant-induced and temperature conformational transition of catalase has been examined by difference spectroscopy for n-decyltrimethylammonium bromide (C10TAB) in different media (pHs 3.2, 6.4 and 10.0). The conformational transition of catalase by C10TAB was followed as a function of denaturant concentration using absorbance measurements at 280 nm and the data were analysed to obtain the Gibbs energy of the transition in water (ΔG0w) and in a hydrophobic environment (ΔG0hc) for saturated protein–surfactant complexes. Changes in absorbance at 280 nm of catalase with temperature were used to determine the parameters characterising the thermodynamics of unfolding, melting temperature (Tm), enthalpy (ΔHm), entropy (ΔSm) and heat capacity (ΔCp). The results suggest that catalase interacts with C10TAB in the media of pHs 6.4 and 10.0 inducing a conformational transition. However, an interaction in acid medium of pH 3.2 was not present. Finally, we compared the interaction of catalase with other cationic surfactants studied by other authors.