The role of counterion on the thermotropic phase behavior of DODAB and DODAC vesicles

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Planar membranes interaction

The system of two parallel planar, arbitrarily charged surfaces immersed in a solution containing only one ionic species, the counterions, is completely analyzed under a mean field Poisson-Boltzmann approach. Results for the pressure, reduced potential, and counterionic concentration are graphically displayed for two dissociating membranes and for a dissociating and an adsorbing membrane. The results indicate that the system of two planar parallel dissociating membranes acts as a buffer for pressure values and for counterionic concentration values in regions interior to and far from the membranes. The results are related to properties of planar or quasiplanar structures in biological cells.

Analysis of the ethidium bromide bound to DNA by photoacoustic and FTIR spectroscopy

Under physiological conditions B-form DNA is an exceedingly stable structure. However, experimental evidences obtained through nuclear magnetic resonance and fluorescence anisotropy suggest that the structure of the double helix fluctuates substantially. We describe photoacoustic phase modulation frequency measurements of ethidium bromide (Eb) with calf thymus, DNA. As in fluorescence phase modulation measurements, we used an intercalating dye as a probe; however, we monitored the triplet excited state lifetime at different ionic strengths. The triplet lifetime of Eb varied from about 0.30 ms, with no DNA present, to 20 ms, (at a DNA:Eb molar ratio of 5). With salt titration, this value falls, to about 2.0 ms. This result suggests, a strong coupling between the phenantridinium ring of the ethidium and the base pairs because of the stacking movement of the DNA molecule under salt effect. This, effect may be understood considering DNA as a polyelectrolyte. The counterions, in the solution shield the phosphate groups, reducing the electrostatic repulsion force between them, hence compacting the DNA molecule. The results from Fourier transform infrared demonstrated two important bands: 3187 cm(-1) corresponding to the symmetric stretching of the NH group of the bases, and 1225 cm(-1) corresponding to the asymmetric stretching of phosphate groups shifted toward higher wavenumbers, suggesting a proximity between the intercalant and base pairs and a modification of the DNA backbone state, both induced by salt accretion.

Using an effective surface charge to explain surface potentials of Langmuir monolayers from dialkyldimethylammonium halides with the Gouy-Chapman theory

The use of an effective surface charge density has allowed the Gouy-Chapman (CC) theory to explain surface potential isotherms of Langmuir monolayers of dioctadecyldimethylammonium bromide (DODAB). The effective surface charge density of DODAB monolayer increases with the electronegativity of the counterions in the subphase. The pressure-area isotherms indicate a very condensed monolayer for DODAB spread on an I--containing subphase, which exhibits the lowest surface charge density, whereas the monolayer on a F-containing subphase is extremely expanded owing to the high surface charge density or electrostatic repulsion between headgroups. (C) 2001 Published by Elsevier B.V. B.V.

Electronic Perspective on the Electrochemistry of Prussian Blue Films

The derivative of the voltabsommetric scans, together with previous nano-electrogravimetric and X-ray diffraction results, allow different electrochemical processes to be distinguished during the Prussian blue (PB) voltammetric scan. Potassium, proton, and hydrated proton counterions involved in PB electrochemistry are related here to the electrochemical reactions of specific Fe sites. Potassium counterions show two different sites for their insertion: one located in the crystalline framework and another in ferrocyanide vacancies. From the monitoring of electroactive Fe sites, the covalent-exchange model is suggested as one of the first approaches to explain the origin of the PB magnetic ordering observed at room temperature during voltammetric scanning.

Disordered distribution of defects in polythiophene

In this work we study the electronic structure associated to a disordered distribution of bipolarons in polythiophene. The polymer chain is modelled by a tight-binding Hamiltonian with explicit treatment of electron-phonon coupling and the elastic energy of the sigma framework. The model also includes the electrostatic interaction due to the counterions. The density of states of the disordered system is obtained by the use of the Negative Factor Counting technique. Our results show that ion-induced conformational disorder can account for the closure of the gap and that the states around the Fermi level are extended. © 1993.

Estudo conformacional de polianfóteros fracos utilizando simulações de Monte Carlo

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Determinação do gradiente de pH na região interna a duas membranas em equilíbrio com eletrólito

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Síntese de microesferas de quitosana para aplicação em liberação controlada de drogas pelo método de atomização e coagulação

Chitosan is a natural biodegradable polymer with great potential for pharmaceutical applications due to its biocompatibility, high charge density , nontoxicity and mucoadhesion. Gel formation can be obtained by the interactions of chitosans with low molecular counterions such as polyphosphates, sulphates and crosslinking with glutaraldehyde. This gelling property of chitosan allows a wide range of applications such as coating of pharmaceuticals and food products, gel entrapment of biochemicals, whole cells, microorganisms and algae. One of its main applications is the synthesis of microspheres for coating of pharmaceuticals , magnetic particles an other substances. In such a way, we can build targeted drug delivery systems. In the present work, we applied the method of spraying and coagulation. The resulting microspheres, then, were characterized by optical microscopy

Estudo dos processos de geração e transferência de cargas em corantes cianinas

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Long-Term Stability at High Temperatures for Birefringence in PAZO/PAH Layer-by-Layer Films

Optical memories with long-term stability at high temperatures have long been pursued in azopolymers with photoinduced birefringence. In this study, we show that the residual birefringence in layer-by-layer (LbL) films made with poly[1-[4-(3-carboxy-4 hydroxyphenylazo)benzene sulfonamido]-1,2-ethanediyl, sodium salt] (PAZO) alternated with poly(allylamine hydrochloride) (PAH) can be tuned by varying the extent of electrostatic interactions with film fabrication at different pHs for PAH. The dynamics of both writing and relaxation processes could be explained with a two-stage mechanism involving the orientation of the chromophores per se and the chain movement. Upon calculating the activation energies for these processes, we demonstrate semiquantitatively that reduced electrostatic interactions in films prepared at higher pH, for which PAH is less charged, are responsible for the longer stability at high temperatures. This is attributed to orientation of PAZO chromophores via cooperative aggregation, where the presence of counterions hindered relaxation.

Base-Paired and Base-Stacked Structures of the Anti-HIV Drug Lamivudine: A Nucleoside DNA-Mimicry with Unprecedented Topology

We have prepared a DNA-mimicry of nucleosides in which the anti-HIV drug lamivudine (beta-L-2',3'-dideoxy-3'-thiacytidine, 3TC) self-assembles into a base-paired and helically base-stacked hexagonal structure. Face-to-face and face-to-tail stacked 3TC=3TC dimers base-paired through two hydrogen bonds between neutral cytosines by either N-H center dot center dot center dot O or N-H center dot center dot center dot N atoms give rise to a right-handed DNA-mimicry of lamivudine with an unusual highly symmetric hexagonal lattice and topology. In addition, a base-paired and base-stacked supramolecular architecture of lamivudine hemihydrochloride hemihydrate was also obtained as a result of our crystal screenings. This structure is formed through partially face-to-face stacked lamivudine pairs held together by protonated and neutral fragments. However, no helical stacking occurs in this structure in which lamivudine also adopts unusual conformations as the C1'-endo and C1'-exo sugar puckers and cytosine orientations intermediate between the anti and syn conformations. As a conclusion drawn from the nucleoside duplex, the hexagonal DNA-mimicry of lamivudine reveals that such double-stranded helices can be assembled without counterions and organic solvents but with higher crystallographic symmetry instead, because only water crystallizes together with lamivudine in this structure.

Interfacial concentrations of chloride and bromide in zwitterionic micelles with opposite dipoles: Experimental determination by chemical trapping and a theoretical description

Interfacial concentrations of chloride and bromide ions, with Li+, Na+, K+, Rb+, Cs+, trimethylammonium (TMA(+)), Ca2+, and Mg2+ as counterions, were determined by chemical trapping in micelles formed by two zwitterionic surfactants, namely N-hexadecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (HPS) and hexadecylphosphorylcholine (HDPC) micelles. Appropriate standard curves for the chemical trapping method were obtained by measuring the product yields of chloride and bromide salts with 2,4,6-trimethyl-benzenediazonium (BF4) in the presence of low molecular analogs (N,N,N-trimethyl-propane sulfonate and methyl-phosphorylcholine) of the employed surfactants. The experimentally determined values for the local Br- (Cl-) concentrations were modeled by fully integrated non-linear Poisson Boltzmann equations. The best fits to all experimental data were obtained by considering that ions at the interface are not fixed at an adsorption site but are free to move in the interfacial plane. In addition, the calculation of ion distribution allowed the estimation of the degree of ion coverage by using standard chemical potential differences accounting for ion specificity. (C) 2012 Elsevier Inc. All rights reserved.

Salts of the anti-HIV drug lamivudine with phthalic and salicylic acids

Salts of the anti-HIV drug lamivudine, with phthalic acid and salicylic acid as counterions, were investigated in this study. Neither the packing of the (lamivudine)(+)(phthalic acid)(-) ion pairs nor the conformation of the lamivudine moiety itself were similar to those found in other multicomponent molecular salts of the drug, such as hydrogen maleate and saccharinate ones, even though all three salts crystallize in the same P2(1)2(1)2(1) orthorhombic space group with similar unit cell metrics. Lamivudine salicylate assumes a different crystal structure to those of the hydrogen maleate and saccharinate salts, crystallizing in the P2(1) monoclinic space group as a monohydrate whose (lamivudine)(+)(salicylic acid)(-) ion pair is assembled through two hydrogen bonds with cytosine as a dual donor to both oxygens of the carboxylate, such as in the pairing of lamivudine with a phthalic acid counterion. In lamivudine salicylate monohydrate, the drug conformation is related to the hydrogen maleate and saccharinate salts. However, such a conformational similarity is not related to the intermolecular interaction patterns. Lamivudine and water molecules alternate into helical chains in the salicylate salt monohydrate.

Kinetische Untersuchungen zur Synthese von Blockcopolymeren mit Methacrylatsegmenten durch neue anionische Polymerisationssysteme

Ein metallfreies Reaktionssystem mit Bis(triphenylphosphoranyliden)ammonium-Kationen als Gegenionen sowie das Additiv Lithium-2-methoxyethoxid wurden auf ihre Eignung zur anionischen Synthese von Blockcopolymeren mit (Meth)acrylatsegmenten bei moderaten Temperaturen im Strömungsrohr-Reaktor untersucht. Das metallfreie System ist zur lebenden Polymerisation von Methacrylaten in THF mit engen Molekulargewichtsverteilungen bei Reaktionszeiten < 1 s bis zu Temperaturen von 0 °C geeignet. In Gegenwart von Metallionen (Li+) findet eine Verlangsamung der Polymerisation unter Verlust der Reaktionskontrolle statt, Lithiumenolate verursachen nun breite, multimodale Molekulargewichtsverteilungen. Eine lebende Polymerisation von Acrylaten ist nicht möglich, massenspektroskopische Untersuchungen der Produkte weisen auf einen komplexen Reaktionsmechanismus mit Abbruch- und Übertragungsreaktionen hin. Die Synthese von Poly(styrol)-block-Poly(1,4-butadien)-block-Poly(methylmethacrylat)-Copolymeren in Toluol ist mit Lithium-2-methoxyethoxid als Additiv für die MMA-Polymerisation bei moderaten Mischtemperaturen (T < 0 °C) im Strömungsrohr-Reaktor möglich, die Effektivität des Wechselschritts von Polybutadien zu PMMA beträgt im Durchschnitt ca. 50 %. Untersuchungen verschiedener Reaktionsparameter, wie z.B. der Endfunktionalisierung des Polybutadiens mit 1,1-Diphenylethylen und der Temperatur während der Verkappung und der MMA-Polymerisation, geben keine eindeutigen Hinweise auf die Ursache dieses Phänomens. MALDI-TOF-Massenspektren des unreagierten Polybutadien Precursors zeigen die Anlagerung von 1-3 Molekülen Methylmethacrylat und keinen Abbruch durch Backbiting, was auf die Ausbildung stabiler Aggregate hindeutet.