DNA adsorption and release from cat-anionic vesicles made of sodium dodecylsulfate-dodecyldimethylammonium bromide (SDS-DDAB) in nonstoichiometric amounts was investigated by different electrochemical, spectroscopic, and biomolecular strategies. The characterization of the vesicular system was performed by dynamic light scattering, which allowed estimating both its size and distribution function(s). The interaction dynamics was followed by dielectric spectroscopy and zeta-potential, as well as by agarose gel electrophoresis, AGE. Also, circular dichroism, CD, measurements were carried out, to ascertain possible structural rearrangements of DNA, consequent to the interactions with the cat-anionic vesicles. CD demonstrates that vesicle-bound DNA retains its native conformation. The results obtained by the aforementioned techniques are consistent and indicate that binding saturation is obtained at a [DNA/vesicles] charge ratio close to 0.8, considering only the excess surface charges on the vesicles. This result is apparently in contradiction with a purely electrostatic approach and is tentatively ascribed to the distance between charges on the biopolymer and the vesicle surface, respectively. A possible interpretation is discussed. The nucleic acid can be completely retrieved from the vesicles upon addition of adequate amounts of SDS, which is the defective surfactant in the vesicular system. Precipitation of the poorly soluble SD-DDA salt results in an almost complete release of DNA.

The interactions between cat-anionic (an acronym indicating surfactant aggregates (micelles and vesicles) formed upon mixing cationic and anionic surfactants in nonstoichiometric amounts) vesicles and DNA have been the subject of intensive studies because of their potential applications in biomedicine. Here we report on the interactions between DNA and cetyltrimethylammonium bromide (CTAB)-sodium octyl sulfate (SOS) cat-anionic vesicles. The study was performed by combining dielectric relaxation spectroscopy, circular dichroism, dynamic light scattering, ion conductivity, and molecular biology techniques. DNA is added to positively charged vesicles until complete charge neutralization of the complex and formation of lipoplexes. This occurs when the mole ratio between the phosphate groups of DNA and positive charges on the vesicle is about 1.8. Above this threshold the nucleic acid in excess remains free in solution. This very interesting new result shows that anionic surfactants are not expelled upon saturation, and therefore, no formation of micelles occurs. Furthermore, vesicle-bound DNA can be released in its native form, as confirmed by dielectric spectroscopy and circular dichroism measurements. The nucleic acid is released upon addition of SOS, which competes with the phosphate groups of the DNA: this results in the demolition of the CTAB-SOS cat-anionic vesicles. These results indicate the possibility of a controlled DNA release and might be of interest in biomedicine.

The X-ray patterns of lithium (LiDC) and potassium (KDC) deoxycholate fibers, drawn from aqueous micellar solutions, have been interpreted by means of a packing of 8/1 helices formed by trimers. Previously, these helices satisfactorily represented the structure of the sodium (NaDC) and rubidium (RbDC) deoxycholate micellar aggregates. Dielectric measurements show that the trend of the average electric dipole moment µ of a NaDC monomer as a function of temperature and concentration supports a two-structure equilibrium. The high µ values (32-58 D) can be explained by the remarkable hydration of the NaDC micellar aggregates. The µ moderate decrease when the size of the aggregates increases can agree with the presence of small helices but disagrees with the existence of aggregates that are disordered or have a center (or a pseudocenter) of symmetry. Formerly, it was observed that sodium taurodeoxycholate (NaTDC) micellar aggregates, represented by 7/1 helices, formed by trimers, behave similarly. The contribution to the electrical conductivity of NaDC and NaTDC in aqueous solutions containing NaCl tends to zero by increasing the NaCl concentration, denoting strong interactions between Na + ions and anion aggregates. According to the similar 7/1 and 8/1 helices, which have the Na + ions in their inner part, the micellar size and the fraction of Na + ions trapped inside the helices increase together. The aggregate apparent hydrodynamic radius (R h ) increases by increasing the ionic strength in the order LiDC > NaDC > KDC > RbDC. Fibers drawn from solutions containing two cations at the same concentration show that the affinity for the anionic structure seems to follow the order Li + > Na + > K + > Rb + at high ionic strength. The R h values vs the mole fractions of Li + and Rb + or Na + and K + at lower ionic strength are fitted by straight lines. Probably, the free energy gains, associated with the cation and anion transfer from the bulk solution to the micellar aggregates, are almost equal for the four salts at lower ionic strength.

Previously, structural models, observed in fibers and crystals, were proposed for sodium deoxycholate (NaDC), glycodeoxycholate (NaGDC), taurodeoxycholate (NaTDC), and taurocholate (NaTC) micellar aggregates, and were verified in aqueous solutions by means of several techniques. Here we report the X-ray analysis of sodium glycocholate (NaGC) fibers, which indicates that NaGC micellar aggregates could be formed by dimers and octamers as in the case of NaTC. Moreover, we present electrolytic conductance and dielectric measurements on NaGDC, NaTC, and NaGC aqueous micellar solutions to verify our micellar aggregate models. Specific conductance values of 0.1 mol dm-3 NaDC, NaTDC, NaGDC, NaTC, and NaGC solutions containing NaCl at concentration ranging from 0 to 0.8 mol dm-3 practically do not depend on the particular bile salt. Comparison with NaCl values shows that bile salt contribution to conductance decreases by increasing NaCl concentration, is nearly zero around the concentration range 0.5−0.6 mol dm-3, and becomes negative at higher concentration. This behavior can be explained if Na+ ions strongly interact with bile salt anions and reinforce their interaction when micellar size increases. Even the inclusion of Na+ and Cl- ions, coming from NaCl, into micellar aggregates cannot be excluded, especially at high ionic strength. NaDC, NaTDC, NaGDC, NaTC, and NaGC present high values of the average electric dipole moment per monomer μ that can be justified by a remarkable hydration of their micellar aggregates. Reasonably, micellar aggregate composition and population change very slightly or do not change at all within the temperature range 15−45 °C, because μ is nearly constant in this interval. Results also suggest that Na+ ions are anchored to anions in dilute solution, thus forming ion pairs in the case of NaTC and NaGC, at least. Dihydroxy and trihydroxy bile salts are characterized by very similar cation−anion interaction strengths, even though their structures are different. The trend of μ, which moderately decreases by increasing bile salt concentration, agrees with our structural models and can be due to coexistence of two structures, at least.

Synthetic vesicles were prepared by mixing anionic and cationic surfactants, aqueous sodium dodecylsulfate with didodecyltrimethylammonium or cetyltrimethylammonium bromide. The overall surfactant content and the (anionic/cationic) mole ratios allow one to obtain negatively charged vesicles. In the phase diagram, the vesicular region is located between a solution phase, a lamellar liquid crystalline dispersion, and a precipitate area. Characterization of the vesicles was performed by electrophoretic mobility, NMR, TEM, and DLS and we determined their uni-lamellar character, size, stability, and charge density. Negatively charged vesicular dispersions, made of sodium dodecylsulfate/didodecyltrimethylammonium bromide or sodium dodecylsulfate/cetyltrimethylammonium bromide, were mixed with lysozyme, to form lipoplexes. Depending on the protein/vesicle charge ratio, binding, surface saturation, and lipoplexes flocculation, or precipitation, occurs. The free protein in excess remains in solution, after binding saturation. The systems were investigated by thermodynamic (surface tension and solution calorimetry), DLS, CD, TEM, 1H NMR, transport properties, electrophoretic mobility, and dielectric relaxation. The latter two methods give information on the vesicle charge neutralization by adsorbed protein. Binding is concomitant to modifications in the double layer thickness of vesicles and in the surface charge density of the resulting lipoplexes. This is also confirmed by developing the electrophoretic mobility results in terms of a Langmuir-like adsorption isotherm. Charges in excess with respect to the amount required to neutralize the vesicle surface promote lipoplexes clustering and/or flocculation. Protein-vesicle interactions were observed by DLS, indicating changes in particle size (and in their distribution functions) upon addition of LYSO. According to CD, the bound protein retains its native conformation, at least in the SDS/CTAB vesicular system. In fact, changes in the alpha-helix and beta-sheet conformations are moderate, if any. Calorimetric methods indicate that the maximum heat effect for LYSO binding occurs at charge neutralization. They also indicate that enthalpic are by far the dominant contributions to the system stability. Accordingly, energy effects associated with charge neutralization and double-layer contributions are much higher than counterion exchange and dehydration terms.