Chiroptical phenomena in reverse micelles: the case of (1R,2S)-dodecyl (2-hydroxy-1-methyl-2-phenylethyl)dimethylammonium bromide (DMEB).

(1R,2S)-Dodecyl(2-hydroxy-1-methyl-2-phenylethyl)dimethylammonium bromide (DMEB) aggregates dispersed in carbon tetrachloride have been investigated by Fourier transform infrared (FT-IR), vibrational circular dichroism (VCD) and (1)H nuclear magnetic resonance (NMR) spectroscopy at various surfactant concentration and water-to-surfactant molar ratio. Experimental data indicate that, even at the lowest investigated concentration and in absence of added water, DMEB molecules associate in supramolecular assemblies. At higher DMEB concentration the aggregates can confine water molecules, making it plausible to think that DMEB form reverse micelles and that water molecules are quite uniformly distributed among them and mainly located in the proximity of surfactant head groups. Moreover, the water state in DMEB reverse micelles has been found to be different from that in pure water, due to system-specific water/surfactant head group interactions. (1)H NMR diffusion measurements of both water and DMEB emphasize their joined translational motion characterized by a diffusion rate one order of magnitude lower than that of free molecules. Finally, VCD allowed us to show some characteristics of the association of optically active DMEB molecules as reverse micelles and water confinement inside; namely, we monitored the vibrational optical activity of deuterated hydroxyl bonds of the self-assembled DMEB molecules and their interaction with D(2)O molecules.

Effects of the net charge on abundance and stability of supramolecular surfactant aggregates in gas phase.

Self-assembling of amphiphilic molecules under electrospray ionization (ESI) conditions is characterized by quite unexpected phenomenology. The noticeable differences with respect to the condensed phase are attributable to the absence of the surfactant-solvent interactions, the presence of net charge in the aggregates, and the strong deviation from equilibrium conditions. Aiming to investigate the effects of the net charge on abundance and stability of supramolecular surfactant aggregates, positively and negatively charged aggregates of sodium bis(2-ethylhexyl)sulfosuccinate (AOT) and sodium methane sulfonate (MetS), butane sulfonate (ButS) and octane sulfonate (OctS) have been studied by ESI mass spectrometry, energy resolved mass spectrometry and density functional theory calculations. The negatively charged aggregates are found to be less stable than their positive counterparts. The results are consistent with a self-assembling pattern dominated by electrostatic interactions involving the counterions and head groups of the investigated amphiphilic compounds while the alkyl chains point outwards, protecting the aggregates from unlimited growth processes.

Self-assembly in surfactant-based liquid mixtures: bis(2-ethylhexyl)phosphoric acid/bis(2-ethylhexyl)amine systems.

Surfactant-based liquid mixtures constitute an interesting class of nanostructured materials with promising potential in specialized applications. Here, structural and conductometric properties of liquid mixtures composed of bis(2-ethylhexyl)amine (BEEA) and bis(2-ethylhexyl)phosphoric acid (HDEHP) have been thoroughly investigated with the aim to correlate structural features with system charge transport capability. The evolution of self-assembled local nanostructures with system composition has been investigated by FT-IR and XRD while the conductometric properties were probed by conventional AC complex impedance. Both pure components exhibit nano-segregation due to their amphiphilic nature but with only very low proton conductivities. However their mixtures, characterized by local organization of polar and apolar domains driven by acid-base interactions between HDEHP PO(4)H and BEEA NH groups, show enhanced structural order and proton conductivity.

Fluorescence emission and enhanced photochemical stability of Zn(II)-5-triethyl ammonium methyl salicylidene ortho-phenylendiiminate interacting with native DNA.

The photophysical and photochemical properties of the cationic Zn(II) complex of 5-triethyl ammonium methyl salicylidene ortho-phenylendiimine (ZnL(2+)) interacting with native DNA were investigated by steady state and time-resolved fluorescence spectroscopies. Experimental results indicate that, in the presence of DNA, ZnL(2+) is efficiently protected from a photochemical process, which occurs when it is in the free state dispersed in aqueous solution. The analysis of the absorption and emission spectra of ZnL(2+), both stored in the dark and after exposure to tungsten lamp light for 24 h, corroborated by quantum chemical calculations, allowed us to point out that ZnL(2+) undergoes a photoinduced two-electron oxidation process. According to this picture, the protective action of DNA toward the intercalated ZnL(2+) was attributed to an effective inhibition of the ZnL(2+) photooxidation. In this context, it can be considered that DNA-intercalated ZnL(2+) is located in a region more hydrophobic than that sensed in the bulk water solvent. Moreover, by a thorough analysis of steady state and time-resolved fluorescence spectra, the interaction process can be consistently explained in terms of a complete intercalation of the complex molecules and that the polarity of the environment sensed by intercalated ZnL(2+) is comprised between that of methanol and ethanol.

Supramolecular aggregates in vacuum: positively monocharged sodium alkanesulfonate clusters.

The formation and structural features of positively monocharged aggregates of sodium bis(2-ethylhexyl) sulfosuccinate (AOT) and sodium methane--(MetS), butane--(ButS) and octane--(OctS) sulfonate molecules in gas phase have been investigated by electrospray ionization mass spectrometry, energy resolved mass spectrometry and density functional theory (DFT) calculations. The experimental results show that the center-of-mass collision energy required to dissociate 50% of these monocharged aggregates scantly depends on the length of the alkyl chain as well as on the aggregation number. This, together with the large predominance of monocharged species in the mass spectra, was rationalized in terms of an aggregation pattern mainly driven by the counter ions and head groups electrostatic interactions while minor effects were attributed to the steric hindrance caused by the size of the surfactant head group and alkyl chain. DFT calculations show that the most favoured structural arrangement of these aggregates is always characterized by an internal polar core constituted by the sodium counter ions and surfactant head groups surrounded by an external layer composed by the surfactant alkyl chains.

Fast proton conduction in hydrogen bonded microheterogeneous systems: bis(2-ethylhexyl)phosphoric acid/N-methyl formamide liquid mixtures.

Structural and dynamical properties of bis(2-ethylhexyl)phosphoric acid (HDEHP)/N-methyl formamide (NMF) liquid mixtures in the whole composition range have been investigated by Fourier Transform Infrared spectroscopy (FT-IR), X-rays Diffraction (XRD), and AC complex impedance spectroscopy. Driven by hydrogen bond interactions among HDEHP PO(4)H group and NMF CO and NH groups, and also by steric effects among the HDEHP alkyl chains, the system microstructure is mainly characterized by the coexistence of spatially separated hydrophilic and hydrophobic nanodomains showing local organization and short-range order. The evolution of this structural feature with system composition has been highlighted by FT-IR and XRD. On the other hand, the thermally activated proton mobility within these hydrophilic domains, as pointed out by AC complex impedance measurements, emphasizes the occurrence of a percolating behaviour characterized by percolation threshold at zero. Due to fast proton conduction across the interconnected network formed by the polar groups, some samples showed very high room-temperature conductivity up to 10(-3) S cm(-1).

Spectroscopic and structural investigation of the confinement of D and L dimethyl tartrate in lecithin reverse micelles.

The confinement of D and L dimethyl tartrate in lecithin reverse micelles dispersed in cyclohexane has been investigated by FT-IR, polarimetry, electronic and vibrational circular dichroism (ECD and VCD), 1H NMR, and small-angle X-ray scattering (SAXS). Measurements have been performed at room temperature as a function of the solubilizate-to-surfactant molar ratio (R) at fixed lecithin concentration. The analysis of experimental data indicates that the dimethyl tartrate molecules are solubilized within reverse micelles in proximity to the surfactant head groups in the same way for the D and L forms. The encapsulation of dimethyl tatrate within lecithin reverse micelles involves changes in its H-bonds, from what is observed in the pure solid or in CCl4 solutions; this is a consequence of the establishment of specific solute-surfactant headgroup interactions and of confinement effects. In the 0 < or = R < or = 1.7 range, SAXS profiles of dimethyl tartrate/lecithin/ cyclohexane micellar solutions are well-described by a model of interacting polydisperse spherical micellar cores whose mean radius does not change appreciably with R (i.e., it changes from about 18 to 20 angstroms). 1H NMR diffusion measurements of both dimethyl tartrates and lecithin were rationalized in terms of collective translational motions of the entire micellar aggregate and of their molecular diffusion among clusters of reverse micelles. The association of optically active lecithin with D and L dimethyl tartrate leads to the formation of self-organized supramolecular aggregates whose interesting chiroptical features are evidenced by polarimetry and CD.

Complex permittivity of FeCl3/AOT/CCl4 microemulsions probed by AC impedance spectroscopy.

The complex permittivity of FeCl(3)/AOT/CCl(4) microemulsions in the 1-10(5) Hz frequency range has been measured by the conventional AC complex impedance technique. Measurements as a function of the volume fraction of the dispersed phase (FeCl(3)+AOT) and temperature at fixed salt-to-AOT molar ratio (R, R = 0.5) show that the entrapment of FeCl(3) clusters significantly enhances the local permittivity of the AOT reverse micelles and the number density of charge carriers resulting from the peculiar state of the confined inorganic salt. An estimate of the apparent static permittivity of the FeCl(3) ionic clusters entrapped in the core of AOT reverse micelles gives the very high and quite surprisingly value of about 237. Moreover, a thorough analysis of conductivity data and of their temperature dependence strongly supports the hypothesis that the charge transport in these systems is mainly sustained by a mechanism of hopping consisting in the continuous jumping of charged species within supra-micellar aggregates of AOT reverse micelles whose aggregation is driven by fluctuating opposite charges on contacting micelles.

Spatially ordered surfactant assemblies in the gas phase: negatively charged bis(2-ethylhexyl)sulfosuccinate-alkaline metal ion aggregates.

The formation and structural features of negatively charged aggregates of sodium bis(2-ethylhexyl)sulfosuccinate (AOTNa) surfactant molecules in the gas phase have been investigated by electrospray ionization mass spectrometry (ESI-MS) and density functional theory calculations. Mainly driven by the interactions of alkali metal ions both with the oxygen atoms of the sulfonate group and with the succinate moiety of the AOT- anion, spatially ordered supramolecular assemblies, characterized by an internal core composed of surfactant counterions and hydrophilic head groups surrounded by the surfactant alkyl chains pointing outwards, are formed. Calculations have shown that surfactant self-organization in the gas phase is energetically favoured, the energy of formation of negatively charged aggregates from isolated AOTNa and AOT- being linearly related to the aggregation number. Information on the chelating properties of AOTNa towards clusters of inorganic salts was achieved by infusion of solutions at various AOTNa/metal salt (NaCl, NaBr, NaI, LiI, KCl, CsI, RbI) ratios in the ESI source of a mass spectrometer. A wide variety of negatively charged AOT-metal aggregates, some of them also incorporating halide (X-) ions, has been observed. Calculations have shown that the capture of a halide anion to give the AOTMX- species is favoured but the energetics of the process depends on the alkali metal and halide types. The use of energy-resolved mass spectrometry has allowed us to evaluate the stability of different complexes and to evaluate the role played by the metal ion. Overall, the present investigation supports the idea that, in the gas phase, mainly driven by electrostatic interactions, surfactant molecules are present as molecular aggregates characterized by a reverse micelle-like organization with an internal core formed by the surfactant counterions and head groups surrounded by the surfactant alkyl chains. These peculiar aggregates are able to incorporate ionic clusters in their hydrophilic core.

The zero field self-organization of cobalt/surfactant nanocomposite thin films.

Cobalt nanostructures have been prepared by a chemical route based on the Co(II) reduction in the confined space of cobalt bis(2-ethylhexyl)sulfosuccinate (Co(DEHSS)(2)) reverse micelles dispersed in n-heptane. This procedure involves the rapid formation of surfactant softly coated Co nanostructures followed by a slow separation process of the magnetic-field responsive Co/surfactant nanocomposites from the liquid phase. The detailed structure of thin films of the Co/surfactant nanocomposites has been investigated by scanning force microscopy (SFM). The thin films were characterized by different anisotropic features. Micrometric long domains of self-aligned ellipsoidal NPs (tens of nanometers in size) have been observed, together with bendable micrometric long homogeneous nanofibers (NFs). The film structures were strongly dependent on the Co/surfactant ratio and, by increasing the Co percentage, the system was forced towards the formation of mutually connected superstructures consisting of anisotropic bands of self-aligned NFs and anisotropic 2D close packed Co-NP super-lattices. Transmission electron microscopy (TEM) showed that the NPs observed by SFM are in effect composed of almost spherical and oxygen-free cobalt nanoparticles, 1-3 nm in size, which typically assemble in larger ellipsoidal systems tens of nanometers in size. Magnetic force microscopy (MFM) demonstrates the magnetic response of these thin films, highlighting the different behavior (attractive/repulsive) of the Co-NPs aggregates towards the oscillating magnetized tip. The above structural findings have been interpreted in terms of nanostructures/matrix interaction along with a fine balance between short-range isotropic repulsions, van der Waals attractions and long-range anisotropic magnetic interactions.

Physico-chemical investigation of nanostructures in liquid phases: nickel chloride ionic clusters confined in sodium bis(2-ethylhexyl) sulfosuccinate reverse micelles.

The confinement of finite amounts of nickel chloride in the hydrophilic core of sodium bis(2-ethylhexyl) sulfosuccinate (AOT) reverse micelles dispersed in n-heptane has been investigated by FT-IR, UV-vis-NIR and fluorescence spectroscopies. The analysis of experimental data consistently leads to hypothesize that NiCl(2) forms small size ionic clusters stabilized by a monolayer of oriented surfactant molecules. Due to confinement and interfacial effects, these ionic clusters show peculiar photophysical properties, which are different from those possessed by the bulk material. From NiCl(2)/AOT/n-heptane solutions, by evaporation of the organic solvent, interesting salt/surfactant nanocomposites at various salt concentrations have been prepared and characterised by WAXS. On the other hand, after mix with Na(2)S-containing dry micellar systems, the formation of NiS nanoparticles have been ascertained by UV-vis spectroscopy.

Spectroscopic study of the interaction of Ni(II)-5-triethyl ammonium methyl salicylidene ortho-phenylendiiminate with native DNA.

The interaction of native calf thymus DNA with the cationic Ni(II) complex of 5-triethyl ammonium methyl salicylidene ortho-phenylendiimine (NiL(2+)), in 1mM Tris-HCl aqueous solutions at neutral pH, has been monitored as a function of the metal complex-DNA molar ratio by UV absorption spectrophotometry, circular dichroism (CD) and fluorescence spectroscopy. The dramatic modification of the DNA CD spectrum, the appearance of a broad induced CD band in the range 350-400nm, the strong increase of the DNA melting temperature (T(m)) and the fluorescence quenching of ethidium bromide-DNA solutions, in the presence of increasing amounts of the NiL(2+) metal complex, support the existence of a tight intercalative interaction of NiL(2+) with DNA, analogous to that recently reported for both ZnL(2+) and CuL(2+). The intrinsic binding constant (K(b)) and the interaction stoichiometry (s), determined by UV spectrophotometric titration, are equal to 4.3x10(6)M(-1) and 1.0 base pair per metal complex, respectively. Interestingly, the value of K(b) is slightly higher and 10 times higher than that relative to the CuL(2+)-DNA and the ZnL(2+)-DNA systems, respectively. Speculations can be performed to rationalize the observed trend, on the basis of the electronic and geometrical structures of the three complexes of the same ligand. Analogously to what previously observed for CuL(2+), the shape of the CD of the NiL(2+)-DNA system at NiL(2+)-DNA molar ratios higher than 0.5 is indicative of the formation of supramolecular aggregates in solutions, as a possible consequence of the electrostatic interaction between the cationic complex and the negatively charged phosphate groups of DNA.

The interaction of native calf thymus DNA with Fe(III)-dipyrido[3,2-a:2',3'-c]phenazine.

The mono and bis dipyrido[3,2-a:2',3'-c]phenazine (dppz) adducts of iron(III) chloride, i.e. [Fe(dppz)]Cl(3) and [Fe(dppz)(2)]Cl(3), have been synthesized and characterized. The interaction of the Fe(III)dppz hydrolyzed aquo complex with native calf thymus DNA has been monitored as a function of the metal complex-DNA molar ratio, by variable temperature UV absorption spectrophotometry, circular dichroism (CD) and fluorescence spectroscopy. The results obtained in solution at various ionic strength values give support for a tight intercalative binding of the Fe(III)dppz cation with DNA. In particular, the appearance of induced CD bands, caused by the addition of Fe(III)dppz, indicate the existence of a rigid metal complex-DNA-binding leading to dominating chiral organization of Fe(III)dppz species within the DNA double helix. The trend of selected CD bands with the molar concentration of Fe(III)dppz emphasizes that the presence of high amounts of metal complex induces also the formation of DNA-Fe(III)dppz supramolecular aggregates in solution. The analysis of fluorescence measurements allowed us to calculate a value of the intercalative binding constant comparable to that obtained by UV spectrophotometric titration. Finally, the temperature dependence of the absorbance at 258nm shows that the metal complex strongly increases the DNA melting temperature already at metal complex-DNA molar ratio equal to 0.25 suggesting that metal complex intercalation effectively hinders DNA denaturation. Overall, the results of the present study point out that the Fe(III)dppz aquo complex has DNA-binding properties analogous to those previously reported for the tris-chelate Fe(II)(phen)(2)dppz complex (phen=1,10-phenantroline).

Confinement effects on the interaction of native DNA with Cu(II)-5-(triethylammoniummethyl)salicylidene ortho-phenylendiiminate in C(12)E(4) liquid crystals.

Confinement effects of native calf thymus DNA interacting with the complex Cu(ii)-5-(triethylammoniummethyl)salicylidene ortho-phenylendiiminate (CuL(2+)) perchlorate in tetraethylene glycol monododecyl ether (C(12)E(4)) liquid crystals have been investigated by UV absorption spectrophotometry, circular dichroism (CD) and small angle X-ray scattering (SAXS). The results indicate the occurrence of dramatic structural changes of both the DNA and the CuL(2+)-DNA system, when going from aqueous solution to C(12)E(4) liquid crystals, due to confinement constrains imposed by the closed structure of C(12)E(4) reverse micelles. Further marked departures from the behaviour observed in aqueous solution have been emphasized by registering the spectral response of DNA and CuL(2+)-DNA confined in C(12)E(4) reverse micelles after thermal treatment. It has been also ascertained that the confinement causes the formation of a more compact and thermoresistant DNA structure accompanied by a transition from the right- to left-handed form while a tight CuL(2+)-DNA binding has been revealed by the appearance of a broad induced CD band in the range 350-450 nm. From a biological point of view, these findings stress the need to account for confinement effects and the peculiarity of drug-DNA interactions occurring within the intra-cellular environment.

Structural characterization of frozen n-heptane solutions of metal-containing reverse micelles.

The microstructure of temperature-quenched solutions of reverse micelles formed by sodium, cobalt, ytterbium, and cobalt/ytterbium bis(2-ethylhexyl)sulfosuccinate in n-heptane has been investigated by SAXS and EXAFS. Some changes in the X-ray absorption spectra with respect to the same systems at room temperature have been observed. The analysis of the SAXS spectra leads to the hypothesis that at 77 K the closed spherical structure of reverse micelles is retained and that during the temperature quench they undergo a clustering process involving the transition from a quite random dispersion to the formation of more or less large clusters of strongly packed reverse micelles. This behavior is attributed to competitive effects caused by the temperature decrease. The prevalence of intermicellar attractive interactions with respect to Brownian motions leading to a collapse to more compact structure is in competition with the rapid decrease of reverse micelle diffusion rate involving a freezing of the local structures. In the case of cobalt, ytterbium, and cobalt/ytterbium bis(2-ethylhexyl)sulfosuccinate reverse micelles, further information from EXAFS measurements indicates that within the reverse micelle core exists a quite ordered nanosized domain composed of water, surfactant counterions, and oxygen atoms of the SO3- head groups. The conservation of local order and inverse structure during the clustering phenomenon that results from the fast freezing with liquid nitrogen of solutions of reverse micelles could have biological implications, i.e., the preservation of tissue samples at cryogenic temperatures.

Study of confined 5-aza[5]helicene in ytterbium(III) bis(2-ethylhexyl) sulfosuccinate reversed micelles.

Some relevant physicochemical properties of 5-aza[5]helicene (H5) in solutions of ytterbium bis(2-ethylhexyl) sulfosuccinate (Yb(DEHSS)3) reversed micelles have been investigated by UV-vis-NIR, photoluminescence, and FT-IR techniques with the aim of emphasizing the role played by specific Yb(III)/H5 interactions and confinement effects as driving forces of its binding to reversed micelles, preferential solubilization site, and local photophysical properties. It has been found that the binding strength of 5-aza[5]helicene to reversed micelles, triggered by steric and orientational constrains as well as the water content, is mainly regulated by its interaction with the Yb(III) counterion. Moreover, when H5 is entrapped in Yb(DEHSS)3 reversed micelles, the combined action of this interaction and of confinement effects leads to marked changes of its photophysical properties with respect to those of H5 molecularly dispersed in apolar medium. The influence of the entrapment of finite amounts of H5 on the reversed micelle structure was investigated by SAXS. The analysis of experimental results brings to the hypothesis that H5 is preferentially solubilized and opportunely oriented in the micellar palisade layer and that its insertion causes an unidimensional growth of reversed micelles. From an analysis of WAXS spectra of H5/Yb(DEHSS)3 composites, obtained by complete evaporation of the volatile components of the H5/water/Yb(DEHSS)3/n-heptane solutions, it was ascertained that also on these systems H5 is dispersed molecularly or in a quite amorphous state in the surfactant liquid crystals without forming a separate crystalline nanophase.

Localization and interactions of melatonin in dry cholesterol/lecithin mixed reversed micelles used as cell membrane models.

The state of melatonin confined in dry cholesterol/lecithin mixed reversed micelles dispersed in CCl4 was investigated using 1H-NMR and FT-IR spectroscopies as a function of the melatonin to lecithin molar ratio (R(MLT)) and of the cholesterol to lecithin molar ratio (R(CHL)). An analysis of experimental results leads to the hypothesis that, independent of R(MLT) and as a consequence of anisotropic melatonin/lecithin, melatonin/cholesterol and cholesterol/lecithin interactions, melatonin is totally solubilized in reversed micelles. Melatonin is mainly located in and oriented in the nanodomain constituted by the hydrophilic groups of cholesterol and lecithin. A competition of melatonin and cholesterol for the hydrophilic binding sites of the reversed micelles was observed by changing the R(CHL). Some possible biological implications of the specific interactions governing the solubilization process, the preferential location and the peculiar properties of melatonin confined in cholesterol/lecithin mixed reversed micelles are discussed.