Gabriella Siani

Disaggregation of single-walled carbon nanotubes (SWNTs) promoted by the ionic liquid-based surfactant 1-hexadecyl-3-vinyl-imidazolium bromide in aqueous solution

Stable homogeneous aqueous dispersions of pristine single-walled carbon nanotubes (SWNTs) are obtained by using a water-soluble long-chain imidazolium ionic liquid (hvimBr) above its critical micelle concentration. The amount of hivmBr and the sonication time are two essential factors to obtain a good dispersion. The effective concentration of exfoliated SWNTs in aqueous solution is determined by simple, convenient and rapid UV-visible spectrophotometric measurements.

Perturbing Effects of Chiral Stationary Phase on Enantiomerization Second-Order Rate Constants Determined by Enantioselective Dynamic High-Performance Liquid Chromatography: A Practical Tool to Quantify the Accessible Acid and Basic Catalytic Sites Bonded on Chromatographic Supports

Second-order rate constants of the diethylamine-promoted enantiomerization of 2-[2-(1-methyl-1H-pyrrol-2-yl)-2-oxo-1-phenylethyl]-isoindole-1,3-dione, a chiral alpha-substituted ketone endowed with high anti-MAO activity type-A, were measured by dynamic high-performance liquid chromatography (DHPLC), stopped-flow high-performance liquid chromatography (sf-HPLC), and a classical method based on enantioselective HPLC as the monitoring tool. The chiral column used in all determinations was the commercial Chiralpak AD. By comparison of the obtained data, perturbing effects of the stationary phase on the DHPLC and sf-HPLC determinations were highlighted and distinguished in indirect (SP(IPC)) and direct (SP(DPC)) type. It was evidenced that SP(DPC) noise effects may be completely erased by simple mathematical treatment of data obtained at different concentrations of the basic catalyst. Perturbations of type SP(IPC) may instead only be partially kept down by modulating the concentration of the basic catalyst. An estimation of the density distribution of catalytic sites covalently bonded to the stationary phase (SP) of the Chiralpak AD was performed exploiting the quantified SP(DPC) effects. Such an approach might be of general application, supplying a useful way to characterize the attitude of SPs to speed acid- or base-catalyzed equilibria possibly active during chromatographic separations.

Characterization of cationic liposomes. Influence of the bilayer composition on the kinetics of the liposome breakdown

The cationic large unilamellar mixed liposomes from 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC) and didodecyldimethylammonium bromide (DDAB) or dioctadecyldimethylammonium bromide (DODAB) were prepared. The influence of the addition of Triton X-100 (TX-100) or octaethylene glycol mono-n-dodecylether (C(12)E(8)) on the membrane integrity was investigated turbidimetrically. The stability of the liposomal systems was estimated by monitoring fluorimetrically at 25 °C the rate of spontaneous and surfactant-induced release of entrapped 5(6)-carboxyfluorescein (CF). In order to evaluate the interaction of the cationic DODAB guest with the host POPC membrane, the main phase transition temperatures (T(m)) were determined by electron paramagnetic resonance spectroscopy (EPR). All the results obtained show that the presence of DODAB and DDAB stabilizes the POPC liposomes. The extent of stabilization depends on the concentration and nature of the cationic guest.

Basicity of Pyridine and Some Substituted Pyridines in Ionic Liquids

The equilibrium constants for ion pair formation of some pyridines have been evaluated by spectrophotometric titration with trifluoroacetic acid in different ionic liquids. The basicity order is the same in ionic liquids and in water. The substituent effect on the equilibrium constant has been discussed in terms of the Hammett equation. Pyridine basicity appears to be less sensitive to the substituent effect in ionic liquids than in water.

Liposome-induced exfoliation of graphite to few-layer graphene dispersion with antibacterial activity

Few-layer graphene aqueous dispersions are obtained by exploiting liposomes as effective exfoliating agents for graphite. Raman measurements evidence the presence of non-oxidized double layer graphene as well as amphiphilic phospholipid molecules organized in bilayers in the samples. TEM analyses confirmed that the obtained homogeneous graphene nanosheets are embedded in the liposomal bilayer. The as-prepared graphene aqueous dispersion is stable for days and demonstrates significant antibacterial activity against both Gram-positive (Staphylococcus aureus) and Gram-negative (Escherichia coli) strains, with a reduction in the growth of S. aureus and E. coli as high as 60 and 78%, respectively.

Kinetics of demetallation of a zinc–salophen complex into liposomes

A Zn-salophen complex has been incorporated into POPC large unilamellar liposomes (LUV) obtained in phosphate buffer at pH 7.4. Fluorescence optical microscopy and anisotropy measurements show that the complex is located at the liposomal surface, close to the polar headgroups. The interaction of the POPC phosphate group with Zn(2+) slowly leads to demetallation of the complex. The process follows first order kinetics and rate constants have been measured fluorimetrically in pure water and in buffered aqueous solution. The coordination of the phosphate group of monomeric POPC with salophen zinc also occurs in chloroform as detected by ESI-MS measurements. The effect of the Zn-salophen complex on the stability of POPC LUV has been evaluated at 25°C by measuring the rate of release of entrapped 5(6)-carboxyfluorescein (CF) in the presence and in the absence of Triton X-100 as the perturbing agent. It turns out that the inclusion of the complex significantly increases the stability of POPC LUV.

Synthesis and aggregation behaviour of a new sultaine surfactant

The aggregation properties of a new sultaine surfactant have been studied in buffered aqueous solution at pH 7.4 under controlled condition of osmolarity. Spontaneously formed sultaine vesicles with a mean diameter of about 1 μm can be observed by optical microscopy. The phase behaviour of the surfactant has been investigated by differential scanning calorimetry (DSC) and Nile Red fluorescence. Two critical vesicular concentrations (CVC(1) and CVC(2)) have been fluorimetrically measured, by using pyrene and Nile Red as the fluorescent probes. The two populations of vesicles behave differently as a consequence of their size. The stability of extruded large unilamellar vesicles (LUV) formed slightly above the CVC(1) has been evaluated in the temperature range 25-75°C by following the rate of spontaneous release of entrapped 5(6)-carboxyfluorescein (CF). The stability of the same vesicles at 70°C has also been investigated under osmotic stress obtained by adding NaCl or sucrose to the bulk solution. At a sultaine concentration above the CVC(2) LUV tend to associate and form stable larger closely packed aggregates as suggested by Dynamic Laser Light Scattering and rheological measurements.

Fine-tuning of POPC liposomal leakage by the use of β-cyclodextrin and several hydrophobic guests

The effect of entrapped β-cyclodextrin (β-CD) on the stability of multilamellar vesicles (MLVs) of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), prepared by the dehydration-rehydration method, was studied by monitoring the release of 5(6)-carboxyfluorescein encapsulated into the liposomes. Different hydrophobic guests, such as Fullerene C60, have been incorporated into the POPC bilayer in order to modify the membrane composition. The kinetic results as well as ESI-MS measurements evidenced that the destabilizing activity of β-CD is due to the formation of β-CD inclusion complexes and the consequent removal of selected bilayer constituents from the liposomal membrane. Hence, when β-CD was added to the liposomes in the form of a strong, water-soluble 2:1 β-CD/C60 inclusion complex, such a destabilizing effect was not observed. However, the same β-CD/C60 inclusion complex does not form as a result of C60 extraction from the bilayer. This may be attributed either to the overwhelming concentration of POPC with respect to C60 and/or to the fact that C60 is largely aggregated in the bilayer. Turbidimetric and fluorimetric determinations of lamellarity and entrapped volume of the studied MLVs provided further evidence of the alteration of the liposomal bilayer as a consequence of the addition of β-CD and/or the presence of the studied guests.

A rational approach to predict and modulate stereolability of chiral α substituted ketones

An effective strategy to assess and modulate the stereolability of chiral alpha substituted ketones (C alpha SKs) is presented. The tendency of C alpha SKs to retain or change their configuration in water is analyzed as a function of thermodynamic proton-release attitude of alpha asymmetric atoms inside the structures by linear Brønsted correlations. A molecular modeling procedure was developed to analyze and suggest chemical modifications of C alpha SKs in view to obtain the desired grade of stereochemical stability. The approach was employed to predict the tendency to enantiomerize in water of two ketones (1 and 2) endowed with inhibitory activity against monoamine oxidases (MAOs) and the results were confirmed by experimental kinetics measurements performed in organic medium. As a demonstration of practical potentialities of the approach, four new structures, conceived as simple chemical modifications of 1 and 2, were designed to improve/reduce the stereostability grade of the starting anti-MAO ketones. The possibility to extend easily the procedure to other classes of C-H acids appears of interest.

Optimizing the Interactions of Surfactants with Graphitic Surfaces and Clathrate Hydrates.

Surfactants are amphiphilic molecules active at the surface/interface and able to self-assemble. Because of these properties, surfactants have been extensively used as detergents, emulsifiers, foaming agents, and wetting agents. New perspectives have been opened by the exploitation of surfactants for their capacity to interact as well with simple molecules or surfaces. This feature article gives an overview of significant contributions in the panorama of the current research on surfactants, partly accomplished as well by our research group. We look at several recent applications (e.g., adsorption to graphitic surfaces and interactions with hydrate crystals) with the eye of physical organic chemists. We demonstrate that, from the detailed investigation of the forces involved in the interactions with hydrophobic surfaces, it is possible to optimize the design of the surfactant that is able to form a stable and unbundled carbon nanotube dispersion as well as the best exfoliating agent for graphitic surfaces. By studying the effect of different surfactants on the capacity to favor or disfavor the formation of a gas hydrate, it is possible to highlight the main features that a surfactant should possess in order to be devoted to that specific application.