Search

Aggregation of β-amyloid peptides into senile plaques has been identified as one of the hallmarks of Alzheimer’s disease. An attractive therapeutic strategy for Alzheimer’s disease is the inhibition of the soluble β-amyloid aggregation using synthetic β-sheet breaker peptides that are capable of bindingAβ but are unable tobecome part of aβ-sheet structure. As the early stages of theAβ aggregationprocess are supposed tooccur close to theneuronalmembrane, it is strategic todefine theβ-sheet breaker peptide positioning with respect to lipid bilayers. In thiswork,we have focused on the interaction between the β-sheet breaker peptide acetyl-LPFFD-amide, iAβ5p, and lipid membranes, studied by ESR spectroscopy, using either peptides alternatively labeled at the C- and at the N-terminus or phospholipids spin-labeled in different positions of the acyl chain. Our results show that iAβ5p interacts directly with membranes formed by the zwitterionic phospholipid dioleoyl phosphatidylcholine and this interaction ismodulated by inclusion of cholesterol in the lipid bilayer formulation, in terms of both peptide partition coefficient and the solubilization site. In particular, cholesterol decreases the peptide partition coefficient between themembrane and the aqueous medium. Moreover, in the absence of cholesterol, iAβ5p is located between the outer part of the hydrophobic core and the external hydrophilic layer of the membrane, while in the presence of cholesterol it penetrates more deeply into the lipid bilayer.

It is well known that the system monoolein (MO) - sodium monooleate (NaO) – water shows the spontaneous formation of unilamellar vesicles in the dilute corner of its phase diagram. These vesicles, with dimensions ranging between 1000 and 8000 Å are nowadays widely investigated because of their potential use as drug carrier. On the other hand, chitosan, a polysaccharide obtained by the alkaline hydrolysis of chitin, can be chemically modified by grafting on it a large variety of molecules with hydrophobic or hydrophilic properties. In the former case it is possible to tune its hydrophobic properties, inserting aliphatic chains of different length and operating on their crowding on the chitosan backbone. The contemporary presence in acidic water of these vesicles and C12-chitosan, obtained by grafting a C12 aliphatic chain, gives rise to the formation of a gel. We think that the aliphatic chains inserted on the chitosan backbone could penetrate into the vesicular double layer of the monoolein (MO) - sodium monooleate (NaO) – water vesicles, with the formation of a system with very different characteristics with respect to those shown by the aqueous vesicles and the C12- chitosan solution. The structural characterization has been performed by means of EPR and SANS measurements allowing obtaining microscopic information about the vesicular aggregates.

Formation of aggregates composed by sodium cholate (NaC), sodium deoxycholate (NaDC) and sodium glycocholate (NaGC), three constituents of bile fluid, either in pure or mixed combination in aqueous solution has been studied by means of intradiffusion, mutual diffusion, small angle neutron scattering and surface tensiometry techniques. The collected data have shown the formation of pure and mixed micelles with characteristics depending on the molecules involved and the relative amount, despite of their similar structure. Diffusion and scattering data has allowed highlighting the presence of ellipsoidal micelles with a relatively small aggregation number (~5÷10) at moderate concentrations c ≤10 mmol dm−3 and a scarce presence of counterions into the Stern layer. When the concentration of the solutes is increased, a micellar growth is observed. Such growth, according to scattering data, occurs along one dimension, leading to very elongated structures. Concerning the mixed systems, in particular NaC–NaDC–water system, experimental data have shown that the presence of deoxycholate promotes the micellization process and when the concentration of both the solutes is increased keeping fixed their ratio, mixed aggregates enrich in cholate molecules.

This paper is a review of the research on diffusion done at the Chemistry Department of Naples University during the last decades. It is proceded by an introduction on the history of Gouy and Rayleigh interferometric techniques for measuring liquid diffusion coefficients, based on a lecture given by Dr. Donald Miller at the XX Meeting of the Italian Association of Physical Chemistry, Firenze (1999)

The effect of poly(acrylic acid) (PAA) and/or of its sodium salt (PANa) on the physico-chemical properties of both poly(vinyl alcohol) (PVA) aqueous solutions and on the corresponding hydrogels formed through the freeze/thaw method has been investigated. These gels could be suitable for delivery of small molecules in transdermal applications. Mutual diffusion, conductivity and density measurements have been used to characterize the sol phase, while small angle neutron scattering has been employed for sol and gel systems. Investigations performed have revealed that PAA/PVA polymers are well soluble in water, at least up to 20% wt of the total amount of solutes. Gel formation through freeze/thaw method has been observed for PAA/PVA systems only if the concentration of the former polymer is smaller than 4% wt. When the PAA amount is increased, a two phase system (a PVA-rich gel phase and a PAA-rich aqueous phase) is formed. Spontaneous gel formation has been observed at room temperature when the total concentration of PAA and PANa is ~2.5% wt and their ratio is 1.5. The study of diffusivity and conductivity of ternary liquid systems allows to have an insight of the driving force implied in the gelation process. SANS reveals that gels can be described, on a microscopic scale length, in terms of a network composed by the presence of small crystallites, highly connected by swollen amorphous tie chains. In conclusion, the presence of different domains in the gel phase as highlighted by SANS measurements, suggests to study these systems even in the presence of suitable hydrophobic domains, to have a multi-domain system useful for the releasing of both hydrophilic and hydrophobic molecules, with particular regard for transdermal applications.

A rewiew (67 references) on the theoretical and experimental investigation of the properties of aqueous mixtures containing at least one ethoxylated surfactant with short hydrophobic tail is presented. For short hydrophobic tail, hydrocarbon chains with less than 12 carbon atoms are intended

publisher marcel Dekker Inc. New York

The ternary system water pentaethyleneglycol monooctyl ether (C8E5)-octyltrimethyl ammonium bromide (C(8)TAB) has been studied at 25degreesC. Accurate c.m.c. values have been determined through spectro fluorimetric measurements, by using DL-tryptophan as a molecular probe. The surfactant intradiffusion coefficients have been measured through the PGSE-NMR technique. Experimental data show the formation of mixed micelles. The compositions of the aqueous and micellar pseudo-phase have been computed over the whole micellar composition range; they indicate that C8E5 has a larger tendency to form aggregates than C(8)TAB. The aggregation number and the Gibbs energy of mixed micelle formation are calculated and interpreted in terms of interactions among the surfactants in the micellar aggregates. The experimental results have been compared with those predicted by the regular solution model finding reasonable agreement.

Aq. solns. of 3 surfactants (Na 1-hexylsulfonate, polyoxyethylene-5 hexyl ether, 1-hexaminium-N,N,N-triethyl bromide) were characterized through viscosity, d. and calorimetry expts. at 25°. The interaction of these surfactants with α-Cyclodextrin was also studied calorimetrically. The enthalpy of inclusion and the inclusion const. were measured.

Diffusion, d., and viscosity data are presented on the binary system monobutylurea-water at 25°C as a function of concn. and on the ternary system (L)-phenylalanine-monobutylurea-water at 25°C at three main concns. The results are briefly discussed.

The mutual diffusion coefficients, Dij, of the ternary system sodium cholate (NaC, component 1) + sodium deoxycholate (NadC, component 2) + water have been determined at five average compositions (c1, c2) keeping c1 constant and varying c2. The possibility to obtain expressions for the four diffusion coefficients in term of the micellization parameters and of the diffusivity of the species in solution is discussed. Data have been qualitatively interpreted. Results for the main term diffusion coefficients have been used in the much more difficult analysis of the cross term diffusion coefficients.

The present work deals with an interpretation of the differences in micellar structure of two surfactants, sodium decyl sulfonate and sodium decyl sulfate, whose only difference in molecular structure is the presence, for sulfate, of an oxygen atom bridging alkyl chains from a common SO 3 - charged head group. Using a simple geometrical approach, a different exposure of the surfactant head groups on the micellar surface is proposed. It is also shown that the different interphase microstructure could be the reason that decyl sulfate forms ellipsoidal aggregates, in contrast to decyl sulfonate, which forms nearly spherical micelles.

Differential scanning calorimetry (DSC), dynamic light scattering (DLS), and turbidity measurements have been used to study the effect of sodium decylsulfate, C10OS, on 1,2-dipalmitoylphosphatidylcholine, DPPC, vesicles. DSC measurements, performed soon after addition of C10OS solutions to DPPC vesicular dispersion, at several C10OS/DPPC molar ratios, showed the contemporary presence of maxima ascribable both to transitions of an unmodified DPPC membrane and to transitions related to membrane domains in which C10OS is also present. In all the considered samples only unilamellar vesicles are present as shown by DLS measurements carried out on aqueous DPPC and C10OS-DPPC systems. Also the aging of the samples confirmed the presence of SUV in the time scale of DSC measurements. On the basis of DSC results the pseudo-phase diagram of the system was drawn. This diagram does not represent a system in equilibrium, from a thermodynamic point of view, but only kinetically probable states. In fact the prolonged aging of the solutions used both for DSC and DLS scans highlight the disappearance of the unmodified DPPC membrane and its conversion into a mixed membrane. Turbidity measurements at 25 degrees C, performed as a function of surfactant composition, showed that C10OS initially contributes to increase the vesicles dimensions and allowed to calculate the saturation ratio C10OS/DPPC in the membrane. In contrast, at 45 degrees C the presence of surfactant gives origin immediately to smaller structures in comparison to vesicles, probably gel-state micelles.

Diffusion coefficients for the binary system n-decyl dimethyl phosphine oxide + water have been studied at 25 °C, with the aim to quantitatively describe the surfactant micellization process in aqueous medium. Accurate mutual diffusion coefficients have been measured by the Taylor dispersion technique. Intradiffusion coefficients of both components have been measured by the pulsed gradient spin echo (PGSE)-FT NMR technique. The data have been compared and interpreted in the framework of the equilibrium model of surfactant self-aggregation. Interpolation of the mutual diffusion coefficients has allowed us to estimate reliable values of the average number of surfactant molecules involved in the formation of a micellar aggregate and the constant of the equilibrium between these aggregates and surfactant unimers. Interpolation of water intradiffusion coefficients has allowed us to estimate the number of water molecules involved in the hydration of surfactants in unimeric and micellized form.

The molar conductivities of NaCl in several poly(ethylene glycol)(PEG)–water “mixed solvents” are presented as a function of the PEG content in solution. Three different PEG samples, a monodispersed one and two polydispersed ones, have been used. The comparison between the molar conductivity values in the mixed solvents and the corresponding ones in pure water shows that the only effect of PEG on the ions’ motions is an obstruction effect despite the variation of the macroscopic dielectric constant induced by PEG itself. A comparison between the molar conductivities and the corresponding mutual main diffusion coefficients shows a tight correlation between the two quantities. This correlation is possible only in the absence of electrostatic effects due to the presence of PEG.

Diffusion coefficients for the binary system glycerol + water have been studied at 25 °C. Accurate mutual diffusion coefficients have been measured by the Taylor dispersion and Gouy interferometric techniques. Intradiffusion coefficients of both components have been measured by the pulsed gradient spin-echo- Fourier transform NMR technique. The collected diffusion coefficients have been combined with data of water excess chemical potential present in the literature to calculate the velocity correlation coefficients. The results have been interpreted in terms of molecular interactions.

Measurements of osmotic coefficients, mutual diffusion coefficients, and conductivity were performed on the binary system sodium n-octyl sulfonate (C8SO3Na)-water at 25degreesC both below and above the micellar composition range. The osmotic coefficient data were obtained through vapor-pressure osmometry, while the Taylor dispersion method was used to measure diffusion coefficients. The mass equilibrium model was applied to this self-aggregating system, taking into account the deviation of the activity coefficients from the Debye-Huckel limiting law by using the Guggenheim corrective terms for mixed electrolyte solutions. The expressions derived from the model fit the experimental osmotic and diffusion coefficient data well, when the same values of aggregation number, fraction of condensed counterions, and equilibrium constant are used. Osmotic coefficients were also used to determine the thermodynamic factor required to compute the solute mobility from diffusion data. Conductivity data were used to test two theoretical models, namely, the Onsager-Fuoss and the Mean Spherical Approximation theories. Both models have been found to yield unsatisfactory fits to our experimental data and some arbitrary terms had to be applied to the theoretical expressions to obtain good agreement between experiment and theory.

Nanostructured hydrogels composed by biocompatible molecules are formulated and characterized. They are based on a polymer network formed by hydrophobically modified chitosans (HMCHIT or CnCHIT) in which vesicles of monoolein (MO) and oleic acid or sodium oleate (NaO), depending on pH , are embedded. The best conditions for gel formation, in terms of pH , length of the hydrophobic moieties of chitosan, and weight proportion among the three components were estimated by visual inspection of a large number of samples. Among all possible combinations, the system C12CHIT–MO–NaO in the weight proportion (1:1:1) is optimal for the formation of a well-structured gel-like system, which is also confirmed by rheological experiments. Electron paramagnetic resonance (EPR) measurements unambiguously show the presence of lipid bilayers in this mixture, indicating that MO–NaO vesicles are stabilized by C12CHIT even at acid pH . A wide small angle neutron scattering investigation performed on several ternary systems of general formula CnCHIT–MO–NaO shows that the length of the hydrophobic tail Cn is a crucial parameter in stabilizing the polymer network in which lipid vesicles are embedded. Structural parameters for the vesicles are determined by using a multilamellar model that admits the possibility of displacement of the center of each shell. The number of shells tends to be reduced by increasing the polymer content. The thickness and the distance between consecutive lamellae are not influenced by either the polymer or MO–NaO concentration. The hydrogel presented in this work, being fully biocompatible and nanostructured, is well-suited for possible application in drug delivery.

Mixed micelle formation by aqueous mixtures of pentaethylene glycol monohexyl ether (C6E5) and diethylene glycol monohexyl ether (C6E2) has been investigated by determining the temperature-composition phase diagrams and measuring the intradiffusion coefficients. In binary aqueous solutions, C6E5 shows a high tendency to micellize, while C6E2, because of its too short hydrophilic head, presents a macroscopic phase separation in a water-rich and a surfactant-rich phases. The temperature-composition phase diagrams obtained for the ternary water-C6E5C6E2 mixtures present a micellar region in which both surfactants co-aggregate. The dimensions of this micellar region depend on the relative ratio between the two surfactants molalities. By analysing the micelle intradiffusion coefficient, measured through the PGSE-NMR technique, the hydrodynamic radius of the aggregates has been estimated at 25 °C. It is similar to that of triethylene glycol monohexyl ether (C6E3) in binary aqueous solution. The experimental evidences suggest that in mixed ethoxylated surfactants the micellar behaviour is mainly determined by the mean number of ethoxylic units per surfactant molecule, independently of their distribution.

A novel ruthenium complex, linked to a cholesterol-containing nucleolipid (named ToThyCholRu), stabilized by lipid aggregates for antineoplastic therapy is presented. In order to retard the degradation kinetics typically observed for several ruthenium-based antineoplastic agents, ToThyCholRu is incorporated into a liposome bilayer formed by POPC. The resulting nanoaggregates contain up to 15% in moles of the ruthenium complex, and are shown to be stable for several weeks. The liposomes host the ruthenium–nucleolipid complex with the metal ion surrounded by POPC lipid headgroups and the steroid moiety inserted in the more external acyl chain region. These ruthenium-containing liposomes are more effective in inhibiting the growth of cancer cells than a model NAMI-A-like ruthenium complex, prepared for a direct evaluation of their anti-proliferative activity. These results introduce new perspectives in the design of innovative transition-metal-based supramolecular systems for anticancer drug vectorization.

The micro- and mesoscopic structure of reverse Pluronic 25R4 in aqueous mixtures has been studied by SANS, SAXS and shear rheology. These techniques have been able to give a deep insight into the complex structure of the system phase diagram, that includes an isotropic water-rich liquid phase L(1), and liquid crystalline phases with hexagonal, E, or lamellar order, D. Particular attention has been paid to the isotropic water-rich phase L(1), which has a large stability region in the temperature-composition phase diagram. This region is crossed by a large "cloudy zone". Below it, namely at low temperature and composition, SANS data show the presence of polymer unimers in a gaussian coil conformation. Above the "cloudy zone", at higher temperature and composition, the L(1) phase is structured as a network of interconnected multimeric micelles. Rheology adds information about the structuring of the L(1) phase showing its incipient hexagonal pre-structuring. This technique is also able to highlight the defective structure of the E phase itself. In the temperature and concentration ranges in which a lamellar phase D is present, SANS and SAXS results are in complete agreement, showing how interlamellar distance is influenced by both polymer composition and temperature according to an "ideal deswelling" or a "not ideal swelling" mechanism. In addition, in the D phase rheology suggests the presence of densely packed vesicles.

Binary mutual diffusion coefficients measured by the Taylor dispersion method in two different laboratories (University of Naples, Federico II, Italy, and University of Coimbra, Portugal) are reported for aqueous solutions of isoniazid at concentrations from (0.000 to 0.100) mol·dm−3 and at two temperatures (298.15 and 310.15) K. The hydrodynamic radii for the isoniazid in aqueous solution are calculated from the experimental results. In addition, the Hartley equation and the experimental diffusion coefficients are used to estimate activity coefficients for aqueous isoniazid at both temperatures.

Lysophosphatidylcholines are widely used as biomembrane mimics. In order to furnish a structural basis for this application, in this work the self-aggregation behaviour of n-acyl-lysophosphatidylcholines (C(n)lysoPC, n=6,8,10,12), in aqueous solution has been investigated by the PGSTE-NMR and spin probing EPR techniques at 25 degrees C. The experimental data show that C(n)lysoPCs behave as zwitterionic surfactants, and permit evaluation of the influence of the acyl chain length on the phospholipid micellization. For all the C(n)lysoPCs considered, the phospholipid intradiffusion coefficient trend shows a slope change corresponding to the critical micelle concentration (CMC). In the micellar composition range, solubilized tetramethylsilane (TMS) molecules were used to determine the micelle intradiffusion coefficient, from which the aggregate radii and the aggregation numbers were obtained. The solvent intradiffusion coefficient in the C(n)lysoPC aqueous mixtures has been also measured. The results show that the C(n)lysoPC micelles present a thick external layer constituted by strongly hydrated glycerophosphocholine groups. The ability of this layer to embed either anionic or cationic guest molecules has been studied by EPR spectroscopy, employing 3-carboxy-PROXYL in its deprotonated form (CP(-)) or TEMPO-choline (TC) as spin probes. In all the considered systems, the nitrogen isotropic hyperfine coupling constant of the spin probe, A(N), decreases and the correlation time, tau(C), increases with increasing phospholipid molality. The results show that C(n)lysoPC micelles can establish a variety of interaction with different guests. In fact, CP(-) anions interact with the C(n)lysoPC choline groups adsorbing on the micelle surface, while TC cations interacting with the C(n)lysoPC phosphate groups are embedded in thick external layer of the micelles.

Nowadays chitosan, CHIT, is becoming one of the most interesting raw materials because of the actual and potential applications in several technological fields due to its chemical versatility, natural abundance and ecological compatibility. By applying a very versatile and quantitative amination reaction, linear aliphatic chains of length variable from five to 12 carbon atoms have been grafted on the chitosan backbone at 10% in mole of the glycosidic units. Newtonian viscosity and fluorescence probe emission have been used to analyze the intra- and inter-aggregation properties of these modified chitosans, C n -CHITs, and to compare them with those of chitosan itself. Viscosity in the dilute concentration range showed that for CHIT and C5-CHIT the behavior is quite similar highlighting that the insertion of this short aliphatic chain does not modify the rigidity and/or the interaction with the solvent. For C6, C8, and C10-CHIT, the increasing length of the pendant promotes a progressively more efficient intra-aggregation of the polymer as shown by the reduction of its hydrodynamic radius. Converging evidences are obtained by the analysis of the fluorescence spectra of pyrene, used as probe, in the aqueous solutions of the grafted polymers, which show how different is the environment in which pyrene is located, if short or long chained polymers are analyzed. In fact, the spectra in CHIT, C5-CHIT, and C6-CHIT are quite similar among them and indicate a scant hydrophobic quality of the aggregates for these short grafted polymers even in concentrated solution. In contrast, the presence of C8-CHIT, C10-CHIT, and C12-CHIT concentrated solutions gives rise to a progressive and evident modification of the pyrene spectra, which resemble those of this fluorescent probe in a hydrocarbon solvent, thus indicating the formation of well-defined hydrophobic pools. However, in the limit of very concentrated solutions, the specificity of the grafted chains of different length is lost.