Evidence of hydration forces between proteins

Proteins are fundamental molecules in biology that are also involved in a wide range of industrial and biotechnological processes. Consequently, many works in the literature have been devoted to the study of protein–protein and protein–surface interactions in aqueous solutions. The results have been usually interpreted within the frame of the classical Derjaguin–Landau–Verwey–Overbeek (DLVO) theory for colloidal systems. However, against the DLVO predictions, striking evidence of repulsive forces between proteins at high salt concentrations has been observed in different works based on the analysis of the second virial coefficient or on the direct measurement of protein interaction with an atomic force microscope. Hydration forces due to the adsorption of hydrated cations onto the negatively charged protein surfaces have been invoked to rationalize this anomalous repulsion. The hydration forces between proteins provide protein-covered particles with a non-DLVO colloidal stability at high salt concentrations, as different studies in the literature has proven. This review summarizes the most relevant results published so far on the presence of hydration forces between proteins and protein-coated colloidal particles.     

Influence of charge density on the swelling of colloidal poly(N-isopropylacrylamide-co-acrylic acid) microgels

The volume phase transition of colloidal poly(N-isopropylacrylamide-co-acrylic acid) microgels depends in a complex way on the effective charge density within the polymer network. A series of monodisperse PNIPAM/AAc microgels with different content of acrylic acid were synthesized by surfactant-free emulsion polymerization employing sonication instead of a conventional stirring technique. Subsequently, the colloids were characterized by dynamic light scattering and electron microscopy. Potentiometric titrations provided the amount of carboxyl groups incorporated into the copolymer. The effective charge density was systematically controlled by the content of acrylic acid monomers, the pH value of the suspension, and the salt concentration. The hydrodynamic dimensions of the microgels have been measured by dynamic light scattering. The swelling/deswelling behavior is determined by the delicate balance between hydrophobic attraction of NIPAM and the repulsive electrostatic interactions of the carboxylate group of the acrylic acid moieties. Compared to their macroscopic counterparts the charged microgel particles show a significantly different swelling/deswelling behavior. This manifests in the occurrence of a two-step volume phase-transition process with increasing acrylic acid content. Hydrogen bonding has to be considered to understand this two step volume phase transition uniquely observed for colloidal microgels. Another interesting phenomenon presented here is the reversible formation of well-defined aggregates at low pH and under high salt conditions.     

Viscometric and light scattering studies on azrechtic acid

Results are reported for viscometric and light scattering studies on azrechtic acid (ARA) in pure aqueous medium, in 0–02 N KCl and in a mixture of 0–02 N KCl, CaCl₂, MgCl₂. The investigations reveal the polyelectrolyte character of the azrechtic acid molecule. The corrected average value of the molecular weight from light scattering measurements is 1–7 × 10⁶. Assuming the polydisperse random coil as a suitable model for ARA molecules, the root-mean-square end-to-end distances have been calculated as 2570 Å in water, 1410 Å in 0–02 N KCl and 1180 Å in a mixture of 0–02 N KCl, CaCl₂, MgCl₂. The contraction of the molecules in the presence of neutral salts has been attributed to partial reduction of electrostatic repulsion due to similarly charged ions.     

Phenomenological surface characterization of cationic-lipid monolayers in the presence of oppositely charged polyions

The thermodynamic properties of monolayers of double chain cationic lipids DOTAP at the air–water interface have been investigated by means of surface pressure and surface potential measurements. We studied the interfacial properties of the film in the liquid-expanded regime during the isothermal compression in the presence of oppositely charged linear polyions (poly(acrylate)sodium salt, [NaPA]) of different molecular weights. The influence of the ionic character of the aqueous subphase on the polyion adsorption has been studied in different environmental conditions, considering different subphase compositions, ranging from a polyion solution at different concentrations to a salty polyion solution, containing different amount of simple added salt [NaCl]. The data are compared to the ones when only NaCl salt is present in the subphase. The results have been analyzed according to an osmotic-type equation of state and the characteristic parameter associated with the water activity has been evaluated as a function the different molecular weight polyion content. The influence of the simple salt in the adsorption process has been discussed in the light of current scaling theories of polyelectrolyte solutions and the critical salt concentrations inducing a polyion desorption in the different experimental conditions investigated have been estimated.     

Oxygen adsorption and photoreduction on fractal melanin particles

The main putative functions of melanins in living cells, that is photoprotection and dark or light-dependent oxido-reductive activity, still requires an interpretation which takes into account the micro–mesoscopic structure of native melanin particles. It is indeed well established that a different chemical composition of melanins, even if derived from different biosynthetic pathways, has only a little influence on the biological and physical properties of the solid aggregates, the common form in which the pigment is found [P.R. Crippa et al., Chemistry of melanins, in: A. Brossi (Ed.) The Alkaloids, vol. 36, Academic Press, New York, 1989, pp. 253–323]. In the present work a model for interfacial electron transfer is proposed describing the process of light induced superoxide formation through a monoelectronic reduction of dioxygen adsorbed on melanin solid surface. This process is presumed to be dependent on the surface fractal characteristics, and its kinetics must be interpreted as a heterogeneous interfacial reaction involving light produced carriers and the adsorbed acceptor, like in colloidal inorganic semiconductors such as TiO₂.     

Electrokinetic behavior and colloidal stability of polystyrene latex coated with ionic surfactants

This work is focused on analyzing the electrokinetic behavior and colloidal stability of latex dispersions having different amounts of adsorbed ionic surfactants. The effects of the surface charge sign and value, and the type of ionic surfactant were examined. The analysis of the electrophoretic mobility (mu(e)) versus the electrolyte concentration up to really high amounts of salt, much higher than in usual studies, supports the colloidal stability results. In addition, useful information to understand the adsorption isotherms was obtained by studying mu(e) versus the amount of the adsorbed surfactant. Aggregation studies were carried out using a low-angle light scattering technique. The critical coagulation concentrations (ccc) of the particles were obtained for different surfactant coverage. For latex particles covered by ionic surfactants, the electrostatic repulsion was, in general, the main contribution to the colloidal stability of the system; however, steric effects played an important role in some cases. For latices with not very high colloidal stability, the adsorption of ionic surfactants always improved the colloidal stability of the dispersion above certain coverage, independently of the sign of both, latex and surfactant charge. This was in agreement with higher mobility values. Several theoretical models have been applied to the electrophoretic mobility data in order to obtain different interfacial properties of the complexes (i.e., zeta potential and density charge of the surface charged layer).     

Colloidal stability and drug incorporation aspects of micellar-like PLA–PEG nanoparticles

The drug delivery properties of a series of poly(lactic acid)–poly(ethylene glycol) (PLA–PEG) micellar-like nanoparticles have been assessed in terms of their colloidal stability and their ability to incorporate a water soluble drug. These studies have focused on a range of PLA–PEG copolymers with a fixed PEG block (5 kDa) and a varying PLA segment (3–110 kDa). In aqueous media, these copolymers formed micellar-like assemblies following precipitation from water miscible solvents. There was a controlled increase in the particle size as the molecular weight of the PLA block was increased. The characteristics of the PEG corona were also highly dependent on the PLA moiety. Copolymers with a low molecular weight PLA block (3–15 kDa) formed highly colloidally stable dispersions, with a complete PEG surface coverage. However, increasing the molecular weight of the PLA block resulted in significantly less colloidally stable nanoparticle dispersions, which flocculated in solvents that were significantly better than θ-solvents for the stabilising PEG chains. This can be attributed to a reduced PEG surface coverage and the probable presence of naked PLA ‘patches’ on the particle surface. These larger PLA–PEG nanoparticles (30:5–110:5) were found to be stabilised in the presence of serum components, which are thought to adsorb into the gaps on the particle surface and prevent flocculation. All of the dispersions were found to be stable under physiological conditions and therefore suitable for in vivo administration. A reasonable loading (3.1% w/w) of the micellar-like PLA–PEG 30:5 nanoparticles with the water soluble drug procaine hydrochloride was achieved. The incorporated drug was found to have no effect on the nanoparticle structure or recovery, which can be attributed to the micellar character of these assemblies and the presence of the stabilising PEG chains.     

Methods for separation of trace amounts of platinum and investigation of the influence of interfering elements during platinum determination in copper ores and copper concentrates by graphite furnace atomic absorption spectrometry

A THGA graphite furnace with Zeeman background correction has been used to determine platinum content in copper ore and copper concentrate at the part per billion (ppb) concentration level. Two different procedures for the separation of trace platinum have been applied: (i) use of an ion exchange resin; and (ii) a two-stage method based on platinum separation on inorganic carriers. The influence of interfering elements in the matrix (Cu, Pb, Fe, Ti, V, Au, Pd, Ir, Rh and Al) has been examined using a graphite furnace. It was found that the presence of Cu (12.5–100 mg l⁻¹), Pb (100–500 mg l⁻¹), Fe (100–2000 mg l⁻¹), Ti (25–100 mg l⁻¹), V (25–100 mg l⁻¹), Au (25–300 mg l⁻¹), Pd (20–250 mg l⁻¹), Ir (0.5–3.5 mg l⁻¹) and Rh (0.025–1 mg l⁻¹) in the samples analyzed has no effect on the platinum absorption signal when using a recommended temperature program (T_pyr=1300°C, T_at=2450°C). Spectral interference was observed, which was due to aluminum, as a result of the close neighborhood of the Pt 265.945-nm and Al 266.039-nm lines. This interference could not be eliminated by the Zeeman background correction.     

Role of anions for the reduction behavior of silver ions in polymer/silver salt complex membranes

The reduction of silver ions to silver nanoparticles is an essential issue in polymer/silver salt complex membranes for facilitated olefin transport, because it has a critical influence on the long-term stability of membrane performance. In this study, the role of anions for the formation of silver nanoparticles in polymer/silver salt complexes was investigated. This role was assessed for the complexes of poly(N-vinyl pyrrolidone) (PVP) with three silver salts including AgBF₄, AgCF₃SO₃, and AgNO₃. Especially, UV irradiation to the membranes was used to clearly investigate the reduction behavior of silver ions. Separation performance test, UV–vis spectroscopy and transmission electron microscopy (TEM) clearly show that the reduction rate of silver ions strongly depends on the counteranions of salt, and has the following order: AgBF₄ > AgCF₃SO₃ > AgNO₃. This behavior of the formation of silver nanoparticles in polymer/silver salt complex membranes is explained in terms of the interaction strength of silver ions with the carbonyl oxygens of polymer, and that of silver ions with counteranions. It is concluded that when the former interaction is strong and the latter one is weak, the reduction rate of silver ions to silver nanoparticles is fast, and vice versa. These interactions were characterized using FT-IR, FT-Raman spectroscopy, and theoretical ab initio calculation.     

Charge and salt-driven reentrant order-disorder and gas-solid transitions in charged colloids

Monte Carlo simulations have been performed for aqueous charged colloidal suspensions as a function of effective charge density (sigma) on the particles and salt concentration C(s). We vary the effective charge density in our simulations over a range where a reentrant solid-liquid transition in suspensions of silica and polymer latex particles has been reported by Yamanaka et al. (Phys. Rev. Lett. 80 (1998) 5806). We show that at low ionic strengths a homogeneous liquid-like ordered suspension undergoes crystallization upon increasing sigma. Further increase in sigma resulted once again in a disordered state, which is in agreement with experimental observations. In addition to this reentrant order-disorder transition, we observe an inhomogeneous-to-homogeneous transition in our simulations when salt is added to the disordered inhomogeneous state. This inhomogeneous-to-homogeneous disordered transition is analogous to the solid-gas transition of atomic systems and has not yet been observed in charged colloids. The reported experimental observations on charged colloidal suspensions are discussed in the light of present simulation results.     

Coagulation kinetics of amorphous colloidal silica suspensions with and without hydroxypropyl cellulose polymer

The coagulation rate constant of submicron silica has been measured as a function of solution pH, salt concentration and hydroxypropyl cellulose (HPC) polymer concentration. Results show that the colloidal stability of silica is dominated by the cation concentration in the presence of salt in the pH range 3–9.5. The stability increases as cation concentration decreases. At low salt concentration and a minimum colloid stability was found in the intermediate pH range 4–8. These results show that differences in the literature values of the critical coagulation constant by relative light-scattering experiments can be explained by the use of the coagulation rate constant analysis. When HPC polymer was present in the solution, the colloid stability of the silica increased. The adsorption of polymer stabilizes the silica suspensions, both at low pH near the isoelectric point and at high ionic strength where it coagulates without the polymer. A monolayer coverage was necessary to provide steric stabilization. At 10^–3 M KCl a smaller equilibrium concentration of HPC in solution is needed to give monolayer coverage and steric stabilization than at 1 M KCl and pH 4.2.     

Photochemical formation of silver-gold (Ag?Au) composite colloids in solutions containing sodium alginate

Photochemical formation of colloidal silver, colloidal gold and silver-gold (Ag-Au) composite colloids under mild conditions has been studied. Irradiation of either aqueous AgCIO₄ or HAuCI₄ solution in the presence of sodium alginate (SA) with 253.7 nm light yielded colloidal silver or gold, whose particle diamter was 10-30 nm or 40-60 nm, respectively. The Ag-Au composite colloids consisting of mixtures of silver and gold domains (particle diameter 30-150 nm) have been prepared and their extinction spectra have been examined on the basis of a conventional Mie theory in combination with an effective medium theory to estimate the optical constants of these colloids. It has been shown that the extinction spectra of the Ag-Au composite colloids are completely different from those of Ag-Au alloy colloids, in that the former have two extinction maxima close to the colloidal extinction bands of pure silver and gold, in contrast to a single extinction maximum of the latter. The importance of natural, high-molecular carboxylic acids such as alginic acid in the photochemical formation of metal colloids and thin films has been stressed.     

Spectral studies on the interaction of Amido black 10B with DNA in the presence of cetyltrimethylammonium bromide

The interaction of Amido black 10B (AB) with DNA in basic medium was studied in the presence of cetyltrimethylammonium bromide (CTMAB) based on the measurements of resonance light scattering (RLS), UV–vis, CD spectra, and RLS imaging. The interaction has been proved to give a ternary complex of CTMAB–DNA–AB in Britton–Robinson buffer of pH 11.55, which exhibits strong negative Cotton effect at 233.3 nm and 642.8 nm, and strong RLS signals characterized at 469 nm. Experiments showed that the enhanced RLS intensities (ΔI_RLS) against the mixture of AB and CTMAB are proportional to the concentration of fish sperm DNA (fsDNA) and calf thymus DNA (ctDNA), respectively over the range of 0.03–1.0 and 0.05–1.5 μg ml⁻¹, with the limits of determination (3σ) of 7.3 ng ml⁻¹ for fsDNA and 7.0 ng ml⁻¹ for ctDNA.     

Aromatic guest inclusion by a tripodal ligand: Fluorescence and structural studies

The newly synthesized simple tripodal ligand tris-[2-(naphthalen-2-yloxy)-ethyl]-amine (L₁) act as a fluorescence signaling system for aromatic guest. It forms inclusion complexes with several electron deficient aromatic compounds. This inclusion phenomenon has been studied by steady-state fluorescence spectroscopy and solid-state structural analysis. Electron-rich L₁ shows dramatic color change and a concomitant quenching of luminescence in solution as well as solid phase when titrated with several other electron deficient aromatic guest molecules. Rather high selectivity towards the picric acid was observed. L₁ simultaneously forms inclusion complex and organic salt co-crystal with the composition [(L₁H⁺) (Pic⁻)]  PicH (PicH = picric acid) when crystallized in the presence of picric acid. In the solid state, it forms a strong π–π, C–Hπ and C–HO type interactions.     

Vapour-liquid equilibrium of the acetone---methanol system in the presence of KSCN and comparison with other salts

The effect of KSCN at salt mole fractions of 0.01–0.05 and at saturation on the VLE of the binary acetone-methanol system has been experimentally investigated at 101.32 kPa using a modified Othmer equilibrium still. The experimental data were correlated by the electrolytic NRTL model [B. Mock, L.B. Evans and C.C. Chen, AIChE J., 32 (1986) 1655–1664], the extended UNIQUAC equation [B. Sander, A. Fredensland and P. Rasmussen, Chem. Eng. Sci., 41 (1986) 1171–1183] and the modified Wilson and NRTL models proposed by Tan [T.C. Tan, AIChE J., 31 (1985) 2083–2085; Chem. Eng. Res. Des., 65 (1987) 355–366; Trans. Inst. Chem. Eng., 68 (1990) 93–102]. The results were compared with those obtained by Tan's predictive models. A crossover between the salting-out and salting-in effects on acetone and a non-azeotropic minimum in the temperature-composition diagram were observed at low and high salt concentrations. Based on the solubility data in the mixture, comparisons with the effect of other salts on the same mixed solvent were made.     

Nano-structured proton exchange membranes molded by polymerizing bi-continuous microemulsion

A unique type of nano-structured proton exchange membrane (PEM) has been fabricated through photo-polymerizing a bicontinuous microemulsion. This microemulsion is constituted by a polymerizable zwitterionic surfimer 3-((11-acryloyloxyundecyl)imidazoyl)propyl sulfonate (AIPS), 3-sulfopropylmethacrylate, potassium salt (SPM), acrylonitrile and water. As the resulting colloidal system maintains quasi-homogeneous state throughout the polymerization course, the inherent bicontinuous microemulsion structure was successfully transferred to the matrix of the polymer membrane. Such membranes are therefore composed of dual phase channels with ca. 1.5–2 nm of the hydrophilic channel breadth. This particular structural trait imparts to the membranes excellent proton conductivities of up to 10⁻¹ S cm⁻¹ as well as low methanol permeability. The DMFC single cell loaded with the demo PEM manifests ca. 20 mW cm⁻² of power output. The attributes of this PEM are elucidated from the bicontinuous structure of microemulsion.     

Molecular mass dependence of adsorbed amount and hydrodynamic thickness of polyelectrolyte layers

Highly charged polyelectrolytes adsorbed on oppositely charged colloidal particles are investigated by electrophoresis and dynamic light scattering. The dependence of the adsorbed amount and of the hydrodynamic layer thickness on the molecular mass and the salt level is analyzed. The adsorbed amount increases with increasing salt level and decreases with increasing molecular mass. The hydrodynamic layer thickness is independent of the molecular mass at low salt levels, but increases with the molecular mass as a power law with an exponent 0.10 ± 0.01 at high salt. The same behavior was observed for different polyelectrolytes and substrates and therefore is suspected to be generic. Due to semi-quantitative agreement with computer simulations carried out by Kong and Muthukumar in 1998, the observed behavior is interpreted with conformational changes of single adsorbed polyelectrolyte chains.     

Os(VIII)-catalysed oxidation of sulfides by sodium salt of N-chlorobenzenesulfonamide

Catalytic activity of Os(VIII) in the oxidation of some twenty organic sulfides with sodium salt of N-chlorobenzenesulfonamide (CAB) has been investigated in alkaline (pH8.7) t-butanol–water (1:1 v/v) medium. Significant retarding influence of [OH⁻] on the reactivity is exhibited. The catalysed reaction is strongly accelerated in the presence of Hg(II). Imperfections are observed in the linear Hammett relationship in the case of –NO₂ substituents.     

Influence of polyelectrolyte and salt on the structure of dispersions containing charged colloidal particles

An integral equation theory has been used as the basis for studying the structure of dispersions containing charged colloidal particles: globular protein molecules with a nonzero dipole moment, a polyelectrolyte and a low-molecular salt. It is demonstrated that there is an effective attraction between charged colloidal particles, which increases in the presence of charged polymer chains. The influence of the length of polyelectrolyte chains and of salt concentration on the partial structure factor of colloidal particles was studied.     

Colloidal behavior of carrier-free125mTe in aqueous solutions

NaCl solutions containing carrier-free^125mTe(IV) and^125mTe(VI) have been prepared. The effect of the presence of various substances on the colloidal behavior of^125mTe in solution has been studied by ultracentrifugation. It has been found that phosphate, sulfide and oxalate ions which complex with various elements decrease the colloidal formation of^125mTe. In order to estimate the size of colloidal particles, the NaCl solutions containing carrier-free^125mTe have been microfiltered through membrane filters with various pore size.