Light scattering studies of the solution properties of chitosans of varying degrees of acetylation

The use of two techniques, differential interferometry and quasi-elastic light scattering (QELS), allowed us to study solutions of chitosan varying in degree of acetylation (DA), degree of dissociation (alpha), and concentration (C(p)). With the first technique, we demonstrated the modification of the electric polarizability of the polymer chains, through a law of behavior of the variation of the refractive index increment dn/dC with DA and alpha. This brought us information on the various kinds of interactions (H-bonds, electrostatic, and hydrophobic) involved in the evolution of the solution properties. QELS experiments performed in dilute regime showed the presence of supramolecular structures depending on DA and alpha. The topology and the nature of these objects are discussed. The typical presence of aggregates and their evolution with concentration was also demonstrated in semidilute regime.     

Synthesis and characterization of ion‐imprinted resin for selective removal of UO2(II) ions from aqueous medium

In this work, uranyl ion‐imprinted resin based on 2‐(((4‐hydroxyphenyl)amino)methyl)phenol was synthesized by condensation polymerization of its uranyl complex in presence of resorcinol and formaldehyde cross‐linkers. Numerous instrumental techniques including elemental analysis, Fourier transform infrared spectroscopy, ultraviolet, ¹H along with ¹³C nuclear magnetic resonance spectroscopy have been employed for complete characterization of the synthesized ligand and its uranyl complex. Additionally, the obtained ion‐imprinted and non‐imprinted resins were investigated using scanning electron microscope and Fourier transform infrared spectroscopy. The effects of various essential parameters such as pH, temperature and contact time on removal of uranyl ions have been examined, and the results indicated that the obtained resin exhibited the optimum activity at pH 5. Furthermore, the adsorption process was spontaneous at all studied temperatures and followed the second‐order kinetics model. Also, Langmuir adsorption isotherm exhibited the best fit with the experimental results with maximum adsorption capacity 139.3 mg/g. Moreover, the selectivity studies revealed that the ion‐imprinted resin exhibited an obvious affinity toward the uranyl ions in presence of other metal ions compared with the non‐imprinted resin. Copyright © 2015 John Wiley & Sons, Ltd.     

Synthesis and characterization of sugar-bearing chitosan derivatives: aqueous solubility and biodegradability

The extended use of chitosan in biomedical fields has been limited by its insoluble nature in a biological solution. To endow the water solubility in a broad range of pH, chitosan derivatives were prepared by the covalent attachment of a hydrophilic sugar moiety, gluconic acid, through the formation of an amide bond. These sugar-bearing chitosans (SBCs) were further modified by the N-acetylation in an alcoholic aqueous solution. Thereafter, the effect of the gluconyl group and the degree of N-acetylation (DA) on the water solubility at different pHs and on the biodegradability of chitosan were investigated. The SBCs showed the water solubility in a broader range of pH than chitosan, whereas they were still insoluble at neutral and alkali pH. The N-acetylation of SBCs significantly affected the water solubility, for example, the SBCs with the DA, ranging from 29% to 63%, were soluble in the whole range of pH. This might result from the improved hydrophilicity by the gluconyl group, accompanied by the role of the N-acetyl group that disturbed the hydrogen bonding between amino groups of chitosan. From the biodegradation tests, determined by the decrease in the viscosity of a polymer solution exposed to lysozyme, it was evident that the gluconyl group attached to chitosan improved the biodegradability. Thus, it was possible to control the biodegradability of chitosan by adjusting the amounts of gluconyl and N-acetyl groups in the chitosan backbone. The N-acetylated SBCs, soluble in the broad range of pH, might be useful for various biomedical applications.     

Magnetic chitosan composite particles: Evaluation of thorium and uranyl ion adsorption from aqueous solutions

Magnetic chitosan composite particles with 40 μm average size and 24 emu/g saturation magnetization obtained by an in situ procedure were evaluated as a new low-cost adsorbent for radioactive wastewater decontamination. Sorbent characterization by SEM, EDX, FTIR and magnetization measurements proved that the target ions were bound and their surface distribution was uniform. The 18 emu/g magnetization of the metal loaded particles was high enough to ensure their easy magnetic field separation and recovery. The parameters influencing the sorption process were optimized with respect to sorbent mass, target ion concentration and contact time. The material under study had superior adsorption capacity both for uranyl (666.67 mg/g) and thorium (312.50 mg/g) ions when compared to other low-cost adsorbents reported in literature. The adsorption process is spontaneous and endothermic. The material may be regenerated and re-used.     

About mechanism of chitosan cross-linking with glutaraldehyde

The regularities of the reaction of aminopolysaccharide chitosan with glutaraldehyde (GA) have been considered. The equilibrium forms of GA in water have been thoroughly studied by NMR spectroscopy. It has been established that at pH 5.6, the exchange of the protons of O=CHCH₂ groups for deuterium occurs, indicating the presence of an anion, a product of the first stage of the aldol reaction; at pH > 7.2, the formation of the products of an aldol reaction and aldol condensation takes place. The kinetics of the reaction between the amino groups of chitosan and GA, the kinetics of gel formation in chitosan solutions in the presence of GA, and the kinetics of changes in the rigidity of gels formed have been studied by UV spectroscopy. IR spectra of cross-linked chitosan have been obtained. It has been shown that chitosan catalyzes the polymerization of GA to form irregular products; in this process, the length of oligomeric chains in modified or cross-linked chitosan and the concentration of conjugated bonds increase with the GA concentration and pH of the reaction medium.     

Rheometric study of the gelation of chitosan in aqueous solution without cross-linking agent

A new process of formation of chitosan physical hydrogels in aqueous solution, without any organic solvent or cross-linking additive, was studied. The three conditions required for the physical gelation were an initial polymer concentration over C*, a critical value of the balance between hydrophilic and hydrophobic interactions, and a physicochemical perturbation responsible for a bidimensional percolating mechanism. The time necessary to reach the gel point was determined by rheometry, and gelations were compared according to different initial conditions. Thus, we investigated the influence of the polymer concentration and the degree of acetylation (DA) of chitosan on gelation. The number of junctions per unit volume at the gel point varied with the initial polymer concentration, i.e., the initial number of chain entanglements per unit volume or the number of gel precursors. The time to reach the gel point decreased with both higher DAs and concentrations. For a chitosan of DA = 36.7%, a second critical initial concentration close to 1.8% (w/w) was observed. Above this concentration, the decrease of the time to reach the gel point was higher and fewer additional junctions had to be formed to induce gelation. To optimize these physical hydrogels, to be used for cartilage regeneration, their final rheological properties were studied as a function of their degree of acetylation and their polymer concentration. Our results allowed us to define the most appropriate gel for the targeted application corresponding to a final concentration of chitosan in the gel of near 1.5% (w/w) and a DA close to 40%.     

Encapsulating live cells with water-soluble chitosan in physiological conditions

A new class of microcapsules was prepared under physiological conditions by polyelectrolyte complexation between two oppositely-charged, water-soluble polymers. The microcapsules consisted of an inner core of half N-acetylated chitosan and an outer shell of methacrylic acid (MAA) (20.4%)-hydroxyethyl methacrylate (HEMA) (27.4%)-methyl methacrylate (MMA) (52.2%) (MAA-HEMA-MMA) terpolymer. Both 400 and 150 kDa half N-acetylated chitosans maintained good water solubility and supplied enough protonated amino groups to coacervate with terpolymer at pH 7.0-7.4, in contrast to other chitosan-based microcapsules which must be prepared at pH <6.5. The viscosity of half N-acetylated chitosan solutions between 80 and 3000 cPas allowed the formation of microcapsules with spherical shape. Molar mass, pH and concentration of half N-acetylated chitosan, and reaction time, influenced the morphology, thickness and porosity of the microcapsules. Microcapsules formed with high concentration of half N-acetylated chitosan exhibited improved mechanical stability, whereas microcapsules formed with low concentration of half N-acetylated chitosan exhibited good permeability. This 3D microenvironment has been configured to cultivate sensitive anchorage-dependent cells such as hepatocytes to maintain high level of functions.     

Chelating derivatives of chitosan obtained by reaction with ascorbic acid

Ascorbic acid immediately dissolves Euphausia superba chitosan upon mixing and forms chitosan ascorbate; during the 6-h period after dissolution in water at pH 5–7, ascorbate is oxidized to dehydroascorbate which undergoes Schiff reaction with the amino groups of chitosan, thus yielding a viscous solution of a polymeric ketimine. The latter is characterized by infrared spectrometry, circular dichroism spectropolarimetry, viscometry and alkalimetry. When brought into contact with transition metal ions, the chitosan ascorbate ketimine yields insoluble metal chelates. Upon reduction with sodium cyanoborohydride, the water-insoluble N-[2-(1,2-dihydroxyethyl)tetrahydrofuryl] chitosan (NDTC) is obtained, which shows enhanced capacity for uranium, up to 800 mg U/g from solutions at pH 4·5.     

Relation between solution properties and degree of acetylation of chitosan: role of aging

Aging of solutions of chitosan varying in degree of acetylation (DA) and degree of dissociation (alpha) was studied using two techniques. The first concerned potentiometric experiments performed during 3 days on solutions having the same concentration of amino groups (5.2% < DA < 70.6% and 0 < alpha < 1.1). The presence of aggregates at low alpha certainly depends on electrostatic interactions for low DA values and on hydrophobic interactions and H-bondings for high values. When alpha increases, the role of the cationicity of the amine groups, which depends on DA, seems to play a more important role on the behavior of the polymer chains. The second regarded capillary viscometric experiments performed during 5 days on solutions of the same polymer concentration (5.2% < DA < 70.6% and 0 < alpha < 0.30). The observations mentioned above and the results obtained in a previous paper (Biomacromolecules 2001, 2 (3), 765) are confirmed, and the influence of the electroviscous effects is discussed.     

Chitosan Interactions with Metal Ions and Dyes: Dissolved-state vs. Solid-state Application

Summary Chitosan is an amino-polysaccharide with highly efficient properties for the binding of metal ions and anionic dyes. Uptake may occur through chelation on free amino functions (at near-neutral pH) or by electrostatic attraction on protonated amino groups (in acidic solutions). The polymer is soluble in acidic solutions and its binding properties can be used in both solid form (sorption) and liquid form (ultrafiltration coupled with chelation, coagulation–flocculation). These properties have been used for the recovery of mercury from dilute solutions at initial pH 5 (which reveals the most efficient pH in the range pH 4–6) and for the recovery of Reactive Black 5 (RB5, anionic dye) at pH 3. While in the case of mercury binding saturation of the biopolymer is only slightly higher when chitosan is used in the liquid form compared to solid-state adsorption, in the case of the coagulation–flocculation of RB5 (using the liquid-form of chitosan) the saturation of the polymer (calculated on the basis of molar ratio of dye vs. amino groups of the polymer) is reached at a significantly greater value than when the polymer is used for the solid-state binding of the dye. There is a much more efficient use of amino groups when chitosan is used in the liquid-form due to a better availability of amino groups (less hydrogen bonds between the chains of the polymer) and to a better accessibility to internal sorption sites (lower diffusion control).     

Highly enhanced adsorption for decontamination of lead ions from battery wastewaters using chitosan functionalized with xanthate

Decontamination of lead ions from aqueous media has been investigated using cross linked xanthated chitosan (CMC) as an adsorbent. Various physico-chemical parameters such as contact time, amount of adsorbent, concentration of adsorbate were optimized to simulate the best conditions which can be used to decontaminate lead from aqueous media using CMC as an adsorbent. The atomic absorption spectrometric technique was used to determine the distribution of lead. Maximum adsorption was observed at both pH 4 and 5. The adsorption data followed both Freundlich and Langmuir isotherms. Langmuir isotherm gave a saturated capacity of 322.6+/-1.2mg/g at pH 4. From the FTIR spectra analysis, it was concluded that xanthate and amino group participate in the adsorption process. The developed procedure was successfully applied for the removal of lead ions from real battery wastewater samples.     

Interaction of U(VI) with Schizophyllum commune studied by microscopic and spectroscopic methods

Biosorption of actinides like uranium by fungal cells can play an important role in the mobilization or immobilization of these elements in nature. Sorption experiments of U(VI) with Schizophyllum commune at different initial uranium concentrations and varying metal speciation showed high uranium sorption capacities in the pH range of 4–7. A combination of high angle annular dark-field and scanning transmission electron microscopy analysis (HAADF-STEM) showed that living mycelium cells accumulate uranium at the cell wall and intracellular. For the first time the fluorescence properties of uranium accumulates were investigated by means of time-resolved laser-induced fluorescence spectroscopy (TRLFS) beside the determination of corresponding structural parameters using X-ray absorption fine structure spectroscopy (EXAFS). While the oxidation state of uranium remained unchanged during sorption, uranium speciation changed significantly. Extra and intracellular phosphate groups are mainly responsible for uranium binding. TRLFS spectra clearly show differences between the emission properties of dissolved species in the initial mineral medium and of uranium species on fungi. The latter were proved to be organic and inorganic uranyl phosphates formed depending on the uranyl initial concentration and in some cases on pH.     

Influence of molecular weight and pH on adsorption of chitosan at the surface of large and giant vesicles

This paper describes the mechanisms of adsorption of chitosan, a positively charged polyelectrolyte, on the DOPC lipid membrane of large and giant unilamellar vesicles (respectively, LUVs and GUVs). We observe that the variation of the zeta potential of LUVs as a function of chitosan concentration is independent on the chitosan molecular weight (Mw). This result is interpreted in terms of electrostatic interactions, which induce a flat adsorption of the chitosan on the surface of the membrane. The role of electrostatic interactions is further studied by observing the variation of the zeta potential as a function of the chitosan concentration for two different charge densities tuned by the pH. Results show a stronger chitosan-membrane affinity at pH 6 (lipids are negatively charged, and 40% chitosan amino groups are protonated) than at pH 3.4 (100% of protonated amino groups but zwitterionic lipids are positively charged) which confirms that adsorption is of electrostatic origin. Then, we investigate the stability of decorated LUVs and GUVs in a large range of pH (6.0 < pH < 12.0) in order to complete a previous study made in acidic conditions [Quemeneur et al. Biomacromolecules 2007, 8, 2512-2519]. A comparative study of the variation of the zeta potential as a function of the pH (2.0 < pH < 12.0) reveals a difference in behavior between naked and chitosan-decorated LUVs. This result is further confirmed by a comparative observation by optical microscopy of naked and chitosan-decorated GUVs in basic conditions (6.0 < pH < 12.0): at pH > 10.0, in the absence of chitosan, the vesicles present complex shapes, contrary to the chitosan-decorated vesicles which remain spherical, confirming thus that chitosan remains adsorbed on vesicles in basic conditions up to pH = 12.0. These results, in addition with our previous data, show that the chitosan-decorated vesicles are stable over a very broad range of pH (2.0 < pH < 12.0), which holds promise for their in vivo applications. Finally, the quantification of the chitosan adsorption on a LUV membrane is performed by zeta potential and fluorescence measurements. The fraction of membrane surface covered by chitosan is estimated to be lower than 40 %, which corresponds to the formation of a flat layer of chitosan on the membrane surface on an electrostatic basis.     

A Stopped-Flow Method for the Determination of Ascorbic Acid in Orange Juice Containing Triose Reductone

The adsorption of uranium by chitin phosphate and chitosan phosphate was investigated to obtain information on uranium recovery from aqueous systems, especially sea water and uranium mine waste water. The adsorption of uranium by chitin phosphate and chitosan phosphate was much greater than copper, cadmium, manganese, zinc, cobalt, nickel, magnesium and calcium. The adsorption of uranium was very rapid during the first 10 min and was affected by pH of the solution, temperature, granule radius and the co-existence of carbonate ion. The amounts of uranium adsorbed on the adsorbents increased linearly as the external uranium concentration increased. Uranium adsorbed on chitin phosphate easily desorbed with diluted sodium carbonate solution. On the other hand, uranyl and cobalt ions were separated from each other by using chitin phosphate.     

Preparation and characterization of chitosan encapsulated Sargassum sp. biosorbent for nickel ions sorption

A new biosorbent - Sargassum sp. encapsulated with epichlorohydrin (ECH) cross-linked chitosan (CS) was investigated for nickel ions removal. The prepared biosorbent with Sargassum sp. to cross-linked chitosan of 3 (weight ratio) had the highest sorption capacity. The biosorption kinetics can be well fitted by the diffusion-controlled model. The organic leaching of CS was 77-88% less than that of algae at different pH. The biosorption capacity of nickel on CS was much higher than that of cross-linked chitosan (CLC) bead and lower than that of raw algae due to encapsulation. In addition, the reusability of CS was further evaluated and confirmed through five adsorption-desorption cycles. Fourier transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS) analysis demonstrated that the nickel ions sequestration mechanism included ion exchange and nickel complexation with the carboxyl, amino, alcoholic and ether groups in CS.     

ELECTRON STAINS : I. Chemical Studies on the Interaction of DNA with Uranyl Salts

Chemical studies have been carried out on the interaction of DNA with uranyl salts. The effect of variations in pH, salt concentration, and structural integrity of the DNA on the stoichiometry of the salt-substrate complex have been investigated. At pH 3.5 DNA interacts with uranyl ions in low concentration yielding a substrate metal ion complex with a UO₂⁺⁺/P mole ratio of about ½ and having a large association constant. At low pH's (about 2.3) the mole ratio decreases to about ⅓. Destruction of the structural integrity of the DNA by heating in HCHO solutions leads to a similar drop in the amount of metal ion bound. Raising the pH above 3.5 leads to an apparent increase in binding as does increasing the concentration of the salt solution. This additional binding has a lower association constant. Under similar conditions DNA binds about seven times more uranyl ion than bovine serum albumin, indicating useful selectivity in staining for electron microscopy.     

Studies on Uranium Removal by the Extracellular Polysaccharide of a Pseudomonas aeruginosa Strain

Extracellular polysaccharide (EPS) produced by a Pseudomonas aeruginosa strain BU2 was characterized for its ability to remove uranium from aqueous solution. The EPS was acidic in nature and found as a potent biosorbent for uranium (U), showing pH dependence and fast saturating metal sorption, being maximum (985 mg U g^{? 1} EPS) at pH 5.0. The polymer showed enhanced uranium sorption capacity and affinity with increasing solution pH, suggesting a preferential sorption of monovalent uranyl hydroxide ions over the nonhydroxylated divalent species. Pseudo-first-order and pseudo-second-order kinetic models were applied to the experimental data, assuming that the external mass transfer limitations in the system can be neglected and biosorption is sorption controlled. Equilibrium metal binding showing conformity to the Freundlich model suggested a multilayer sorption involving specific binding sites with affinity distribution. The presence of two types of metal binding sites corresponding to strong and weak binding affinity was interpreted from the Scatchard model equation. Uranium sorption by EPS was unaffected or only slightly affected in the presence of several interfering cations and anions, except iron and thorium. Fourier transform infrared (FTIR) spectroscopy ascertained the strong binding of uranium with the carboxylic groups of uronic acids of bacterial EPS at pH 5.0, whereas at lower pH, amino and hydroxyl groups played a major role in metal binding.     

Why is chitosan mucoadhesive?

Chitosan is a biocompatible and biodegradable amino polysaccharide, which is soluble in aqueous solutions at pH < 6.5. It has been widely used for developing drug delivery systems because of its excellent mucoadhesive properties. Although many studies report on chitosan being mucoadhesive, the nature of interactions between chitosan and mucin remains poorly defined. Here, we have examined the role of primary amino groups and the role of electrostatic attraction, hydrogen bonding, and hydrophobic effects on aggregation of gastric mucin in the presence of chitosan. Reducing the number of amino groups through their half acetylation results in expansion of chitosan's pH-solubility window up to pH 7.4 but also reduces its capacity to aggregate mucin. We demonstrated that electrostatic attraction forces between chitosan and gastric mucin can be suppressed in the presence of 0.2 mol/L sodium chloride; however, this does not prevent the aggregation of mucin particles in the presence of this biopolymer. The presence of 8 mol/L urea or 10% v/v ethanol in solutions also affects mucin aggregation in the presence of chitosan, demonstrating the role of hydrogen bonding and hydrophobic effects, respectively, in mucoadhesion.     

Zum chemischen Status der Deoxyribonucleinsäure im Bakteriennucleoid

Zusammenfassung Der in einer früheren Mitteilung beschriebene Effekt der Uranylsalze auf die Struktur des Bakteriennucleoids tritt bereits bei wesentlich niedrigeren Uranylionenkonzentrationen ein, wenn die Zellen bereits mit Uranylacetat und ÄDTE (successiv) vorbehandelt wurden. ÄDTE allein verursacht eine geringere Senkung der Grenzkonzentration. Durch Anwendung eines Gemisches von Calcium-und Magnesiumionen und Polyaminen wird diese Erscheinung größtenteil wieder rückgängig gemacht.Außerdem wurde versucht, die Bindungsverhältnisse der Uranylionen an DNS in vitro genauer zu charakterisieren.Alle Befunde sprechen dafür, daß die Uranylionen natürliche Liganden von den Phosphatgruppen der DNS zu verdrängen vermögen. Diese Liganden sind, soweit sie mit Hilfe von ÄDTE allein ausgetauscht werden können, Metallionen, im übrigen sind sie organischer Natur. Die Experimente sind als erster Schritt zu einer indirekten Charakterisierung dieser Liganden aufzufassen.Der Struktureffekt der Uranylionen am Nucleoid tritt ein, wenn etwa die Hälfte der DNS-Phosphatgruppen durch Uranylionen neutralisiert ist.

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Summary The limiting concentration of uranyl ions which is necessary to induce a fibrillar appearance in bacterial nucleoids as observed in ultrathin section, was decreased markedly by pretreatment of the cells with solutions of uranyl salt and subsequent removal of uranyl ions by EDTA, in a lesser extent by pretreatment with EDTA alone. If the pretreatment was followed by application of a mixture of calcium and magnesium ions and polyamines, its effect was reversed to a great extent.In in vitro experiments the association of uranyl ions to DNA was studied in some detail.All observations are in accordance with the view that uranyl ions displace ligands from the phosphate groups of intracellular DNA which are partly (as far as they are removed by EDTA alone) divalent metal ions, the remainder being of organic nature and possibly of low molecular weight.The experiments may be considered as a first step to an indirect characterization of these ligands.The effect of uranylions on nucleoid structure is induced as soon as about 50% of the phosphate groups of DNA are neutralized by uranyl ions.

     

Chemical preparation and structural characterization of a homogeneous series of chitin/chitosan oligomers

The preparation of a homogeneous series of chitin/chitosan oligomers (chito-oligomers) with the same distribution of degrees of polymerization (DP) ranging from 2 to 12, but with various average degrees of N-acetylation (DA) from 0 to 90% is described. This DA-series was obtained according to a two-step chemical process involving (i) the production of a well-defined mixture of glucosamine (GlcN) oligomers obtained by acid hydrolysis of a fully N-deacetylated chitosan and after selective precipitations of the hydrolysis products, and (ii) the partial N-acetylation of the GlcN units of these oligomers from a hydro-alcoholic solution of acetic anhydride in a controlled manner. The characterization of this series of samples with different DAs by proton nuclear magnetic resonance (1H NMR) spectroscopy and matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) allowed us to determine their average DA and identify the main oligomer structures constituting each mixture. Furthermore, MALDI-TOF MS was particularly helpful to study the distribution evolution of the diverse oligomers as a function of DA for the main DPs from 3 to 7. The modeling of these distributions by means of a binomial law displayed that the chemical N-acetylation of low DP GlcN oligomers, produced in a homogeneous medium, occurs randomly along the oligosaccharide chains in accordance with a statistical (Bernoullian) arrangement. In this case, the relative proportion of each chito-oligomer present in the mixture can be estimated precisely as a function of DA considering oligomers of same DP.