Search

Ionotropic gelation is particularly appealing for the formation of hydrogels because it takes place under mild conditions, is not thermoreversible, and does not involve toxic chemicals. A well-known example is the gelation of alginate in the presence of calcium ions, which is at the base of numerous applications involving this polymer. In this study, alginate-derived oligosaccharides were converted into acrylamide- and methacrylamide-type macromonomers in two steps without resorting to protective group chemistry. They were then copolymerized with 2-hydroxyethylmethacrylamide in aqueous solution to yield high molar mass biohybrid glycopolymers containing between 25 and 52% by mass of oligosaccharide graft chains. A comparative kinetic study showed that both acrylamide- and methacrylamide-type macromonomers reacted since the early stages of the copolymerization, but that the mole fraction in the polymer was smaller than in the feed up to 50-60% conversion and increased markedly afterward. This effect was slighter for the methacrylamide-type macromonomer though. Copolymers carrying oligosaccharide chains with 16-20 repeating units were synthesized and used for a gelation experiment: When dialyzed against CaCl(2) 0.5 mol L(-1), the polymer carrying (1→4)-α-l-guluronan residues led to a soft isotropic self-standing transparent hydrogel, while the polymer carrying (1→4)-β-d-mannuronan residues gave a loose opaque gel. This study demonstrates that alginate-extracted oligosaccharides and aqueous radical polymerization can be combined for the flexible design of biohybrid glycopolymers capable of ionotropic gelation under very mild conditions.

Chitosan-g-N-isopropylacrylamide (NIPAm) water-soluble copolymers were synthesized and characterized by FTIR and (1)H NMR spectroscopies combined with conductometric and potentiometric titrations. Their thermoresponsive, fully reversible, behavior in aqueous solutions was characterized by means of microcalorimetry and rheology. During heating of copolymer solutions there is a well-known endothermic effect, which coincides with a marked increase in G' and a moderate decrement in G'' due to the formation of a hydrophobic network at the expense of the net amount of sol fraction. It was also found that a straight dependence between the values of G' above the LCST and the enthalpies associated with the transition reflecting that the connectivity in the gel network is governed by the net number of formed enthalpic-hydrophobic driven-junctions. Both the LCST and the enthalpy change vary with the ionic strength of copolymer solutions, but no dependence was found with the neutralization of the polyelectrolyte chain.

In this paper we extend our previous experimental work on interaction between polyelectrolytes and liposomes. First, the adsorption of chitosan and alkylated chitosan (cationic polyelectrolytes) with different alkyl chain lengths on lipid membranes of liposomes is examined. The amount of both chitosans adsorbed remains the same even if more alkylated polysaccharide has to be added to get saturation if compared with unmodified chitosan. It is demonstrated that alkyl chains do not specifically interact with the lipid bilayer and that electrostatic interaction mechanism governs the chitosan adsorption. The difference observed between unmodified and alkylated chitosans behavior to reach the plateau can be interpreted in terms of a competition between electrostatic polyelectrolyte adsorption on lipid bilayer and hydrophobic autoassociation in solution (which depends on the alkyl chain length). Second, interaction of liposomes with hyaluronan (HA) and alkylated hyaluronan (anionic polyelectrolytes) is analyzed. The same types of results as discussed for chitosan are obtained, but in this case, autoassociation of alkylated HA only occurs in the presence of salt excess. Finally, a first positive layer of chitosan is adsorbed on the lipid membrane, followed by a second negative layer of HA at three different pHs. This kind of multilayer decoration allows the control of the net charge of the composite vesicles. A general conclusion is that whatever the pH and, consequently, the initial charge of the liposomes, chitosan adsorption gives positively charged composite systems, which upon addition of hyaluronan, give rise to negatively charged composite 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.

The conformational behavior of hyaluronan (HA) polysaccharide chains in aqueous NaCl solution was characterized directly at the single-molecule level. This communication reports on one of the first single-chain atomic force microscopy (AFM) experiments performed at variable temperatures, investigating the influence of the temperature on the stability of the HA single-chain conformation. Through AFM single-molecule force spectroscopy, the temperature destabilization of a local structure was proven. This structure involved a hydrogen-bonded network along the polymeric chain, with hydrogen bonds between the polar groups of HA and possibly water, and a change from a nonrandom coil to a random coil behavior was observed when increasing the temperature from 29 +/- 1 to 46 +/- 1 degrees C. As a result of the applied force, this superstructure was found to break progressively at room temperature. The use of a hydrogen-bonding breaker solvent demonstrated the hydrogen-bonded water-bridged nature of the network structure of HA single chains in aqueous NaCl solution.

This paper describes the behavior of large and giant unilamellar vesicles (LUVs and GUVs, respectively) in the presence of chitosan, a positively charged polyelectrolyte. Variation of the zeta-potential of LUVs as a function of chitosan concentration is studied for two different molecular weights (MW) after a preliminary study devoted to pH and salt effects on zeta-potential in order to discriminate among the effects of protons, salt, and chitosan concentrations. The difference observed between pH and salt effects on the one hand and chitosan on the other allows us to conclude there is a strong LUV-chitosan interaction. In presence of chitosan, the zeta-potential of LUVs becomes positive and two distinct regimes of variation are suggested and interpreted as follows: a first step consists of chitosan adsorption flat on the membrane (independent of MW) followed by a possible reorganization of the polymer of higher molecular weight on the surface, giving rise to loops. Then a comparative observation of the effect of pH and salt by optical microscopy is made on naked and chitosan-decorated GUVs. Results further confirm a membrane-chitosan interaction and are interpreted in the light of the results obtained for LUVs in terms of both electrostatic and hydrophobic interaction. A large majority of decorated vesicles remain stable down to pH = 1 while in the absence of chitosan they burst quickly at pH between 2 and 3. Osmotic pressure and net charge change due to addition of HCl results in a decrease in the diameter of the decorated vesicles, which remain spherical while forming tubes of lipids. In presence of NaCl, a higher resistance of decorated vesicles is also evidenced (they are stable for NaCl concentrations up to 10-1 M while naked vesicles burst when [NaCl] is between 10-2 and 10-3 M). At higher salt concentration, aggregation of decorated vesicles occurs, which is attributed to the screening of electrostatic repulsions between vesicles covered by the positively charged chitosan. Finally, adhesion of vesicles on a positively charged surface is investigated. In absence of chitosan, the vesicles immediately burst when they come in contact with the surface. On the contrary, suspension of chitosan-vesicles remain stable down to pH = 1.5. Under gentle flow vesicles move: they do not adhere on the substrate, probably due to the repulsion between positively adsorbed charged chitosan and substrate; spherical deflation occurs, but in this case daughter vesicles are formed instead of lipid tubes.

A new synthetic route to beta-cyclodextrin-linked hyaluronic acid (HA-CD) was developed. This was based on the preparation of a HA derivative selectively modified with adipic dihydrazide (HA-ADH) and a beta-cyclodextrin derivative possessing an aldehyde function on the primary face, followed by their coupling by a reductive amination-type reaction. The CD-polysaccharide was fully characterized in terms of chemical integrity and purity by high-resolution NMR spectroscopy. The complexation ability of the grafted CD was further demonstrated by isothermal titration calorimetry using sodium adamantane acetate (ADAc) and Ibuprofen as model guest molecules. The thermodynamic parameters for the complexation of these negatively charged guest molecules by the beta-CD grafted on negatively charged HA were shown to be largely influenced by the ionic strength of the aqueous medium.

This paper concerns the bulk and interfacial properties of a series of alkylated chitosans having different alkyl chain lengths grafted randomly along the main chitosan chain. Chitosan has a low degree of acetylation (5%); on chitosan derivatives, the role of the degree of grafting and of length of the alkyl chains are examined. The optimum alkyl chain length is C12 and the degree of grafting 5% to get physical gelation based on the formation of hydrophobic domains. The cross-linking is essentially controlled by the salt concentration: it is shown that 0.025 M AcONa is needed to screen electrostatic interchain repulsions. Hydrophobic interactions produce highly non-Newtonian behavior with large thinning behavior; this behavior is suppressed in the presence of cyclodextrins able to cap the hydrophobic alkyl chains. The interfacial properties of the chitosan derivatives were tested for the air/aqueous solution interfaces. Specifically, the role of their structure on the kinetic of film formation was examined showing that excess of external salt favors the stabilization of the interfacial film. The derivatives with a higher degree of substitution and longer alkyl chains are more efficient and give a higher elastic modulus compared to the model surfactant as a result of the chain properties.

In this paper, we have examined the behavior of hyaluronan solutions at different pH values. A slight degradation is observed in acidic conditions (pH = 1.6) and basic medium (pH = 12.6) from molecular weight distribution analysis, but the rheological behavior is relatively not influenced much by the pH at the exclusion of two domains: around pH = 2.5, a gel-like behavior is shown and is attributed to cooperative interchain interactions due to the reduction of the polymer net charge and may be the protonation of the acetamido groups; for pH > 12, the decrease of viscosity is mainly attributed to a reduction of the stiffness of the polymeric backbone in alkaline conditions due to the partial breakage of the H-bond network.

This paper concerns the influence of the chemical structure on the physical properties of some polysaccharides. Especially, we proposed to discuss the role of the substituents on these properties. In some cases, non-carbohydrate substituents play a minor role on rheological properties in the presence of a salt excess as shown on xanthan and succinoglycan. The rheology of aqueous solution of these stereoregular polysaccharides is controlled by the conformation (helical conformation) whose stability is not largely influenced by these substituents. On the other hand, the interaction between galactomannan and xanthan depends on the presence of acetyl substituents on xanthan but also on the xanthan conformation. However, for polymers such as gellan, XM-6 or BEC 1615, complete deacetylation induces the ability to form physical gels in given thermodynamic conditions. The presence of carbohydrate substituents or short side chains was also examined. Especially in the gellan family, the role of position of substitution (position 3 on the glucose unit C or position 6 on the A glucose) was presented. It is concluded that the substituents giving the higher stability for the helical conformation (higher DeltaH and Tm values) also cause a lower salt sensitivity for the helical stability. The role of the substituents on the properties is also described for natural polymers and their chemically or enzymatically modified derivatives.

Maleic copolymers with different contents of galactose moieties and dodecyl chains were synthesized and used as both a stabilizer and a surface coating for the preparation of poly(epsilon-caprolactone) nanoparticles by the emulsification-diffusion technique. The size of the nanoparticles was controlled by varying the initial concentration of the modified maleic copolymers. As the concentration of the latter increased, the particle size decreased, indicating that the copolymers serve as a stabilizer. Moreover, surface modification of nanoparticles was confirmed by xi-potential measurements. Nanoparticles were also shown to be recognized by a galactose-specific lectin, demonstrating the presence of galactose units on the particle surface. This approach offers opportunities for the production of novel targeted drug delivery systems.

The chemical structure of a polysaccharide named Fucogel was characterized and the position of acetylation was identified by NMR. A conformational analysis was performed on this 3-sugar repeating unit. From this, the persistence length, characterizing the stiffness of the polysaccharide, was determined and the role of the presence of acetyl group, reducing the stiffness, was pointed out. The helical conformations were also predicted, one of these being in agreement with X-ray data obtained on a similar polysaccharide. Experimental characterization of the native and deacetylated polysaccharides was developed. SEC experiments allowed us to determine the molar mass and the persistence length on the deacetylated polysaccharide. The value is in good agreement with that predicted from the molecular modeling. Microcalorimetry, rheology, and fluorescence spectroscopy demonstrated respectively that no helical conformation exists in solution but that loose interchain interactions due to the acetyl substituents exist in dilute solutions.

The chemical structure and the rheological behavior of the Klebsiella polysaccharide ATCC 12657 was studied and compared with data described in the literature and obtained for similar polysaccharides. The acetylated polysaccharide presents in solution a normal viscoelastic behavior with no evidence of an ordered conformation whatever the experimental conditions are. The deacetylated form can induce the formation of physical gels, in the presence of salt excess or ethanol. Microcalorimetry, optical rotation, and rheology experiments demonstrate that a thermally reversible and highly cooperative conformational transition occurs at the same temperature than a sol-gel transition. The melting of the gel and the conformational transition temperatures are dependent on the nature of cations and ionic concentration, whereas the gel strength is only influenced by polymer concentration.

Water-soluble artificial glycoconjugate polymers were synthesized from poly(N-vinylpyrrolidone-co-maleic anhydride) by amidation with an amine-containing galactose derivative. The glycopolymers having different galactose contents were fully characterized in terms of chemical structure by NMR and potentiometric titrations, and their aqueous behavior was studied by viscometric measurements. Their specific binding properties were examined by enzyme-linked lectin assays using RCA(120) lectin. Whatever the glycopolymer, the grafted galactoses were shown to behave similarly to free galactose.

The aggregation phenomena in aqueous solutions of hydrophobically modified (HM) chitosan, containing 4 mol % of n-dodecyl side chains, were studied by viscometry and fluorescence spectroscopy with pyrene as a probe. The results are compared with those for unmodified chitosan. Surprisingly, fluorescence data reveal the appearance of intermolecular hydrophobic aggregates both in chitosan and in HM chitosan. Nevertheless, these polymers exhibit quite different rheological properties: upon the formation of aggregates the viscosity of HM chitosan sharply increases, while that of unmodified chitosan raises only slightly. The aggregation models for both chitosan and its hydrophobic derivative were proposed. It was shown that in solutions of HM chitosan two types of hydrophobic domains exist: hydrophobic domains typical for different associating polymers with hydrophobic side chains and hydrophobic domains inherent to chitosan itself.

The degree of acetylation (DA) of various samples has been evaluated by 1H liquid-state NMR and 13C and 15N CP-MAS solid-state NMR over the whole range of DA. A good agreement has been found for all the experiments. The 13C and 15N CP-MAS experiments have permitted the evaluation of the chitin DA and content in the structural polysaccharides in a fungus named Aspergillus niger. The fungus structural carbohydrates mainly consist in pure chitin associated with glucans. Comparison of the 13C CP-MAS spectra with standard (1-->3)-beta-D-glucans strongly suggests that chitin and glucans are linked via covalent bonds.