On the physicochemical properties of gellan gum

Development of re-usable yeast-gellan gum micro-bioreactors for potential application in continuous fermentation to produce bio-ethanol

The objectives of this study were to investigate the feasibility of encapsulating yeast cells using gellan gum by an emulsification method and to evaluate the fermentation efficiency and the reusability of the micro-bioreactors produced. It was found that yeast cells could be successfully encapsulated to form relatively spherical micro-bioreactors with high specific surface area for mass transfer. Cell viability was found to be reduced by one log reduction after the emulsification process. The ethanol yield of the micro-bioreactors was comparable to that of free yeast in the first fermentation cycle. The micro-bioreactors remained intact and could be re-used up to 10 cycles of fermentation. Despite cell breakthrough, relatively high ethanol yields were obtained, indicating that the micro-bioreactors also functioned as regenerative reservoirs of yeast.

l-Carnosine: multifunctional dipeptide buffer for sustained-duration topical ophthalmic formulations

Objectives

The use of L-carnosine as an excipient in topical ophthalmic formulations containing gellan gum, a carbohydrate polymer with in-situ gelling properties upon mixing with mammalian tear fluid, was developed as a novel platform to extend precorneal duration. Specific utilisation of L-carnosine as a buffer in gellan gum carrying vehicles was characterised.

Methods

Buffer capacity was evaluated using 7.5, 13.3, and 44.2 mM L-carnosine in a pH range of 5.5–7.5. Accelerated chemical stability was determined by HPLC at L-carnosine concentrations of 5–100 mM. Combinations of 7.5 mM L-carnosine with 0.06–0.6% (w/v) gellan gum were characterised rheologically. L-Carnosine-buffered solutions of gellan gum were tested for acute topical ocular tolerance in vivo in pigmented rabbits. A unique formulation combining timolol (which lowers intraocular pressure) in L-carnosine-buffered gellan gum was compared with Timoptic-XE in normotensive dogs.

Key findings

L-Carnosine exhibited optimal pharmaceutical characteristics for use as a buffer in chronically administered topical ocular formulations. Enhancement trends were observed in solution-to-gel transition of L-carnosine-buffered vehicles containing gellan gum vs comparators. Topical tolerability of L-carnosine-buffered gellan gum formulations and lowering of intraocular pressure were equivalent with timolol and Timoptic-XE.

Conclusions

Functional synergy between excipients in gellan gum formulations buffered with L-carnosine has potential for topical ocular dosage forms with sustained precorneal residence.

Study of Formulation Variables on Properties of Drug-Gellan Beads by Factorial Design

The aim of the present study was to obtain cross-linked calcium-gellan beads containing diclofenac sodium as model drug, using full 3(3) factorial design. Drug quantity, pH of cross-linking solution, and speed of agitation were selected as variables for factorial design. The resultant beads were evaluated by scanning electron microscopy (SEM), percent yield, entrapment efficiency, micromeritic properties, swelling and drug release studies. The drug-loaded beads were spherical with size range of 0.85-1.8 mm. Percent yield and entrapment efficiency of various batches were in the range of 86.48-98.28% w/w and 72.52-92.74% w/w, respectively. Calcium-gellan beads containing diclofenac sodium showed pH-dependent swelling and drug release properties. Swelling and drug release were significantly higher in pH 7.4 phosphate buffer than 0.1N HCl. The swelling ratio for beads was up to 22 and 3 for phosphate buffer and 0.1N HCl, respectively. Cumulative diclofenac sodium release from calcium-gellan beads was 12-35% in 0.1N HCl within 2 h, whereas complete drug release was observed within 3-4 h in pH 7.4 phosphate buffer.

Non-Covalent Interactions in Polysaccharide Systems

[Chemical structure: see text] This paper describes the behavior of some polysaccharides with well-known chemical structures and in which the influence of cooperative secondary interactions play an important role. The roles played by hydrophobic and ionic interactions (including ionic selectivity) on polysaccharide conformation and gelation are discussed. Electrostatic attractions are also important in the complexes formed between surfactants and polyelectrolytes of opposite charge. Finally, van der Waals dipolar interactions and particularly hydrogen-bond formation are examined. The role of hydrogen bonds in solubility, conformation, and especially the local stiffness of polysaccharides, but also in polymer-polymer complexes frequently obtained with polysaccharides, is developed. Repeat unit for a number polysaccharides.

Synergistic Interaction of Xyloglucan and Xanthan Investigated by Rheology, Differential Scanning Calorimetry, and NMR

A new synergistic interaction between tamarind seed xyloglucan and xanthan was found and investigated by rheology, differential scanning calorimetry (DSC), and NMR. The effect of the acetyl and pyruvate groups in the side chain in xanthan on the synergistic interaction was also examined. The shear moduli G' and G' ' of the mixture solution of xyloglucan and native (or acetate-free) xanthan increased steeply at around 22 degrees C upon cooling. An exothermic DSC peak appeared at the same temperature. A drastic decrease in the of the acetyl and pyruvate groups of the xanthan side chain was observed from 1H NMR spectra only in the mixture at low temperatures (<25 degrees C). It was found that the pyruvate group is more restricted in the mixture solution compared with the acetyl group. The mixture of xyloglucan and pyruvate-free xanthan showed no synergistic interaction. We concluded that this synergistic interaction is caused by the intermolecular binding between xyloglucan and xanthan, and, in the heterotypic junction zones, the xanthan side chain becomes a new state that is different from both the coil and helix states.

The reinfocement of gellan gel network by the immersion into salt solution

The effect of immersion into salt solutions on rheological properties of gellan gels was investigated. The storage Young's modulus of gellan gels increased with time during the immersion into salt solutions. The increase of the storage Young's modulus can not be explained solely by change in the concentration of gellan. The ellipticity at 202 nm decreased by the immersion, suggesting the formation and aggregation of gellan helices. It was considered that during immersion cations penetrated into gellan gels to induce the formation and aggregation of gellan helices in gels, resulting in reinforcement of the gel network.

Thermally induced coil-to-helix transition of sodium gellan gum with different molar masses in aqueous salt solutions

Using 5 samples of well-purified Na-gellans (Na-gellans G1-G5, weight-average molar mass M(w) = 120 x 10(3)-32 x 10(3) at 40 degrees C), the effects of molar mass on the coil-to-double-helix transition in aqueous solutions with 25 mM NaCl were studied by light scattering and circular dichroism (CD) measurements, viscometry, and differential scanning calorimetry (DSC). From the temperature dependence of M(w), molar ellipticity at 201 nm [theta]201, intrinsic viscosity [eta], and DSC exothermic curves, it was found that the coil-to-double-helix transitions for G1-G5 samples took place at almost the same temperature. The [eta] and M(w) obtained in the temperature range from 40 to 25 degrees C can be explained by a simple coil/double-helix equilibrium model using the double-helix contents determined from CD data. The van't Hoff's transition enthalpy deltaH(vH) of Na-gellans depended on M(w). It is concluded that the coil-to-double-helix transitions of Na-gellans are all-or-none type transitions, and are accelerated with increasing M(w).

Role of Substituents on the Properties of Some Polysaccharides

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.

Binding Effect of Cu2+ as a Trigger on the Sol-to-Gel and the Coil-to-Helix Transition Processes of Polysaccharide, Gellan Gum

The binding effect of divalent cation Cu(2+) on the gelation process with a coil-helix transition in Cu(2+)/gellan aqueous solutions has been successfully elucidated by EPR, CD, and viscoelasticity measurements. Generally, Na-type gellan gum in aqueous solution can make gel when accompanied by an intrinsic coil-helix formation induced by hydrogen bonding between chains without any additional cations at T(ch)(-)(in) ( approximately 29 degrees C) with cooling temperature. An extrinsic coil-helix transition, induced by additional divalent cations in advance of the intrinsic sol-gel transition of gellan gum, is separately detected by CD measurement. The extrinsic coil-helix transition temperatures T(ch)(-)(ex) (>47 degrees C), which increased with the Cu(2+) concentration added, were nearly identical to the sol-gel transition temperature, T(sg), determined by the viscoelasticity measurement. Judging from the molar ellipticity by CD measurement and quantitative analysis of EPR spectra, it was elucidated that the helix forming process via divalent cations is composed of two steps ascribed to the different origins, i.e., a chemical binding effect via Cu(2+) ions in the initial stage and hydrogen bonds subsequently. Finally, we propose the coil-helix and the sol-gel transition mechanism initiated by the binding effect with the divalent cation, in which the partial chelate formation can cause local formation of helices and junction zones in the vicinity of the chelates at the initial stage of the process and stabilize the helices and the junction zones. On the other hand, the stabilized helices and junction zones can induce further formation and further stabilization of the Cu(2+)-gellan chelates. The mutual stabilization promotes the formation of three-dimensional network structure at the higher temperature than the intrinsic temperature for network formation.

Solution Properties of Gellan Gum: Change in Chain Stiffness between Single- and Double-Stranded Chains

Nine samples of gellan gum in the sodium form, ranging in weight-average molar mass from 3.47 x 10(4) to 1.15 x 10(5) at 40 degrees C, were investigated by static and dynamic light scattering and viscometry in 25 mM aqueous NaCl both at 40 and at 25 degrees C. The ratios of the molar mass at 25 degrees C (in the ordered state) to that at 40 degrees C (in the disordered state) were in the range of 1.99 to 2.07, supporting the scheme of the conformational transition of gellan gum between a disassociated single chain and an associated chain composed of two molecules. Focusing on the effects of polydispersity, the intrinsic viscosities, radii of gyration, and hydrodynamic radii were analyzed on the basis of unperturbed wormlike chain models. The persistence lengths were evaluated as 9.4 nm at 40 degrees C and 98 nm at 25 degrees C.

Structure and properties of a bacterial polysaccharide from a Klebsiella strain (ATCC 12657)

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.

The structure of gellan in dilute aqueous solution

A full assignment of high-field nmr spectra of gellan was obtained in dilute aqueous solution by performing a series of selective one-dimensional nmr experiments. The observed nuclear Overhauser effects (NOEs) cannot be interpreted assuming that each sugar residue is intrinsically rigid and in a chair conformation. In fact, the rhamnose residue gives strong NOE contacts coherent only with an equilibrium involving both a chair as well as a boat (or a hemiboat) conformation. Molecular dynamic calculations performed on a heptamer with a central rhamnose support the above finding, and show a structure based on a very stiff single chain in which it is present a flipping of the rhamnose residue. At low temperatures (5-20 degrees C) in very dilute solutions (0.018 mg/mL) nmr spectra show a splitting of the resonance due to the methyl group of rhamnose residue, thus confirming the presence of a slow equilibrium among different conformers.

A new bacterial polysaccharide (YAS34). I. Characterization of the conformations and conformational transition

This paper concerns the study of the conformational transition of a new exopolysaccharide (YAS34) using experimental techniques such as optical rotation, conductimetric and microcalorimetric measurements as a function of temperature. The behaviors of this polysaccharide in the acid or sodium salt form are compared; a deacetylated sample is also prepared to demonstrate the role of substituents. For the native structure (never heated), a conformational transition is observed but the deacetylated polysaccharide exhibits no ordered conformation. Multidetection size exclusion chromatography (SEC) analyses and conductimetric experiments allowed to determine the nature of each conformation and the molecular dimensions. From these results, it is suggested that the native conformation is a double helix which by heating over T(m) (temperature corresponding to half conformational transition) dissociates into disordered single chains. In the acid and sodium salt forms, by cooling below T(m), an ordered conformation is restored. This conformation seems to be an intramolecular double helix 'hairpin-like turn' (called renatured conformation). Nevertheless an irreversible denaturation is obtained progressively in the sodium salt form when the time of heating over T(m) increases. The conformation of the deacetylated polysaccharide corresponds to that of a single flexible chain (disordered conformation). The conformational transition for the native conformation was studied also in relation to the polyelectrolytic character of the polysaccharide: stability as a function of salt nature and salt and polymer concentrations was investigated for the polymer initially in the sodium and acid forms.

Conformational transition of native and modified gellan

This paper concerns the characterisation of native gellan by differential scanning calorimetry (DSC) and rheology. The stability of the double helix is characterised by Tm and the enthalpy of conformational change. The role of the external salt concentration is investigated; it is shown that Tm is only slightly modified. At ambient temperature, in 10(-2) M NaCl, native gellan behaves as a loose gel (G' > G''). This behaviour disappears when temperature is larger than 60 degrees C. The comparison with deacylated gellan (commercial sample) shows that the position of conformational transition is much more influenced by the salt concentration; the helical structure is less stable and the conformational transition presents a hysteresis between heating and cooling runs when the external salt concentration increases. The rheological behaviour is that corresponding to a solution (G' < G'') at ambient temperature and in 10(-2) M NaCl. When the salt excess increases, then a stronger gel is formed. The differences between the two types of samples are clearly established as well as the relations between the conformation and the rheology of the systems.

Rheological studies of the gelation of deacetylated gellan gum (Gelrite) in physiological conditions

Gels have been successfully used to increase the mucosal contact time and hence the bioavailability of nasal and ophthalmic formulations. The use of in situ gelling polymers requires a rapid sol-gel transition that produces a strong gel for an optimal contact time. In this study, the rheological behaviour of deacetylated gellan gum (Gelrite) was analysed in order to better understand the reasons for the good performance in humans. Thermal scans were used to study gel formation and other changes in the structure of the samples when the macromolecular and ionic contents were altered. The effect the different ions in tear fluid (Na+, K+, Ca2+) had on the gel strength and the consequences of dilution due to the ocular protective mechanisms were examined. Na+ was found to be the most important gel-promoting ion in vivo. It was also found that gels are formed in tear fluid even when the concentration of Gelrite) is only 0.1%. Samples with concentrations of Gelrite of 0.5-1% do not require more ions than 10-25% of those in tear fluid to form gels. These two findings can partly explain the good performance of Gelrite in vivo. Gels with a high elastic modulus can thus be formed even though dilution of instilled drops takes place.

Effect of the formulation on the in-vitro release of propranolol from gellan beads

Gellan gum beads of propranolol hydrochloride, a hydrophilic model drug, were prepared by solubilising the drug in a dispersion of gellan gum and then dropping the dispersion into calcium chloride solution. The droplets formed gelled beads instantaneously by ionotropic gelation. Major formulation and process variables which might influence the preparation of the beads and the drug release from gellan gum beads were studied. Very high entrapment efficiencies were obtained (92%) after modifying the pH of both the gellan gum dispersion and the calcium chloride solution. The beads could be stored for 3 weeks in a wet or dried state without modification of the drug release. Oven-dried beads released the drug somewhat more slowly than the wet or freeze-dried beads. The drug release from oven-dried beads was slightly affected by the pH of the dissolution medium. Gellan gum could be a useful carrier for the encapsulation of fragile drugs and provides new opportunities in the field of bioencapsulation.