Gellan Molecules Research Articles

Conformation and hydration property of low-acetyl gellan gum under microwave irradiation: Experiments and molecular dynamics simulations

The effect of microwave irradiation on the conformation and hydration properties of low-acetyl gellan gum was investigated to reveal the mechanism using experimental methods and molecular dynamics (MD) simulations. The experimental data showed that cross-sectional radius of gyration of the gellan chain treated by microwave or water bath were identical and microwave treatment could improve the water holding capability and restrict water mobility. On the other hand, results of root-mean-square deviation (RMSD) and root-mean-square fluctuation (RMSF) showed that microwave irradiation had little impact on the structural conformation of the low-acetyl gellan molecule. The analysis of radial distribution function and interaction energy demonstrated that microwaves modified the gellan-water molecules structure by energy transfer, making hydration shells of the gellan molecule become more structured. In addition, there was an enhancement of the average hydrogen bond number and lifetime under microwave irradiation due to rearrangement and reorient of molecules. These findings can explain the mechanism of microwave-promoted gellan gum gelation to some extent, which enhance industrial applications of low-acetyl gellan.

Effects of the conjugation of whey proteins with gellan polysaccharides on surfactant-induced competitive displacement from the air-water interface

Whey proteins can be used to stabilize foams and emulsions against coalescence because of their ability to form viscoelastic films at the interface that resist film rupture on collision between colloidal particles. However, whey proteins are competitively displaced from the interface if small-molecule surfactants are added, leading to destabilization of the entire system. This is because surfactants are more effective in molecular packing at the interface, and they lower interfacial tension to a greater degree than whey proteins do, but their interfacial films are poor in viscoelasticity. We hypothesized that whey proteins would become more resistant to surfactant-induced competitive displacement if they were conjugated with network-forming polysaccharides. The protein moiety of the conjugate would be expected to enable its adsorption to the interface, and the polysaccharide moiety would be expected to form self-assembled networks, strengthening the interfacial film as a whole. In this study, whey proteins were conjugated with gellan polysaccharides using the Maillard reaction. Atomic force microscopy images of interfacial films formed by the whey protein–gellan conjugate at the air-water interface and transferred onto mica sheets using the Langmuir-Blodgett method revealed that gellan did form self-assembled networks at the interface and that interfacial films also contained a large number of unconjugated whey protein molecules. Following the addition of a small-molecule surfactant (Tween 20) to the sub-phase, surface pressure increased, indicating spontaneous adsorption of surfactants to the interface. Atomic force microscopy images showed decreases in interfacial area coverage by whey proteins as surface pressure increased. At a given surface pressure, the interfacial area coverage by whey protein–gellan conjugates was greater than coverage by unconjugated whey proteins, confirming that whey proteins became more resistant to surfactant-induced displacement after conjugation with gellan. Furthermore, gellan molecules added to the sub-phase after the formation of a monolayer of whey proteins at the air-water interface did not adsorb to the interfacial protein film. These results provide a molecular basis for designing interfacial structures to enhance the stability of colloidal systems.

Influence of pH on formation and properties of gellan gels

The effect of pH (3.5, 5.3 and 7.0) on the formation of deacylated gellan gels was studied using oscillatory shear measurements and microcalorimetric trials in heating–cooling cycles at 1°Cmin−1. In addition, the mechanical properties and structure of the gels formed at the different pH values were evaluated by uniaxial compression and scanning electron microscopy, respectively. An increase in pH led to an increase in temperature and decrease in enthalpy of the coil–helix transition. These results were attributed to a decreased formation of junction zones at higher pH values due to greater electrostatic repulsion between the gellan molecules. As a consequence, gels formed at neutral pH were more fragile and deformable than acid gels. All the samples showed thermo-reversible rheological behavior, presenting the same rheological properties at 20°C, before and after heating to 90°C.

Adsorption of protein-coated lipid droplets onto gellan gum hydrogel surfaces

Abstract The formation of coatings on hydrogel surfaces due to adsorption of lipid droplets is important in a number of natural and industrial processes, e.g., foods, cosmetics, and pharmaceuticals. The adsorption of cationic protein-coated lipid droplets to the surfaces of anionic hydrogels was examined in this study. An oil-in-water emulsion containing cationic β-lactoglobulin-coated lipid droplets was prepared (d4,3 = 0.3 μm, ζ = +67 mV, pH 3.0). An anionic hydrogel consisting of 1 wt% gellan gum (pH 3.0) was prepared. Emulsions containing different lipid droplet concentrations (0.1–10 wt%) were brought into contact with the hydrogel surfaces for different times (0–24 h). The adsorption of lipid droplets to the hydrogel surfaces could not be explained by a typical adsorption isotherm. We found that the electrical charge on the non-adsorbed lipid droplets became less positive or even became negative in the presence of the hydrogel, and that extensive droplet aggregation occurred, which was attributed to the ability of gellan molecules released from the hydrogels to interact with the lipid droplets. These results may have important consequences for understanding certain industrial and biological processes, as well as for the design of controlled or triggered release systems.

Effects of Alkaline Metal Salts on Viscosity of Gellan Aqueous Solutions.

Effects of alkaline metal salts on zero-shear viscosity (η0) of gellan aqueous solutions were investigated. The temperature (T) dependence curves of η0 showed a steep change due to the coil-helix transition of the gellan molecules. The transition temperature (TTr) depended on the salt concentration while it was independent of the cationic species. At temperatures higher than TTr, η0 decreased with increasing salt concentration. At temperatures lower than TTr, T-η0 curves at various salt concentrations produced a kind of envelope corresponding to the T-η0 curve for gellan with double-helical conformation. The addition of alkaline salts enhanced the aggregate formation at temperatures lower than TTr, and the efficiency for the aggregate formation depended on the cationic species used. At a fixed salt concentration, the degree of aggregation increased in the order of Cs +>Rb+>K+>Na + >Li. Defining the temperature at which the double helices start to aggregate as Ta, both TTr and Ta were not affected by the type of halogens, Cl −, Br− and I−.

Microbial System for Polysaccharide Depolymerization: Enzymatic Route for Gellan Depolymerization by Bacillussp. GL1

A bacterium-producing polysaccharide lyase (gellan lyase) was isolated from soil samples and identified to be Bacillussp. The lyase was purified from the culture fluid of the bacterium (designated Bacillussp. GL1) grown in the presence of gellan as a carbon source. The purified gellan lyase depolymerized deacetylated gellan and gave a single oligosaccharide product with a molecular weight of 646, which was determined by fast atom bombardment mass spectrometry. The structure of the product was determined by the combination of mass spectrometry, HPLC analysis, and high-resolution proton nuclear magnetic resonance spectroscopy to be a tetrasaccharide of glucuronyl-glucosyl-rhamnosyl-glucose with unsaturated glucuronic acid at the nonreducing terminal. When incubated in cell extracts of Bacillussp. GL1, the tetrasaccharide was first converted to trisaccharide without unsaturated glucuronyl residue, and the trisaccharide was then converted to hydrolyzed monosaccharides glucose and rhamnose. These results show that, in the bacterium Bacillussp. GL1, gellan is first depolymerized to give a tetrasaccharide, a repeating unit in gellan molecule, by an extracellular gellan lyase and then the tetrasaccharide is hydrolyzed to monosaccharides by successive actions of intracellular exoglycosidases.

Interactions of paramagnetic metal ions with gellan gum studied by ESR and NMR methods

The characterization of the gellan-paramagnetic metal ion complex has been investigated by ESR and NMR methods in aqueous solution of sodium-type gellan. The resonance of the carboxyl carbon in the d-glucuronate unit in 13C-NMR spectra of gellan is selectively broadened with the addition of the Mn(II) ion in sol state. This is due to the paramagnetic interaction between the carbon and the Mn(II) ion. The resonances of other units of gellan are not affected, showing that Mn(II) ions interact with the carboxyl groups in the d-glucuronate unit. Water proton relaxation measurements indicate that water molecules are well coordinated to the gellan-Mn(II) ion complex in gel state. ESR measurements of the Mn(II) ion give the dissociation constant of 3.2 × 10 −5 and 6.5 × 10 −4 M, and the number of the binding site of 0.12 and 0.33 per repeated unit in sol and gel states, respectively. The latter suggests that in sol state a Mn(II) ion is surrounded by several residues in a randomly coiled gellan polymer and in gel state a Mn(II) ion binds two or three d-glucuronate units on neighboring gellan molecules.

Ultrasonic velocity and circular dichroism in aqueous gellan solutions

Circular dichroism spectra and ultrasonic velocities of aqueous gellan solutions with and without added salts were measured at various temperatures. At higher temperatures the molecular ellipticity at 204 nm [ θ] 204 is almost independent of the temperature but decreases steeply at a given temperature ( T c) with further lowering temperature. With decreasing temperature, the difference in velocities between aqueous gellan solution and solvent ( ΔV) increases in the same manner as that of most aqueous systems, followed by a slight maximum or shoulder at T c, and then increases again. These abrupt changes in sound velocity and circular dichroism at T c may be caused by a thermally induced conformational change of gellan molecule in aqueous solutions. The decrease in the ultrasonic velocity was explained in terms of the dehydration of the water molecules. Effects of addition of some monovalent cations on the ultrasonic velocities were also studied.

Viscosities, densities, and sound velocities of dilute aqueous gellan solutions

The viscosities, densities, and sound velocities for dilute aqueous solutions of soluble gellan were measured over the temperature range from 5 to 45°C. With decreasing temperature, the solution viscosities increase very steeply at a certain temperature. This characteristic temperature increases with increasing gellan concentration. It is supposed that the gellan solution constructs a ‘pre-gel’ network structure, though rather loose, at lower temperature. From the density and sound velocity data the apparent specific volumes and the apparent specific adiabatic compressions of gellan in water were estimated at various temperatures. No pronounced transition was found in these thermodynamic properties at a ‘pre-gel’ temperature. The pre-gel network structure may be caused by the disruption of the hydrogen bond between gellan and water and the reconstruction of hydrogen bond between gellan molecules. It is considered that this process is accompanied by no significant change in volume or compression.

Gel-sol transition in gellan gum solutions. I. Rheological studies on the effects of salts

The mechanical spectra of sodium gellan gum solutions with and without sodium chloride, potassium chloride, calcium chloride and magnesium chloride were observed at the frequency range from 10 −2 to 10 rad/sec and at the temperature range from 30 to 0°C. At any temperature from 30 to 0°C, the viscoelastic behaviour of a 1% gellan gum solution without salt was typical of a dilute solution. However, that of a 2% gellan gum solution varied from a dilute solution to a weak gel with decreasing temperature. The behaviour of a 3% gellan gum solution tended towards that of a weak gel, even at the relatively high temperature of 25°C. Storage shear modulus G′ for 1% gellan gum solutions increased on addition of salts at lower temperatures where gellan molecules take helical conformations whilst it decreased on addition of salts at higher temperatures where gellan molecules take coil conformations. Salts promote aggregation of helices at lower temperatures whilst they reduce the coil dimension at higher temperatures. The increase in G′ for gellan gum solutions by the addition of cations at lower temperatures was attributed to the electrostatic shield of the repulsions between carboxyl groups, and more remarkable for potassium ions than sodium ions, and for calcium ions than magnesium ions. On addition of fairly lower concentrations of NaCl, CaCl 2 or MgCl 2 (except for KCl), G′ became slightly smaller than that without salt. This appears to be related to the fact that these ions deprive the hydrated water molecules surrounding carboxyl side groups in gellan gum molecules, so that the repulsion of these ‘naked’ carboxyl groups becomes stronger by the presence of these ions. However, G′ at a constant temperature and at a constant frequency for gellan gum solutions as a function of added salt concentration showed a complicated behaviour. G′ for a 1% gellan gum solution in the presence of 75 mmol/dm 3 NaCl as a function of temperature increased abruptly around 55°C on cooling and showed a maximum around 35°C. On heating G′ showed a maximum around 45°C, and levelled off, and then decreased rapidly about 70°C. On the other hand, G′ for a 1% gellan gum solution in the presence of 5.1 mmol/dm 3 CaCl 2 began to rise rapidly at 35°C on cooling, and decreased with decreasing temperature. On heating G′ did not change remarkably up to 90°C in contrast to the situation in the presence of NaCl. This shows that the mechanism of gel formation in gellan gum with divalent cations is markedly different from that with monovalent cations.

Ultrasonic velocities in aqueous gellan solutions

The velocity of ultrasonic waves propagating through aqueous gellan solution and gel with and without sodium chloride was measured over a temperature range from 15 to 81°C. On the whole, the difference in velocities between aqueous gellan solution and solvent, Δ V, increases with cooling in a manner similar to most aqueous systems. On cooling, however, Δ V values do not increase, but decrease in some cases, at a certain temperature ( T v). The gelling temperature of the solution was determined by a falling ball method. T v is in the vicinity of the gelling temperature and almost independent of the gellan concentration. While the gelling temperature increases with gellan concentration, on heating Δ V values decrease steadily. A hysteresis loop can be observed in measuring the velocity in aqueous gellan solution with falling and rising temperatures. The behavior of velocities was tentatively interpreted in terms of random coil-to-helix conformational change of the gellan molecule followed by dehydration of the water molecule. Similar results were obtained for gellan solutions containing sodium chloride.

Light scattering study of gellan gum

Gellan, an extracellular microbial polysaccharide from Pseudomonas elodea, has been studied in 75 mmol/dm 3 tetramethylammonium chloride by a light scattering technique. Various molecular parameters characterizing gellan have been evaluated. The gellan molecule shows the behaviours characteristics of the stiff polymer chain. Persistence length and the diameter were evaluated considering the polydispersity.

X-Ray fibre diffraction studies of members of the gellan family of polysaccharides

X-Ray fibre diffraction studies are reported for gellan gum and the family of related polysaccharides S-130, S-198, S-88 and S-194. Whereas the linear gellan molecules yield highly crystalline patterns, the branched polysaccharides yield well-aligned but poorly crystalline patterns. These patterns are consistent with the proposed 3-fold double helical structure of gellan with small changes in pitch dependent upon type and position of branches.

Light scattering studies of tetramethyl ammonium gellan

Tetramethyl ammonium (TMA) gellan does not gel. Light scattering studies suggest that in solutions of TMA gellan, in tetramethyl ammonium chloride (TMACI), the gellan molecules assemble end to end to produce elongated fibrous structures. Such fibrils are envisaged as resulting from double-helix formation between the ends of neighbouring gellan molecules. Fibrils with molecular weights ranging from (1.06 ± 0.06) × 10 5 to (4.5 ± 0.1) × 10 6 have been observed. The molecular weights obtained depended upon the pore size of the filters used to clarify the solutions. The formation of strong gels, in the presence of gel promoting cations, is attributed to a localized ordered lateral association, or crystallization of regions of these fibrils. It is suggested that such a model for gelation may be of general applicability to a number of polysaccharide systems.