Helical conformations of gellan gum

Dopamine-Functionalized Gellan Gum Hydrogel as a Candidate Biomaterial for a Retinal Pigment Epithelium Cell Delivery System

In this study, dopamine-functionalized gellan gum (DFG) hydrogel was prepared as a carrier for retinal pigment epithelium (RPE) cell delivery via a carbodiimide reaction. The carboxylic acid of gellan gum (GG) was replaced with catechol in a 21.3% yield, which was confirmed by NMR. Sol fraction and weight loss measurements revealed that dopamine improved degradability in the GG hydrogel. Measurements of the viscosity, injection force, and compressibility also showed that dopamine-functionalized GG hydrogels had more desirable rheological/mechanical properties for improving injectability. These characteristics were confirmed to arise from the GG's helix structure loosened by the dopamine's bulky nature. Moreover, dopamine's hydrophilic characteristics were confirmed to create a more favorable microenvironment for cell growth by promoting swelling capability and cell attachment. This improved biocompatibility became more pronounced when the hydrophilicity of dopamine was combined with a larger specific surface area stemming from the less porous structure of the dopamine-grafted hydrogels. This effect was apparent from the live/dead staining images of the as-prepared hydrogels. Meanwhile, the nonionic cross-linked DFG (DG) hydrogel showed the lowest protein expression in the immunofluorescence staining images obtained after 28 days of culture, supporting that it had the highest degradability and associated cell-releasing ability. That tendency was also observed in the gene expression data acquired by real-time polymerase chain reaction (RT-PCR) analysis. RT-PCR analysis also revealed that the DG hydrogel carrier could upregulate the visual function-related gene of RPE. Overall, the DG hydrogel system demonstrated its feasibility as a carrier of RPE cells and its potential as a means of improving visual function.

Application of double network of gellan gum and pullulan for bone marrow stem cells differentiation towards chondrogenesis by controlling viscous substrates

Hydrogels have a large amount of water that provides a cartilage-like environment and is used in tissue engineering with biocompatibility and adequate degradation rates. In order to differentiate stem cells, it is necessary to adjust the characteristics of the matrix such as stiffness, stress-relaxing time, and microenvironment. Double network (DN) hydrogels provide differences in cellular biological behavior and have interpenetrating networks that combine the advantages of the components. In this study, by varying the viscous substrate of pullulan (PL), the DN hydrogels of gellan gum (GG) and PL were prepared to determine the cartilage differentiation of bone marrow stem cell (BMSC). The characteristics of GG/PL hydrogel were investigated by examining the swelling ratio, weight loss, sol fraction, compressive modulus, and gelation temperature. The viability, proliferation, and toxicity of BMSCs encapsulated in hydrogels were evaluated. Cartilage phenotype and cartilage differentiation were confirmed by morphology, GAG content, and cartilage-specific gene expression. Overall results demonstrate that GG/PL hydrogels can form cartilage differentiation of BMSCs and can be applied for tissue engineering purposes.

Rigid, Fibrillar Quaternary Structures Induced by Divalent Ions in a Carboxylated Linear Polysaccharide

Polysaccharides are ubiquitous in nature; they serve fundamental roles in vivo and are used for a multitude of food, pharmaceutical, cosmetic biomaterials, and biomedical applications. Here, the structure-property function for low acetylated Gellan gum hydrogels induced by divalent ions was established by means of optical, rheological, and microscopic techniques. The hydrogels interacted with visible light as revealed by birefringence and multiple scattering, as a consequence of quaternary, supramolecular fibrillar structures. The molecular assembly and structure were elucidated by statistical analysis and polymer physics concepts applied to high-resolution AFM height images and further supported by FTIR. This revealed intramolecular coil-to-single helix transitions, followed by lateral aggregation of single helices into rigid, fibrillar quaternary structures, ultimately responsible for gelation of the system. Calcium and magnesium chloride were shown to lead to fibrils up to heights of 6.0 nm and persistence lengths of several micrometers. The change in molecular structure affected the macroscopic gel stiffness, with the plateau shear modulus reaching ∼10⁵ Pa. These results shed light on the two-step gelation mechanism of linear polysaccharides, their conformational molecular changes at the single polymer level and ultimately the macroscale properties of the ensued gels.

Dynamics of carbohydrate strands in water and interactions with clay minerals: influence of pH, surface chemistry, and electrolytes

Carbohydrate hydrogels are extensively used in pharmaceuticals and engineered biomaterials. Molecular conformations, assembly, and interactions of the carbohydrate strands with stabilizers such as clay minerals in aqueous solution are difficult to quantify in experiments and the hydrogel properties remain largely a result of trial-and-error studies. We analyzed the assembly of gellan gum in aqueous solution and interactions with dispersed clay minerals in all-atomic detail using molecular dynamics simulation, atomic force microscopy (AFM), and comparisons to earlier measurements. Gellan strands associate at low pH values of 2 and gradually disassemble to double strands with weak association of -0.4 kcal per mole carbohydrate ring as the pH values increases to 9. Ionization of the carbonic acid side groups in the backbone extends the chains and accelerates the conformational dynamics via rapidly changing intramolecular ion bridges. Gellan interactions with clay minerals depend on the strength of electric triple layers between clay, cations, and anionic polymer strands, as well as weaker hydrogen bonds along the edges, which are tunable as a function of the clay surface chemistry, local ionic strength, and pH values. Interaction energies range from -4 to +6 kcal per mol carbohydrate ring and were most favorable for electric triple layers with high charge mobility, which can be achieved for intermediate cation exchange capacity of the clay mineral and high pH values to increase ionization of the clay edges and of the polymer. The findings provide understanding and help control the dynamics and stabilization of carbohydrate hydrogels by clay minerals.

Role of the Drying Technique on the Low-Acyl Gellan Gum Gel Structure: Molecular and Macroscopic Investigations

The effect of three drying processes (freeze, oven and supercritical CO₂ drying) on CP Kelco low-acyl gellan gum gel was investigated, highlighting the role of the water removal mechanism (i.e. sublimation, evaporation and solvent replacement/extraction) and the process parameters on the gel structure, rather than focusing on the drying kinetics. It is the first time that a research paper not only compares the drying methods but also discusses and investigates how the molecular and macroscopic levels of gellan gum are affected during drying. Specifically, the dried gel structures were characterised by bulk density and shrinkage analyses as well as scanning electron microscope (SEM) and micro-computed tomography (μCT) microscopy. Micro-differential scanning calorimetry (μDSC) was used in a novel way to investigate the effect of the drying technique on the polymer disorder chains by partial melting of the gel. The resulting water uptake during rehydration was influenced by the obtained dried structure and, therefore, by the employed drying process. It was found that freeze-dried (FD) structures had a fast rehydration rate, while both oven-dried (OD) and supercritical CO₂-dried (scCO₂D) structures were slower. After 30 min, FD samples achieved a normalised moisture content (NMC) around 0.83, whereas OD and scCO₂D samples around 0.33 and 0.19, respectively. In this context, depending on the role of the specific hydrocolloid in food (i.e. gelling agent, thickener, carrier), one particular dried-gel structure could be more appropriate than another. Graphical abstractFrom left to right: unprocessed hydrogels; μ-CT images of dried gels and unprocessed hydrogel; DSC curves after drying process.

Characterization, Antimicrobial Properties and Coatings Application of Gellan Gum Oxidized with Hydrogen Peroxide

The effect of hydrogen peroxide (H₂O₂) oxidation on the physicochemical, gelation and antimicrobial properties of gellan gum was studied. The oxidized gellan gum (OGG) was characterized by measuring the carboxyl/carbonyl group contents, Fourier transform infrared spectroscopy (FTIR) and proton nuclear magnetic resonance (¹H-NMR) spectroscopy. The H₂O₂ oxidation resulted in a large increase in the carboxyl groups in gellan gum. The OGG lost gelation ability by oxidation even in the presence of metal ions. The antimicrobial activities of the OGG against Gram-positive bacteria (Staphylococcus aureus), Gram-negative bacteria (Escherichia coli), and fungal (Aspergillus niger) were tested. The OGG could inhibit the growth of both bacteria and fungal, and the activity was improved with an increase in the oxidation level. Finally, the application of the OGG as an active coatings material to extend the storage of apples was tested.

Characterisation of bacterial polysaccharides: steps towards single-molecular studies

Techniques used in studies of polysaccharides, including chemical composition, linkage pattern, and higher order structures are in constant development. They provide information necessary for understanding of the polysaccharide properties and functions. Here, recent advancements in studies of the polysaccharides at the single-molecule level are highlighted. Over the last few years, single-molecule techniques such as force spectroscopy have improved in sensitivity and can today be used to detect forces in the pN range. In addition, these techniques can be used to investigate properties of single molecules close to physiological conditions. The challenges in the interpretation of the observations are aided by control experiments using well-characterised polysaccharides and by data provided by complementary methods. This field is expected to have increasing impact on the further advancement of the molecular understanding of the role of polysaccharides in various biological processes such as recognition and cell adhesion.

Direct visualization of changes in deacylated Na(+) gellan polymer morphology during the sol-gel transition

Changes in gellan polymer morphology during the sol-gel transition were directly visualized by transmission electron microscopy and a model incorporating these changes and existing physical data is proposed. Our observations suggest that the most thermodynamically stable conformations of gellan polymers in solution, in the absence of added cations, are the double helix and double-helical duplexes. We have demonstrated two forms of lateral aggregation of gellan helices in the presence of Ca(2+) and K(+) ions. One type forms junction zones that lead to network formation and gelation, while the second type leads to the formation of isolated fibers of aggregated helices and inhibition of gelation. The proposed model of gellan gelation is based on these observations where thermoreversibility, gel strength, and endothermic transitions of gellan gels can be explained.

X-ray study of beijeran sodium salts, a new galacturonic acid-containing exo-polysaccharide

X-Ray fiber diffraction patterns were obtained from oriented films of sodium salts of a new uronic acid-containing polysaccharide (beijeran) both in its native, poly [-->3)-alpha-D-GalA-(1-->3)-beta-L-Rha-(1-->3)-alpha-D-Glc-O6Ac-(1 -->], and deacetylated forms. Initially the stretched films of both polysaccharides were amorphous, but the crystallinity was much improved by annealing at high temperature. The deacetylated specimen had higher crystallinity than the native. Both films showed similar X-ray fiber patterns indicating that these polysaccharides had similar unit cell dimensions and that the O-acetyl groups in the native beijeran chain did not disturb the regular array in the crystal having space group P21. All the visible reflections could be indexed in terms of a monoclinic unit cell with dimensions a = 1.277, b = 1.611, c (fiber axis) = 2.437 nm, and gamma = 96.79 degrees. The fiber axis length and the presence of (002) and (006) reflections indicated that the conformation was made up of two trisaccharide residues, in an extended two-fold helix.

Structure-function relationships in microbial exopolysaccharides

Sufficient well-characterized microbial exopolysaccharides are now available to permit extensive studies on the relationship between their chemical structure and their physical attributes. This is seen even in homopolysaccharides with relatively simple structures but is more marked when greater differences in structure exist, as are found in several heteropolysaccharides. The specific and sometimes unique properties have, in the case of several of these polymers, provided a range of commercial applications. The existence of "families" of structurally related polysaccharides also indicates the specific role played by certain structures and substituents; the characteristics of several of these microbial polysaccharide families will be discussed here. Thus, microbial exopolysaccharides frequently carry acyl groups which may profoundly affect their interactive properties although these groups often have relatively little effect on solution viscosity. Xanthan with or without acylation shows marked differences in synergistic gelling with plant gluco- and galacto-mannans, although the polysaccharides with different acylation patterns show similar viscosity. Similarly "gelrite" from the bacterium originally designated as Auromonas (Pseudomonas)elodea is of greater potential value after deacetylation, when it provides a valuable gelling agent, than it is as a viscosifier in the natural acylated form. The Klebsiella type 54 polysaccharide only forms gels when it, too, has been chemically deacetylated to give a structure equivalent to the Enterobacter XM6 polymer. Both these polysaccharides form gels due to the enhanced interaction with cations following deacylation and to the conformation adopted after removal of the acyl groups. Recent work in our laboratory suggests that deacetylation of certain bacterial alginates also significantly increases ion binding by these polysaccharides, making them more similar in their properties to algal alginates even although the alginates from some Pseudomonas species lack poly-L-guluronic acid sequences. The existence within families of polysaccharides of types in which monosaccharides are altered within a specific structure, or with varying side-chains, also gives an indication of the way in which such substituents affect the physical properties of the polymers in aqueous solution.

X-ray and computer modeling studies on gellan-related polymers: molecular structures of welan, S-657, and rhamsan

The primary structures of the four bacterial polysaccharides gellan, welan, S-657, and rhamsan are the same with respect to their backbones, but have different side-chains. This difference has a profound influence on their behavior in aqueous media. Solutions of gellan gum form stable aqueous gels under appropriate ionic conditions. By contrast, welan, S-657, and rhamsan do not gel but give very viscous solutions over a wide range of thermal, pH, and salt conditions. X-Ray fiber diffraction analysis and computer modeling of these branched polysaccharides demonstrate that they all have the same half-staggered, double-helical conformations as in the unbranched gellan, suggesting, therefore, that the side chains are responsible for diminishing gelling behavior. Depending on the size and location, the side chains shield the carboxylate groups to varying degrees; this shielding is substantial in welan and S-657, but less in rhamsan. In all cases, side-chain-main-chain interactions within the double helix prevent the carboxylate-mediated aggregation of double helices that is necessary for the gelation.

On the physicochemical properties of gellan gum

This paper concerns the behavior in dilute and demidilute solutions of deacetylated gellan. The conformational transition, controlled by temperature and ionic strength, is investigated. It corresponds to a double-helix single-chain transition. Large ionic selectivity is observed in the helical conformation th at controls the degree of aggregation upon gelation. Potentiometry and conductivity measurements are interpreted in terms of the Manning polyelectrolyte theory in the sol state.

The dietary effects of gellan gum in humans

Following a 7-day control period, five female and five male volunteers consumed a weight of gellan gum corresponding to 175 mg/kg body weight for 7 days, followed by 200 mg gellan gum per kg body weight for a further 16 days. Measurements before and at the end of the 23-day test period showed that the gellan gum acted as a faecal bulking agent for the male volunteers and for four of the females. Dietary transit time increased for 2 females and 2 males, and decreased for 3 females and 3 males. Faecal bile acid concentrations increased for 4 females and for 4 males; the average increases were from 0.69 to 0.83 mmol/24 h (females) and from 1.22 to 1.44 mmol/24 h (males). Gellan gum ingestion had no significant effect on (a) plasma biochemistry parameters; (b) haematological indices; (c) urinalysis parameters; (d) blood glucose and plasma insulin concentrations; (e) breath hydrogen concentrations. There were no significant changes in HDL cholesterol, triglyceride or phospholipid concentrations. Serum cholesterol concentrations decreased significantly (P less than 0.1) by 13% on average for females, and by 12%, on average, for males. The data indicate that the ingestion of gellan gum at a high level for 23 days caused no adverse dietary or physiological effects in any of the volunteers. In particular, the enzymatic and other indicators of adverse toxicological effects remained unchanged.