The role of poly-M and poly-GM sequences in the metal-mediated assembly of alginate gels

Preparation of hyaluronic acid-loaded liquid-core hydrogel beads with acceptable mechanical properties and thermal stability

BACKGROUND

Hyaluronic acid liquid-core hydrogel beads (HA-LHB) is a good way for oral intake of HA. However, HA may affect the reaction-diffusion of sodium alginate (SA) and Ca2+ leading to poor mechanical properties, since HA is a polyanionic electrolyte having electrostatic effect and a certain spatial site-blocking effect.

RESULTS

The mechanical properties of HA-LHB were modified from bathing solution, core solution and secondary calcium bath time. The mechanical properties varied with the SA structure and concentration in bathing solution, where SA with high G (guluronic acid) segment compounded with SA with high M (mannuronic acid) segment at a mass ratio of 7:3 with a 11 g kg-1 concentration showed the best mechanical properties. The secondary calcium bath can greatly improve the mechanical properties due to the tight network formed by bidirectional crosslinking, and 15 min reaction reached the plateau if Ca2+ is sufficient. And the mechanical properties were positively correlated with calcium lactate concentration only at <70 g kg-1 in core solution, but the diffusion of Ca2+ was hindered by the tight gel network at higher concentrations. Moreover, the mechanical properties can be maintained during heat treatment, due to the rearrangement of alginate network structure.

CONCLUSION

Our results suggested that the problem of poor mechanical properties of LHB in the presence of high HA concentration can be avoided by process control, which may broaden the development of HA and popping boba market. © 2024 Society of Chemical Industry.

Alginate-metal cation interactions: Macromolecular approach

Alginates are a broad family of linear (unbranched) polysaccharides derived from brown seaweeds and some bacteria. Despite having only two monomers, i.e. β-d-mannuronate (M) and its C5 epimer α-l-guluronate (G), their blockwise arrangement in oligomannuronate (..MMM..), oligoguluronate (..GGG..), and polyalternating (..MGMG..) blocks endows it with a rather complex interaction pattern with specific counterions and salts. Classic polyelectrolyte theories well apply to alginate as polyanion in the interaction with monovalent and non-gelling divalent cations. The use of divalent gelling ions, such as Ca2+, Ba2+ or Sr2+, provides thermostable homogeneous or heterogeneous hydrogels where the block composition affects both macroscopic and microscopic properties. The mechanism of alginate gelation is still explained in terms of the original egg-box model, although over the years some novel insights have been proposed. In this review we summarize several decades of research related to structure-functionships in alginates in the presence of non-gelling and gelling cations and present some novel applications in the field of self-assembling nanoparticles and use of radionuclides.

How Accurate Is the Egg-Box Model in Describing the Binding of Calcium to Polygalacturonate? A Molecular Dynamics Simulation Study

We performed molecular dynamics (MD) simulations of octameric galacturonate, GalA₈, chains in the presence of Ca²⁺ in a ratio of R = [Ca²⁺]/[GalA] = 0.25 in order to determine to which extent the popular "egg-box model" (EBM) is able to describe the association between Ca²⁺ cations and polygalacturonate (polyGalA) chains. To this aim, we slightly revised the empirical parameters for the interaction between Ca²⁺ and the carboxylate oxygen atoms of GalA units so as to reproduce the experimental Ca²⁺-GalA association constant. We also defined an ad hoc order parameter, referred to as the egg-box score (EBS), that quantifies any deviation of the local coordination geometry of calcium cations with respect to an "ideal" EBM coordination geometry. The results reveal that the local coordination geometry of Ca²⁺ cations bound to polyGalA chains differs from that of the EBM. Moreover, polyGalA chains exhibit significant conformational disorder, and the cross-link angles formed between polyGalA chain axes are broadly distributed. Overall, the present study suggests that the EBM fails to describe accurately the association modes between calcium and polyGalA chains at a molar ratio R of 0.25.

Polyguluronate simulations shed light onto the therapeutic action of OligoG CF-5/20

Chronic mucoid P. aeruginosa cystic fibrosis (CF) lung infections are associated with the development of a biofilm composed of anionic acetylated exopolysaccharide (EPS) alginate, electrostatically stabilised by extracellular Ca2+ ions. OligoG CF-5/20, a low molecular weight guluronate rich oligomer, is emerging as a novel therapeutic capable of disrupting mature P. aeruginosa biofilms. However, its method of therapeutic action on the mucoid biofilm EPS is not definitively known at a molecular level. This work, utilising molecular dynamics (MD) and Density-Functional Theory (DFT), has revealed that OligoG CF-5/20 interaction with the EPS is facilitated solely through bridging Ca2+ ions, which are not liberated from their native EPS binding sites upon OligoG CF-5/20 dispersal, suggesting that OligoG CF-5/20 does not cause disruptions to mature P. aeruginosa biofilms through breaking EPS-Ca2+-EPS ionic cross-links. Rather it is likely that the therapeutic activity arises from sequestering free Ca2+ ions and preventing further Ca2+ induced EPS aggregation.

Atomic-scale interactions between quorum sensing autoinducer molecules and the mucoid P. aeruginosa exopolysaccharide matrix

Mucoid Pseudomonas aeruginosa is a prevalent cystic fibrosis (CF) lung coloniser whose chronicity is associated with the formation of cation cross-linked exopolysaccharide (EPS) matrices, which form a biofilm that acts as a diffusion barrier, sequestering cationic and neutral antimicrobials, and making it extremely resistant to pharmacological challenge. Biofilm chronicity and virulence of the colony is regulated by quorum sensing autoinducers (QSAIs), small signalling metabolites that pass between bacteria, through the biofilm matrix, regulating genetic responses on a population-wide scale. The nature of how these molecules interact with the EPS is poorly understood, despite the fact that they must pass through EPS matrix to reach neighbouring bacteria. Interactions at the atomic-scale between two QSAI molecules, C₄-HSL and PQS—both utilised by mucoid P. aeruginosa in the CF lung—and the EPS, have been studied for the first time using a combined molecular dynamics (MD) and density functional theory (DFT) approach. A large-scale, calcium cross-linked, multi-chain EPS molecular model was developed and MD used to sample modes of interaction between QSAI molecules and the EPS that occur at physiological equilibrium. The thermodynamic stability of the QSAI-EPS adducts were calculated using DFT. These simulations provide a thermodynamic rationale for the apparent free movement of C₄-HSL, highlight key molecular functionality responsible for EPS binding and, based on its significantly reduced mobility, suggest PQS as a viable target for quorum quenching.

Competing Effects of Hydration and Cation Complexation in Single-Chain Alginate

Alginic acid, a naturally occurring anionic polyelectrolyte, forms strong physically cross-linked hydrogels in the presence of metal cations. The latter engage in electrostatic interactions that compete with intra- and intermolecular hydrogen bonds, determining the gel structure and properties of the system in aqueous media. In this study, we use all-atom molecular dynamics simulations to systematically analyze the interactions between alginic acid chains and Na⁺ and Ca²⁺ counterions. The formed alginates originate from the competition of intramolecular hydrogen bonding and water coordination around the polyelectrolyte. In contrast to the established interpretation, we show that calcium cations strongly bind to alginate by disrupting hydrogen bonds within (1 → 4)-linked β-d-mannuronate (M) residues. On the other hand, Na⁺ cations enhance intramolecular hydrogen bonds that stabilize a left-hand, fourfold helical chain structure in poly-M alginate, resulting in stiffer chains. Hence, the traditionally accepted flexible flat-chain model for poly-M sequence is not valid in the presence of Na⁺. The two cations have a distinct effect on water coordination around alginate and therefore on its solubility. While Ca⁺ disrupts water coordination directly around the alginate chains, mobile Na⁺ cations significantly disrupt the second hydration layer.

Revisiting Cation Complexation and Hydrogen Bonding of Single-Chain Polyguluronate Alginate

Modifying the properties of bio-based materials has garnered increasing interest in recent years. In related applications, the ability of alginates to complex with metal ions has been shown to be effective in liquid-to-gel transitions, useful in the development of foodstuff and pharma products as well as biomaterials, among others. However, despite its ubiquitous use, alginate behavior as far as interactions with cations is not fully understood. Hence, this study presents a detailed comparison of alginate's complexation with Na⁺ and Ca²⁺ and the involved intramolecular hydrogen bonding and biomolecular chain geometry. Using all-atom molecular dynamics simulations, we find that in contrast to accepted models, calcium cations strongly bind to alginate chains by disruption of hydrogen bonds between neighboring residues, stabilizing a left-hand, 3-fold helical chain structure that enhances chain stiffness. Hence, while present, the traditionally accepted egg-box binding mode was a minor subset of possible conformations. For a single chain, most of the cation binding occurred as single-cation interaction with a carboxyl group, without the coordination of other alginate oxygens. The monovalent Na⁺ ions were found to be mostly nonlocalized around alginate and therefore do not compete with intramolecular hydrogen bonding. The different binding modes observed for Na⁺ and Ca²⁺ contribute toward explaining the different solubility of sodium and calcium alginate.

Cation complexation by mucoid Pseudomonas aeruginosa extracellular polysaccharide

Mucoid Pseudomonas aeruginosa is a prevalent cystic fibrosis (CF) lung colonizer, producing an extracellular matrix (ECM) composed predominantly of the extracellular polysaccharide (EPS) alginate. The ECM limits antimicrobial penetration and, consequently, CF sufferers are prone to chronic mucoid P. aeruginosa lung infections. Interactions between cations with elevated concentrations in the CF lung and the anionic EPS, enhance the structural rigidity of the biofilm and exacerbates virulence. In this work, two large mucoid P. aeruginosa EPS models, based on β-D-mannuronate (M) and β-D-mannuronate-α-L-guluronate systems (M-G), and encompassing thermodynamically stable acetylation configurations–a structural motif unique to mucoid P. aeruginosa–were created. Using highly accurate first principles calculations, stable coordination environments adopted by the cations have been identified and thermodynamic stability quantified. These models show the weak cross-linking capability of Na⁺ and Mg²⁺ ions relative to Ca²⁺ ions and indicate a preference for cation binding within M-G blocks due to the smaller torsional rearrangements needed to reveal stable binding sites. The geometry of the chelation site influences the stability of the resulting complexes more than electrostatic interactions, and the results show nuanced chemical insight into previous experimental observations.

High strength and strain alginate fibers by a novel wheel spinning technique for knitting stretchable and biocompatible wound-care materials

Alginate fibrous materials have been applied as wound dressing to enhance wound healing due to its nontoxic, biodegradable, and hemostatic nature. Conventional nonwoven fabrication tactics, however, showed weakness in inflammation, degradation stability and mechanical properties. Herein, the wet-spun alginate fibers were prepared by a novel wheel spinning technique, then knitted into wound dressing. Benefiting from optimized wet spinning parameters and the agglomeration of alginate multimers, the fibers were endowed with elevated mechanical performances and biodegradability, which allowed for the feasibility of knitting wound-care materials. Using the new wheel spinning technique, high strength alginate fibers with 173 MPa were produced with breaking strain up to 18% and toughness of 16.16 MJ*m^-3. Meanwhile, alginate fibers with high breaking strain reaching 35% were produced with tensile strength of 135 MPa and toughness of 37.47 MJ*m^-3. The overall mechanical performances of these alginate fibers with high breaking strain are significantly higher (up to 2 times) than those published in the literature in term of toughness. In vitro degradation evaluation revealed that this wet spun fibrous dressing had good aqueous absorbency (50%) and sustained biodegradation properties. Furthermore, the consequent cell viability study also proved that this alginate knitted fabric is biocompatible for being applied as wound dressing.

Egg-box model-based gelation of alginate and pectin: A review

Alginate and pectin are emblematic natural polyuronates that have been widely used in food, cosmetics and medicine. Ca-dependent gelation is one of their most important functional properties. The gelation mechanisms of alginate and pectin, known as egg-box model, were believed to be basically the same, because their Ca-binding sites show a mirror symmetric conformation. However, studies have found that the formation and the structure of egg-box dimmers between alginate and pectin were different. Very few studies have reviewed those differences. Therefore, this study was proposed to first summarize the intrinsic and extrinsic factors that can influence the gelation of alginate and pectin. The differences in the effect of these factors on the gelation of alginate and pectin were then discussed. Meanwhile, the similarity and difference in their gelation mechanism was also summarized. The knowledge gained in this review would provide useful information for the practical applications of alginate and pectin.

EPS Glycoconjugate Profiles Shift as Adaptive Response in Anaerobic Microbial Granulation at High Salinity

Anaerobic granulation at elevated salinities has been discussed in several analytical and engineering based studies. They report either enhanced or decreased efficiencies in relation to different Na⁺ levels. To evaluate this discrepancy, we focused on the microbial and structural dynamics of granules formed in two upflow anaerobic sludge blanket (UASB) reactors treating synthetic wastewater at low (5 g/L Na⁺) and high (20 g/L Na⁺) salinity conditions. Granules were successfully formed in both conditions, but at high salinity, the start-up inoculum quickly formed larger granules having a thicker gel layer in comparison to granules developed at low salinity. Granules retained high concentrations of sodium without any negative effect on biomass activity and structure. 16S rRNA gene analysis and Fluorescence in Situ Hybridization (FISH) identified the acetotrophic Methanosaeta harundinacea as the dominant microorganism at both salinities. Fluorescence lectin bar coding (FLBC) screening highlighted a significant shift in the glycoconjugate pattern between granules grown at 5 and 20 g/L of Na⁺, and the presence of different extracellular domains. The excretion of a Mannose-rich cloud-like glycoconjugate matrix, which seems to form a protective layer for some methanogenic cells clusters, was found to be the main distinctive feature of the microbial community grown at high salinity conditions.

Fast anaerobic sludge granulation at elevated salinity

It is commonly accepted that high salt concentrations negatively affect microbial activity in biological wastewater treatment reactors such as upflow anaerobic sludge blanket (UASB) reactors. Microbial aggregation in such reactors is equally important. It is well documented that anaerobic granules, when exposed to high salinity become weak and disintegrate, causing wash-out, operational problems and decreasing process performance. In this research, the possibility of microbial granule formation from dispersed biomass was investigated at salinity levels of 5 and 20 g Na⁺/L. High removal efficiencies of soluble influent organics were achieved at both salinity levels and this was accompanied by fast and robust formation of microbial granules. The process was found to be stable for the entire operational period of 217 days. As far as we know this is the first time it has been demonstrated that stable granule formation is possible at a salinity level as high as 20 g Na⁺/L. Methanosaeta was identified as the dominant methanogen at both salinity levels. Streptococcus spp. and bacteria belonging to the family Lachnospiraceae were identified as the dominant microbial population at 5 and 20 and g Na⁺/L, respectively.

Structural Characterization of Sodium Alginate and Calcium Alginate

Alginate readily aggregates and forms a physical gel in the presence of cations. The association of the chains, and ultimately gel structure and mechanics, depends not only on ion type, but also on the sequence and composition of the alginate chain that ultimately determines its stiffness. Chain flexibility is generally believed to decrease with guluronic residue content, but it is also known that both polymannuronate and polyguluronate blocks are stiffer than heteropolymeric blocks. In this work, we use atomistic molecular dynamics simulation to primarily explore the association and aggregate structure of different alginate chains under various Ca(2+) concentrations and for different alginate chain composition. We show that Ca(2+) ions in general facilitate chain aggregation and gelation. However, aggregation is predominantly affected by alginate monomer composition, which is found to correlate with chain stiffness under certain solution conditions. In general, greater fractions of mannuronic monomers are found to increase chain flexibility of heteropolymer chains. Furthermore, differences in chain guluronic acid content are shown to lead to different interchain association mechanisms, such as lateral association, zipper mechanism, and entanglement, where the mannuronic residues are shown to operate as an elasticity moderator and therefore promote chain association.