Ion-Induced Polysaccharide Gelation: Peculiarities of Alginate Egg-Box Association with Different Divalent Cations

Abstract

Structural aspects of polysaccharide hydrogels based on sodium alginate and divalent cations Ba2+, Ca2+, Sr2+, Cu2+, Zn2+, Ni2+ and Mn2+ was studied using data on hydrogel elemental composition and combinatorial analysis of the primary structure of alginate chains. It was shown that the elemental composition of hydrogels in the form of freezing dried microspheres gives information on the structure of junction zones in the polysaccharide hydrogel network, the degree of filling of egg-box cells by cations, the type and magnitude of the interaction of cations with alginate chains, the most preferred types of alginate egg-box cells for cation binding and the nature of alginate dimers binding in junction zones. It was ascertained that metal–alginate complexes have more complicated organization than was previously desired. It was revealed that in metal–alginate hydrogels, the number of cations of various metals per C12 block may be less than the limiting theoretical value equal to 1 for completely filled cells. In the case of alkaline earth metals and zinc, this number is equal to 0.3 for calcium, 0.6 for barium and zinc and 0.65–0.7 for strontium. We have determined that in the presence of transition metals copper, nickel and manganese, a structure similar to an egg-box is formed with completely filled cells. It was determined that in nickel–alginate and copper–alginate microspheres, the cross-linking of alginate chains and formation of ordered egg-box structures with completely filled cells are carried out by hydrated metal complexes with complicated composition. It was found that an additional characteristic of complex formation with manganese cations is the partial destruction of alginate chains. It has been established that the existence of unequal binding sites of metal ions with alginate chains can lead to the appearance of ordered secondary structures due to the physical sorption of metal ions and their compounds from the environment. It was shown that hydrogels based on calcium alginate are most promising for absorbent engineering in environmental and other modern technologies.

Diversity of Bioinspired Hydrogels: From Structure to Applications

“…The most studied ion-induced gelation is crossinking Alg macromolecules in the presence of Ca 2+ . The intermolecular junctions are formed by chelation of Ca 2+ between G-blocks by a bimolecular mechanism (the gel strength increases with the square of Alg concentration) [200] through the so-called "eggbox" association [7,194,[208][209][210][211]. Strong interactions of cations with COO − groups of guluronic acid from different chains occur in the cavities formed by pairing up of the successive G sequences forming two-fold structures that create cavities to hold Ca 2+ in the binding sites [7,194,210,212].…”

“…The intermolecular junctions are formed by chelation of Ca 2+ between G-blocks by a bimolecular mechanism (the gel strength increases with the square of Alg concentration) [200] through the so-called "eggbox" association [7,194,[208][209][210][211]. Strong interactions of cations with COO − groups of guluronic acid from different chains occur in the cavities formed by pairing up of the successive G sequences forming two-fold structures that create cavities to hold Ca 2+ in the binding sites [7,194,210,212]. Ca 2+ cations coordinate with six oxygens (O2, O3, O6) from two neighbouring G units and with one to three oxygens of H 2 O and form a stable "egg-box" structure [210,213].…”

“…The nanoporous network, which is formed by ionic crosslinking, is inert to cells and provides an appropriate mechanical microenvironment for cells [198], drug delivery matrices for therapeutic agents, and applications in wound dressing and 3D bioprinting [227]. In addition, among the divalent cations, calcium is the most suitable for high-absorbent alginate hydrogels used in various environmental and biomedical applications [7].…”

“…The physical crosslinking can be provided by different intraand intermolecular interactions, such as hydrogen bonding, van der Waals interactions, hydrophobic interactions, or physical entanglements, without covalent crosslinking. On the other hand, chemically crosslinked hydrogels are formed due to the covalent or ionic bonds between the polymeric chains, and this involves the use of some crosslinking agents (molecules that contain two or more reactive sites able to be attached to specific functional groups) [4][5][6][7].…”

“…Natural or synthetic macromolecules, as well as various combinations of them, can be used to create hydrogels [6][7][8][9][10][11][12][13]. Hydrogels prepared from natural macromolecules, including polysaccharides and proteins, are suitable for a variety of applications due to their biochemical similarity with the human extracellular matrix (ECM).…”