Alginate

PRONOVATM sodium alginates

This NovaMatrix product group belongs to a family of highly purified and well-characterized alginates developed for use in biomedical and pharmaceutical applications.


Chemistry
 


Alginate is a linear copolymer with homopolymeric blocks of (1®4)-linked b-D-mannuronate (M) and its C-5 epimer a-L-guluronate (G) residues, respectively, covalently linked together in different sequences or blocks.



The monomers can appear in homopolymeric blocks of consecutive G-residues (G-blocks), consecutive M-residues (M-blocks), alternating M and G-residues (MG-blocks) or randomly organized blocks [1-4]. The relative amount of each block type varies with the origin of the alginate.

Alternating blocks form the most flexible chains and are more soluble at lower pH than the other blocks. G-blocks form stiff chain elements, and two G-blocks of more than 6 residues each form stable cross-linked junctions with divalent cations (e.g. Ca2+, Ba2+, Sr2+ among others) leading to a three-dimensional gel network [5, 6].

At low pH, protonized alginates will form acidic gels. In these gels, it is mostly the homopolymeric blocks that form the junctions, where the stability of the gel is determined by the relative content of G-blocks [7].

Determination of primary structure is today possible by NMR techniques by analyzing the spectrum using appropriate statistical considerations [8, 9].

Gelation

In contrast to most other polysaccharide gels, alginate gels can develop and set at constant temperature. This unique property is particulary useful in applications involving fragile materials like cells or tissue with low tolerance for higher temperatures.

An alginate gel will develop instantaneously in the prescence of divalent cations like Ca2+, Ba2+ or Sr2+ and acid gels may also develop at low pH. Gelling occurs when the divalent cations take part in the interchain ionic binding between guluronic acids blocks (G-blocks) in the polymer chain giving rise to a three dimensional network. Such binding zones between the G-blocks are often referred to as egg-boxes, and consequently alginates with a high content of G-blocks induce stronger gels. Gels made of M-rich alginate are softer and more fragile, and may also have a lower porosity. This is due to the lower binding strength between the polymer chains and to the higher flexibilities of the molecules.

The gelling process is highly dependent on diffusion of gelling ions into the polymer network and there are essentially two main classical methods for the preparation of alginate gels: the dialysis/diffusion method, and the internal gelling method.  (NovaMatrix self-gelling technology provides an exciting alternative.)

In the dialysis/diffusion method (diffusion setting) gelling ions are allowed to diffuse into the alginate solution. This method is most commonly used in biotechnology for immobilization of living cells in alginate gel.

An alginate solution can also be solidifed by internal gelation method/internal setting, i.e. in situ gelling. Here a calcium salt with limited solubility, or complexed Ca2+-ions are mixed with an alginate solution into which the calcium ions are released, usually by the generation of acidic pH with a slowly acting acid such as D-glucono-a-lactone (GDL). This method is chosen if the purpose is to create a homogenous, non-syneretic alginate macrogel to fill the space of a given container. The main difference between internal and diffusion setting is the gelling kinetics. Consequently, the gel network will also be different.



References
[1] Smidsrød, O. and Draget, K.I., Carbohydr. Eur., 1996, 14, 6
[2] Ertesvåg, H., Valla, S. and Skjåk-Bræk, G., Carbohydr. Eur., 1996, 14, 14
[3] Skjåk-Bræk, G. and Espevik, T., Carbohydr. Eur., 1996, 14, 19
[4] Onsøyen, E., Carbohydr. Eur., 1996, 14, 26
[5] Stokke, B.T., Smidsrød, O., Bruheim, P. and Skjåk-Bræk, G., Macromolecules, 1991, 24, 4645
[6] Grant, G.T., Morris, E.R., Rees, D.A., Smith, P.J.C. and Thom, D., FEBS Lett, 1973, 32, 195
[7] Draget, K.I., Skjåk-Bræk, G., Christensen, B.E., Gåserød, O. and Smidsrød, O., Carbohydr. Polym., 1996, 29, 209
[8] Grasdalen, H., Larsen, B. and Smidsrød, O., Carbohydr. Res., 1981, 89, 179
[9] Grasdalen, H., Carbohydr. Res., 1983, 118, 255

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