New Hypothesis on the Role of Alternating Sequences in Calcium−Alginate Gels

Abstract

The availability of mannuronan and mannuronan C-5 epimerases allows the production of a strictly alternating mannuronate-guluronate (MG) polymer and the MG-enrichment of natural alginates, providing a powerful tool for the analysis of the role of such sequences in the calcium-alginate gel network. In view of the calcium binding properties of long alternating sequences revealed by circular dichroism studies which leads eventually to the formation of stable hydrogels, their direct involvement in the gel network is here suggested. In particular, 1H NMR results obtained from a mixed alginate sample containing three polymeric species, G blocks, M blocks, and MG blocks, without chemical linkages between the block structures, indicate for the first time the formation of mixed junctions between G and MG blocks. This is supported by the analysis of the Young's modulus of hydrogels from natural and epimerized samples obtained at low calcium concentrations. Furthermore, the "zipping" of long alternating sequences in secondary MG/MG junctions is suggested to account for the shrinking (syneresis) of alginate gels in view of its dependence on the length of the MG blocks. As a consequence, a partial network collapse, macroscopically revealed by a decrease in the Young's modulus, occurred as the calcium concentration in the gel was increased. The effect of such "secondary" junctions on the viscoelastic properties of alginate gels was evaluated measuring their creep compliance under uniaxial compression. The experimental curves, fitted by a model composed of a Maxwell and a Voigt element in series, revealed an increase in the frictional forces between network chains with increasing length of the alternating sequences. This suggests the presence of an ion mediated mechanism preventing the shear of the gel.

Analysis of G-Block Distributions and Their Impact on Gel Properties of in Vitro Epimerized Mannuronan

“…21,22 This is thought to be a consequence of increased elastic segment flexibility 23 and formation of secondary MG/MG junctions through a calcium dependent zipping mechanism. 24 There was a big difference in the syneresis in Ca-limited gels and Ca-saturated gels in this study. No syneresis could be observed in the Ca-limited gels (data not shown) hence the syneresis was due to increased crosslinking following increased calcium concentrations.…”

Drying and Rehydration of Calcium Alginate Gels

“…before drying) only relatively long guluronate segments are involved in junction zone formation. Upon drying, shorter guluronate segments or mixed mannuronic-guluronic blocks may join in and become involved in junction zone formation 18 . It is noted that multimer structures are not formed in the absence of calcium ions (i.e.…”

Photochemical Patterning of Ionically Cross-Linked Hydrogels

“…These residues are present in the form of homopolymeric GG and MM blocks as well as heteropolymeric GM blocks. In the presence of polyvalent metal cations, particularly Ca 2+ , GG blocks of alginate chains form "egg-box" domains which ionically cross-link two neighboring GG blocks [1][2][3]. The resultant hydrogels are non-immunogenic, porous, and thermally stable.…”