Journal
GELS
Volume 8, Issue 12, Pages -Publisher
MDPI
DOI: 10.3390/gels8120784
Keywords
alginate; pectate; condensed and bound ions; egg-box model
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Pectate and alginate are important biopolymers that can form ionotropic gels when di- or multivalent counterions are added. Both polysaccharides exhibit similar gelation mechanisms with calcium ions, the most relevant counterion type. Specific Ca2+/polyion interactions play a crucial role in the formation of ion-mediated chain/chain cross-links. The affinity of Ca2+ for pectate is higher than for alginate. Calcium ions selectively accumulate near the polyanion(s), promoting the formation of chemical bonds and charge neutralization. Dilatometric and microcalorimetric changes are larger for pectate than for alginate. Light-scattering experiments confirm the intermolecular bonding of calcium ions from the early stages of linker addition.
Pectate and alginate are among the most important biopolymers able to give rise to ionotropic gelation upon the addition of di- or multivalent counterions. The two ionic polysaccharides exhibit several common aspects of the gelation mechanism with calcium ions, the physiologically and commercially most relevant counterion type. The first one pertains to the role that specific Ca2+/polyion interactions play in the establishment of the ion-mediated chain/chain cross-links. Such interactions include both a specific affinity of the territorially condensed Ca2+ counterions for the polyuronate(s) and the formation of long-lasting chemical bonding (inner ion-sphere complex) of specific interchain sites accompanied by high conformational ordering. As to the first mechanism, it is dominated by the strong desolvation of the interacting ionic species, with concomitant positive variations in both enthalpy and entropy, the contribution of the latter prevailing over the former due to the favorable liberation of a very large number of water molecules of hydration. Both dilatometric and microcalorimetric data point to the higher affinity of Ca2+ for pectate than for alginate. The selective accumulation of calcium ions close to the polyanion(s) favors the onset of the second-chemical bonding-mode, which is associated with charge neutralization at the bonding site. This mode coincides with the largely accepted egg-box model for the calcium-mediated interchain junction of pectate and alginate. A new approach was devised for the calculation of the fraction of chemically bound divalent ions; it was based on the available circular dichroism data (further supported by scattering and viscosity results) and successfully tested by comparison with an independently determined fraction in the case of pectate. In detail, the strong bonding mode manifests in two sequential bonding modes. The first one (at low concentrations of added Ca2+ ions) entails a cross-link in which only one calcium ions is bracketed in a twisted egg-box between two chains; upon further counterion addition, a series of nearest-neighboring perfect egg-box structures develops. Both dilatometric and microcalorimetric changes associated with the latter chemical bonding modes are quantitatively larger for pectate than for alginate; clearly the latter polyuronate suffers from the relevant presence of the weakly calcium-binding mannuronic acid repeating units. Light-scattering experiments provided a clear-cut demonstration of the intermolecular bonding of calcium ions from the very beginning of the linker addition.
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