Using SAXS to reveal the degree of bundling in the polysaccharide junction zones of microrheologically distinct pectin gels

The results of microrheological studies carried out on ionotropic pectin gels, particularly the manifest power law behavior observed at high frequencies, indicate that by using different assembly conditions gels can be formed in which the elementary network strands have different stiffnesses. It has been hypothesized that these differences reflect different network architectures, the extreme cases of which might be described as (i) dimeric calcium-chelating junction-zones of limited extent, linked by considerably longer, flexible, single-chain sections, or (ii) semiflexible bundles consisting of extensively aggregated dimeric junction zones that latterly become entangled and cross-linked. To test this hypothesis directly, microrheologically distinct pectin gels have been generated using different assembly modalities, in particular by using different concentrations of polymer and cross-linking ions and by contrasting the controlled-release of ions or ion-binding groups, and the resulting systems have been studied by small-angle X-ray scattering. The results straightforwardly reveal that gels that are clearly more semiflexible from a microrheological point-of-view contain larger scattering entities than those with a more flexible character. Furthermore, a more detailed interpretation of the scattering data with the aid of molecular modeling suggests that for the gels formed here those with a semiflexible microrheological signature consist predominantly of network filaments consisting of four or more chains, whereas those with a more flexible signature are predominantly single-chain sections linked by dimeric associations with no more that a few percent of the chains bundled to any higher extent. The ability to generate differing network architectures from the same polymer that fulfill different functional requirements, either in vivo in the plant cell wall, where pectin plays a crucial structural and mechanical role, or in vitro in a myriad of applications, makes these biomimetic biopolymer networks of considerable interest.