Reexamining the Egg-Box Model in Calcium−Alginate Gels with X-ray Diffraction

Abstract

The structure of the Ca--alginate junction zones was investigated with X-ray scattering on gels prepared with different methods. Fiber diffraction reveals the popular egg-box model may not be the only possible structure for the junction zones. The (001) reflection, which should be extinguished due to 2/1 helical conformation in the egg-box model, was observed. This was further confirmed by the measurements on Ca--alginate gel beads prepared at different pH where large pieces were formed through a relatively slow process, which leads to a higher crystallinity and a more perfect ordering. The results suggest a 3/1 helical conformation is more proper for Ca--alginate gels formed slowly. This does not exclude the possibility for the 2/1 helical conformation in fast gelatinized Ca--alginate in which the 2/1 helix is a metastable form. Comparing the X-ray scattering results of the fresh, dehydrated, and rehydrated gels, a reversible aggregation of junction zones is found during dehydration and rehydration. The different stabilities of initial bonds and bonds formed during drying are speculated to be the contribution of MG block or short G blocks in the junction zones.

Drying and Rehydration of Calcium Alginate Gels

“…14 , the peaks at 2θ=13°and 21.5°are labelled as (110) and (002), respectively (the fibre axis is defined as c-axis). Note that a small shoulder can be observed at 2θ=10.6°in the diffraction pattern in Figure 7b; the origin of this shoulder has been studied in detail in our laboratory and is discussed in another paper 15 . Figure 8 displays X-ray scattering patterns of air-dried alginate samples with different calcium concentrations (the samples were prepared using the dialysis membrane method as explained in the "Materials and Methods").…”

Hierarchical cross-linking in physical alginate gels: a rheological and dynamic light scattering investigation

“…Within this picture, the sample is endowed with a given amount of internal stress unavoidably generated during gelation, since gels form in a microscopically heterogeneous configuration. The links resulting from ionic bonds are expected to be reversible in time 44 ; thus, bonds break and reform with time, thereby locally relaxing internal stress. One therefore expects the dynamics to slow down with sample age, because the driving force, viz the internal stress, would decrease progressively.…”

Some properties of alginate gels derived from algal sodium alginate

“…Donati et al (2005) reported that the frictional force between the calcium-alginate gel network chains increases with increasing length of the alternating mannuronate-guluronate (MG) sequences. Li et al (2007) reported that MG block or short G blocks in calcium-alginate junction zones contribute gel bond stability.…”