Evidence for egg-box-compatible interactions in calcium-alginate gels from fiber X-ray diffraction

Area of Science:

• Polymer Science
• Materials Science
• Biochemistry

Background:

• Alginates are polysaccharides with diverse applications.
• Understanding alginate structure is crucial for controlling its properties.
• Guluronic acid-rich alginates are key components in hydrogel formation.

Purpose of the Study:

• To investigate the structural details of guluronic-acid-rich alginate in acid and calcium forms.
• To clarify discrepancies in reported chain axis repeats.
• To elucidate the structural basis of the egg-box model in calcium alginate.

Main Methods:

• Fiber X-ray diffraction was employed to analyze alginate fibers.
• Structural data were collected for alginate in both acid and calcium (Ca2+) forms.
• Analysis focused on chain axis repeat and lateral packing of polymer chains.

Main Results:

• A chain axis repeat of c = 0.87 nm was confirmed for acid-form alginate, contradicting previous reports.
• Calcium (Ca2+) induced dimerization of alginate chains, consistent with the egg-box model.
• Divalent cation coordination reduced lateral crystallographic packing of dimers, with chains packed on a hexagonal lattice (a = 0.66 nm).

Conclusions:

• The study confirms a specific chain axis repeat for acid-form alginate.
• The egg-box model accurately describes calcium-mediated dimerization in alginate.
• Disordered cations, water, and hydrogen bonding mediate interactions between dimers, influencing overall structure.