Competing Effects of Hydration and Cation Complexation in Single-Chain Alginate.

Area of Science:

• Polymer Science
• Materials Science
• Computational Chemistry

Background:

• Alginic acid, an anionic polyelectrolyte, forms hydrogels via cross-linking with metal cations.
• Cation interactions with alginic acid compete with hydrogen bonds, influencing gel structure and properties in aqueous solutions.
• Existing models often describe alginate chain flexibility without fully considering specific cation effects.

Purpose of the Study:

• To systematically analyze the interactions between alginic acid chains and sodium (Na+) and calcium (Ca2+) counterions using all-atom molecular dynamics simulations.
• To elucidate the distinct roles of Na+ and Ca2+ in modulating alginate chain conformation, hydrogen bonding, and water coordination.

Main Methods:

• All-atom molecular dynamics (MD) simulations were employed to model alginic acid in the presence of Na+ and Ca2+.
• Analysis focused on electrostatic interactions, hydrogen bond dynamics, and water coordination shells around the polyelectrolyte.

Main Results:

• Calcium cations (Ca2+) strongly bind to alginate by disrupting intramolecular hydrogen bonds within β-d-mannuronate (M) residues.
• Sodium cations (Na+) enhance intramolecular hydrogen bonds, stabilizing a left-hand, fourfold helical structure in poly-M alginate, leading to stiffer chains.
• Distinct effects of Ca2+ and Na+ on water coordination were observed: Ca2+ disrupts direct hydration, while Na+ significantly affects the second hydration layer.

Conclusions:

• The study challenges the traditional flexible flat-chain model for poly-M alginate in the presence of Na+.
• Cation-specific interactions significantly influence alginate chain conformation, hydrogen bonding networks, and hydration, thereby affecting solubility.
• Understanding these cation-dependent mechanisms is crucial for designing alginate-based materials with tailored properties.