Thermorheological mapping and molecular insights into salt-dependent gel-like network formation in Mortierella alpina chitin-like exopolysaccharide solutions

Abstract

Fungal chitin-like exopolysaccharides are promising fermentation-derived materials, however the molecular-level determinants of their salt- and temperature-dependent gel-like behavior remain poorly understood. Here, a previously characterized Mortierella alpina exopolysaccharide was re-examined by integrating oscillatory rheology with all-atom molecular dynamics simulations. Frequency sweeps across 5-15 mg mL-1, two ionic media (0.154 mol L-1 NaCl and 0.28 mol L-1 LiCl), and 10-60 C were used to map liquid-like, transitional, and elastic-dominated regimes. Increasing concentration strengthened network behavior, but the two salts followed distinct pathways: NaCl promoted a more coherent progression toward elastic dominance and more thermorheologically compatible relaxation behavior, whereas LiCl produced a high-dissipation intermediate regime and stronger temperature dependence. Simulations of a minimal multichain model comprising eight GlcNAc10 oligomers at approximately 15.8 mg mL-1 showed that NaCl favored larger, more compact assemblies despite weak and short-lived Na+-oxygen coordination. LiCl formed sharper, longer-lived first-shell contacts but maintained smaller and more expanded assemblies. Ion-mediated interchain bridges were sparse and transient. The combined results show that local cation binding strength does not directly predict collective network formation and support a dynamic physical-network mechanism governed by salt-specific coupling among hydration, ion exchange, chain packing, and reversible interchain association.