Structure, network density and domain architecture of transition-metal (Cu2+, Fe3+) cross-linked alginates as seen by paramagnetic solid-state nmr spectroscopy

Owing to the regenerative nature of metal-coordination bonds, transition metal-crosslinked alginates are of increasing technological relevance. However, due to their amorphous character and paramagnetic nature, their atomic-resolution structure, which is crucial for optimizing physicochemical properties, has long remained elusive. By employing very fast (VF) magic angle spinning (MAS) nuclear magnetic resonance (NMR) we overcame experimental challenges and obtained previously unreported data on the structure, network density, and domain architecture of alginates crosslinked by paramagnetic Cu²⁺ and Fe³⁺ ions, and modified by lactic acid - a pharmaceutically relevant ingredient that alters pH and crosslinking mechanism. We identified a high proportion of carboxyl units COOH coexisting with deprotonated carboxylate COO^- groups, nanoscale acidic domains, and strongly hyperfine-coupled nuclei within the paramagnetic matrix, revealing the specific coordination geometry of the Cu²⁺ and Fe³⁺ ions utilizing hydroxyl groups. These results provide new insight into the structural complexity of amorphous paramagnetic biopolymers and demonstrate the analytical power of VF/MAS NMR in challenging environments. The present research thus aims to provide one of the first experimentally confirmed representations of the complex interactions in alginates crosslinked with paramagnetic transition metal ions. These findings have direct implications for the synthesis of novel functional materials based on carbohydrate polymers.