Paramagnetic Probing of Humic Acid Supramolecular Structure: Iron (III)-Induced Rearrangements Revealed by...

Humic acids (HAs), key components of soil organic matter, undergo significant conformational rearrangements upon complexation with metal ions, yet the molecular-scale dynamics of these interactions remain poorly understood. In this study, solid-state 13C CPMAS NMR spectroscopy was employed to probe Fe (III)-induced structural changes in a soil-derived HA, focusing on cross-polarization time constants (T_CH) and proton rotating-frame relaxation times (T_₁ρ[H]) as indicators of spatial and dynamic reorganization. Exponential decay analysis of T_CH versus Fe/C ratios revealed distinct binding hierarchies: carboxyl groups (187-163 ppm) exhibited the strongest response (decay constant k_TCH = 3.5), reflecting Fe (III)-driven local compaction and rigidification, whereas aromatic (163-92 ppm; k_TCH = 2.5) and oxygenated aliphatic (92-46 ppm; k_TCH = 1.5) domains showed intermediate sensitivity. Aliphatic carbons (46-0 ppm; k_TCH ≈ 0.05) remained inert, confirming their exclusion from metal coordination. Complementary T_1ρ(H) data highlighted the role of Fe (III) in restricting molecular mobility, particularly in carboxyl and aromatic regions. These findings support a model in which Fe (III) acts as a supramolecular cross-linker, selectively rigidifying HA domains through primary carboxylate binding and secondary aromatic interactions, whereas aliphatic regions retain dynamic freedom. The study demonstrates that paramagnetic Fe (III) not only perturbs NMR detectability but also serves as a structural probe, with T_CH and T_1ρ(H) providing quantitative metrics for metal-induced reorganization. This work advances the mechanistic understanding of organo-mineral interactions in soils and highlights NMR relaxation parameters as powerful tools for characterizing environmental organic matter.