Analytical and thermodynamic data, EPR, FTIR, solution 1 H and solid-state 13 C cross polarization magic angle spinning NMR and solid-state extended X-ray absorption fine structure (EXAFS) and X-ray absorption nearedge structure (XANES) spectra have been recorded for purified humic acids (HAs) isolated from a German peat (GHA), an Irish peat (IHA), an unpolluted New Hampshire bog soil (NHA) and their tightly bound copper(), iron() and manganese() forms. Brief water washing of partly or fully metal-loaded HAs leaves 'tightly' bound metal in the isolated freeze-dried solids. Most of this metal is removed by washing with 0.1 HCl, indicating acidic HA functional groups as principal metal binding sites. The number of nearest-neighbour atoms coordinated to tightly bound Cu II (four), Fe III (six, probably with distorted geometry) and Mn II (six, undistorted) in solid GHA, IHA and NHA were determined by XANES and EXAFS spectroscopy with reference standards. Isotherms measured at 20.0 ЊC and pH 2.4-3.2 with [M] total = 0.18-25.8 m for tight, reversible Cu 2ϩ (aq), Fe 3ϩ (aq), and Mn 2ϩ (aq) binding by solid IHA and NHA fit the Langmuir model and give the pH-independent stoichiometric site capacities ν i and equilibrium constants K i for metal binding at specific HA sites i = A, B and C. Tight binding sites A, B and C of IHA are occupied by Cu II , sites A and B by Fe III and site A by Mn II , while only identical metal binding site A in NHA is tight enough to resist metal removal by brief water washing. A new helical HA molecular model based on the empirical formula C 36 H 30 N 2 O 15 ؒxH 2 O visualizes metal binding and the likely roles of HAs in biomineralization. Site A is suggested to be carboxylate, mixed ligands probably constitute site B, and site C is tentatively assigned as the interior of the HA helix. Binding free energies and EPR evidence suggest that Cu 2ϩ (aq), Fe 3ϩ (aq) and Mn 2ϩ (aq) rapidly transfer between specific HA binding sites. This affects rates of metal release and transfer to minerals.
Multiple structural models of humic acid (HA) building blocks have been reported. In this work, the modeling is based on two structural motifs: (i) the Steelink structure and (ii) a new humic acid [TNB] building block, which incorporates more fully the results of experimental data and retro-biosynthetic analyses. Both have significant conformational freedom complicated by their stereochemistry. A molecular modeling approach for the analysis of complex molecules with significant conformational freedom is described as it relates to the newly proposed humic acid building block. A potential energy surface for various conformers of the low-energy stereoisomer has been generated. Included in this discussion is the relationship of the stereochemistry to conformation and secondary structure.
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