Tight metal binding by humic acids and its role in biomineralization †

Abstract

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.

Metal Cation Complexation with Natural Organic Matter in Aqueous Solutions: Molecular Dynamics Simulations and Potentials of Mean Force

“…2,3,[26][27][28] The TNB model fragment used in our simulations contains three carboxylic groups, three carbonyl groups, two phenolic groups, two amine groups, and four other R-OH alcohol groups and has a total molecular weight of 753 Da. [38][39][40] The size, composition, molecular weight, degree of aromaticity, and total charge density of this model are in good agreement with available experimental characterization of NOM 26,27,29 , the results of computer assisted 3-D structure elucidation, 42 and stochastic biogeochemical modeling. 33 The composition of the TNB model is also quite close to the composition of Suwannee River NOM, which is often used experimentally.…”

“…37 Despite the well recognized uncertainties of the NOM composition and structure, such methods have been successfully used over the last decade to investigate NOM in molecular-scale detail. [38][39][40][41][42][43][44][45][46][47] The TNB (Temple-Northeastern-Birmingham) model of an NOM molecular fragment [38][39][40] used in our work provides a good structural and compositional analog of the Suwannee River NOM commonly used in experimental studies. [43][44][45] Carboxylic groups are the most important interaction sites for metal binding to these molecules, as they are for many other organic and bioorganic molecules.…”

Effect of fulvic and humic acids on iron and manganese homeostasis in rats

“…Dose-dependent responses were also observed in an acute iron toxicity study (Whittaker et al, 2002). Because it was believed that the absorption of HA from the intestine is poor (1-5%) (Islam et al, 2005) and the binding between Fe and HA is tight (Davies et al, 1997), we hypothesised that the HA-Fe complex would be absorbed poorly from the intestine. Contrary to this hypothesis, the Fe concentration in the large intestinal content of HA rats remained at the control level and did not follow the relative increment of Fe concentration of the diets, suggesting that the efficiency of iron absorption from the intestine increased with the dose of HA (Fig.…”

“…An explanation for the different effects of FA and HA on iron homeostasis may be that the binding of Fe to HA is much tighter than to FA (Davies et al, 1997). This is why HA did not provide iron either to the liver or to the other investigated organs.…”

Studies of Cu(II) in soil by X-ray absorption spectroscopy

“…This and other recent publications (e.g., Davies et al 1997;Manceau et al 1996;Sery et al 1996;Hesterberg et al 1997) that describe the use of EXAFS/XANES to probe the state of heavy metals in soils, sediments and minerals, demonstrate the enormous, unrealized potential of these methods in environmental research. In this article, we will demonstrate, using soil contaminated with Cu as an example, how information about the chemical state of a metal and its bonding geometry can be obtained via the application of state-of-the-art instrumentation and analysis techniques.…”