Spectral Characteristics Related to Chemical Substructures and Structures Indicative of Organic Precursors from Fulvic Acids in Sediments by NMR and HPLC-ESI-MS.

Verónica Gisela López-Martínez,Mario Alfonso Murillo-Tovar,Jorge Ernesto Solá-Pérez,José Gustavo Ronderos-Lara,Hugo Saldarriaga-Noreña,Ismael León-Rivera,Jorge A Guerrero-Álvarez

doi:10.3390/molecules26134051

Abstract

The aim of this work was to determine Fulvic Acids (FAs) in sediments to better know their composition at the molecular level and to propose substructures and structures of organic precursors. The sediment samples were obtained from a priority area for the conservation of ecosystems and biodiversity in Mexico. FAs were extracted and purified using modifications to the International Humic Substances Society method. The characterization was carried out by 1D and 2D nuclear magnetic resonance (NMR) and high-performance liquid chromatography-electrospray ionization-mass spectrometry (HPLC-ESI-MS) in positive (ESI+) and negative (ESI−) modes. Twelve substructures were proposed by the COSY and HSQC experiments, correlating with compounds likely belonging to lignin derivatives obtained from soils as previously reported. The analysis of spectra obtained by HPLC-ESI-MS indicated likely presence of compounds chemically similar to that of the substructures elucidated by NMR. FAs studied are mainly constituted by carboxylic acids, hydroxyl, esters, vinyls, aliphatics, substituted aromatic rings, and amines, presenting structures related to organic precursors, such as lignin derivatives and polysaccharides.

Highlights

The natural organic matter (NOM) present in sediments, soil, and water is considered a very complex chemical matrix, constituted by organic compounds from animal and/or plant origin
The humic substances (HS) are classified into fulvic acids (FAs), humic acids (HAs), and humins (HUs), which are mainly constituted by carbon, hydrogen, and oxygen
Spectral resolution in Nuclear Magnetic Resonance (NMR) experiments was improved through the following: (1) DMSO-d6 was chosen as the solvent because it prevents the aggregation of FAs components through intermolecular interactions in solution [21,28,29]; (2) the amount of 3 mg in 600 μL used in the preparation of the FABC1 sample contributed considerably to decreasing signals overlap as the mass quantities in a range of 15 to 99.6 mg in DMSO-d6 showed overlapping of signals or cross peaks in 1D and 2D spectra, respectively [5,6,21,24,25,30]; (3) the increased number of scans in all the experiments compared to that reported in other studies increasing sensitivity [20,21]; (4) the adjustment of acidic to neutral pH in the final eluate decreased selectively the number of signals in the NMR spectra [31]

Summary

Introduction

The natural organic matter (NOM) present in sediments, soil, and water is considered a very complex chemical matrix, constituted by organic compounds from animal and/or plant origin. The HS are classified into fulvic acids (FAs), humic acids (HAs), and humins (HUs), which are mainly constituted by carbon, hydrogen, and oxygen. FAs and HAs include heteroatoms in their structure, such as nitrogen, sulfur, and phosphorus [1,2,3,4]. In natural and polluted environmental compartments, the physicochemical properties of FAs are of special interest, since they are soluble in the aqueous medium at any pH range and could form hydrogen bonds with water and undergo transfer from the sediment to the aqueous medium. The acidic behavior of FAs is provided by a variety of functional groups such as COOH in fatty and aromatic acids, or substituents from amino acids and phenolic OH groups that come from tannin and lignin structures [5,6,7,8]

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