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Secondary Structures in a Freeze-Dried Lignite Humic Acid Fraction Caused by Hydrogen-Bonding of Acidic Protons with Aromatic Rings
journal contribution
posted on 2016-02-09, 17:07 authored by Xiaoyan Cao, Marios Drosos, Jerry A. Leenheer, Jingdong MaoA lignite humic acid
(HA) was separated from inorganic and non-HA
impurities (i.e., aluminosilicates, metals) and fractionated by a
combination of dialysis and XAD-8 resin. Fractionation revealed a
more homogeneous structure of lignite HA. New and more specific structural
information on the main lignite HA fraction is obtained by solid-state
nuclear magnetic resonance (NMR) spectroscopy. Quantitative 13C multiple cross-polarization (multiCP) NMR indicated oxidized phenyl
propane structures derived from lignin. MultiCP experiments, conducted
on potassium HA salts titrated to pH 10 and pH 12, revealed shifts
consistent with carboxylate and phenolate formation, but structural
changes associated with enolate formation from aromatic beta keto
acids were not detected. Two-dimensional 1H–13C heteronuclear correlation (2D HETCOR) NMR indicated aryl-aliphatic
ketones, aliphatic and aromatic carboxyl groups, phenol, and methoxy
phenyl ethers. Acidic protons from carboxyl groups in both the lignite
HA fraction and a synthetic HA-like polycondensate were found to be
hydrogen-bonded with electron-rich aromatic rings. Our results coupled
with published infrared spectra provide evidence for the preferential
hydrogen bonding of acidic hydrogens with electron-rich aromatic rings
rather than adjacent carbonyl groups. These hydrogen-bonding interactions
likely result from stereochemical arrangements in primary structures
and folding.