title: Secondary Structures in a Freeze-Dried Lignite Humic Acid Fraction Caused by Hydrogen-Bonding of Acidic Protons with Aromatic Rings.
abstract: A 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 (13)C 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 (1)H-(13)C 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.
authors.affiliation: Department of Chemistry and Biochemistry, Old Dominion University , 4541 Hampton Blvd, Norfolk, Virginia 23529, United States.
authors.forename: Xiaoyan
authors.initials: X
authors.lastname: Cao
authors.affiliation: Centro Interdipartimentale di Ricerca sulla Risonanza Magnetica Nucleare per l' Ambiente, l' Agroalimentare ed i Nuovi Materiali (CERMANU), Università di Napoli "Federico II", Via Università 100, 80055 Portici, Italy.
authors.forename: Marios
authors.initials: M
authors.lastname: Drosos
authors.affiliation: 4024 Douglas Mountain Drive, Golden, Colorado 80403, United States.
authors.forename: Jerry A
authors.initials: JA
authors.lastname: Leenheer
authors.affiliation: Department of Chemistry and Biochemistry, Old Dominion University , 4541 Hampton Blvd, Norfolk, Virginia 23529, United States.
authors.forename: Jingdong
authors.initials: J
authors.lastname: Mao
mesh_terms: D000143:Acids; D005591:Chemical Fractionation; D003031:Coal / Q000032:analysis*; D005612:Freeze Drying; D006812:Humic Substances / Q000032:analysis*; D006859:Hydrogen / Q000737:chemistry; D006860:Hydrogen Bonding; D015394:Molecular Structure; D010636:Phenols; D011522:Protons
chemical_list: D000143:Acids; D003031:Coal; D006812:Humic Substances; D010636:Phenols; D011522:Protons; D006859:Hydrogen