Synthesis of Model Humic Substances by Oxidative Coupling of Phenylpropanoic Monomer and Hydroquinone: Mechanistic Study Using Controllable H/D Exchange and Fourier Transform Ion Cyclotron Resonance Mass Spectrometryстатья
Статья опубликована в высокорейтинговом журнале
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Дата последнего поиска статьи во внешних источниках: 22 июля 2015 г.
Аннотация:The products of oxidative coupling of phenols are frequently used as synthetic analogues to natural humic substances (HS) for biomedical research. However, their molecular compositions and exact structures remain largely unknown. The objective of this study was to develop novel approach to molecular-level analysis of phenolic polymerisates capable of inventorying molecular constituents and resolving their distinct structural formulas. For this purpose, we have synthesized the model HS using oxidative coupling of specifically designed phenylpropanoic monomer - 3-(4-hydroxy-3-methoxyphenyl)-3-oxopropionic acid to hydroquinone. We have characterized thus synthesized model HS using high resolution Fourier transform ion cyclotron resonance mass spectrometry (FTICR MS), 1H NMR spectroscopy, and controllable H/D exchange. We succeeded in molecular inventory of the model HS. The assigned molecular formulas occupied substantial space of CHO compositions in Van Krevelen diagram with maximum density in the regions of tannins and lignins resembling those of natural HS. To identify exact structural formulas of individual constituents of the model HS, we have applied selective H/D exchange of non-labile backbone protons by a choice of basic or acidic catalytic conditions followed by FTICR MS. The determined formulas allowed us to verify the proposed pathways of hydroxylation and carboxylation in the course of phenolic coupling and to identify acetylation of aromatic rings as an important side reaction. The conclusion was made that the proposed analytical approach might be used for identifying molecular carriers of biological activity within the phenolic polymerisates and, eventually, within the natural HS.