Chromophores from hexeneuronic acids: identification of HexA-derived chromophores
journal contributionposted on 06.03.2018 by Thomas Rosenau, Antje Potthast, Nele Sophie Zwirchmayr, Hubert Hettegger, Felix Plasser, Takashi Hosoya, Markus Bacher, Karin Krainz, Thomas Dietz
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© 2017, The Author(s). Hexeneuronic acids (HexA) have long been known as triggers for discoloration processes in glucuronoxylan-containing cellulosic pulps. They are formed under the conditions of pulping from 4-O-methylglucuronic acid residues, and are removed in an “A stage” along the bleaching sequences, which mainly comprises acidic washing treatments. The chemical structures of HexA-derived chromophoric compounds 4–8, which make up 90% of the HexA-derived chromophores, are reported here for the first time. The compounds are ladder-type, mixed quinoid-aromatic oligomers of the bis(furano)-[1,4]benzoquinone and bis(benzofurano)-[1,4] benzoquinone type. The same chromophoric compounds are generated independently of the starting material, which can be either a) HexA in pulp, b) the HexA model compound methyl 1- 13 C-4-deoxy-β-L-threo-hex-4-enopyranosiduronic acid (1) or c) a mixture of the primary degradation intermediates of 1, namely 5-formyl-furancarboxylic acid (2) and 2-furancarboxylic acid (3). Isotopic labeling ( 13 C) in combination with NMR spectroscopy and mass spectrometry served for structure elucidation, and final confirmation was provided by X-ray structure analysis. 13 C-Isotopic labeling was also used to establish the formation mechanisms, showing all the compounds to be composed of condensed, but otherwise largely intact, 2-carbonylfuran and 2-carbonylfuran-5-carboxylic acid moieties. These results disprove the frequent assumption that HexA-derived or furfural-derived chromophores are linear furanoid polymers, and might have a direct bearing on structure elucidation studies of “humins”, which are formed as dark-colored byproducts in depolymerization of pentosans and hexosans in different biorefinery scenarios.
Open access funding provided by University of Natural Resources and Life Sciences Vienna (BOKU). This work was supported by the Austrian Forschungsforderungsgesellschaft (FFG) through Project 847169 (‘‘Chromophores II’’) and through the project FLIPPR 2 (Future Lignin and Pulp Processing Research), along with its partner companies.