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Enzymatic sulfation helps solve lignin’s solubility problem

Adding sulfate groups to phenols improves the recalcitrant material’s solubility in water and should aid production of aromatic compounds

by Stephen K. Ritter
May 8, 2017 | A version of this story appeared in Volume 95, Issue 19

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Credit: Designed by Fengxia Yue, Fachuang Lu, and John Ralph/GLBRC
Lignin in softwoods is derived mainly from radical coupling reactions of coniferyl alcohol, which form various dimeric building blocks (colored segments) that make up a phenol-based biopolymer like that shown. Lignins have no defined structural sequence, meaning the chance of encountering two identical lignin molecules is rather small.
A structure of lignin.
Credit: Designed by Fengxia Yue, Fachuang Lu, and John Ralph/GLBRC
Lignin in softwoods is derived mainly from radical coupling reactions of coniferyl alcohol, which form various dimeric building blocks (colored segments) that make up a phenol-based biopolymer like that shown. Lignins have no defined structural sequence, meaning the chance of encountering two identical lignin molecules is rather small.

Lignin produced by plants is nature’s greatest source of aromatic compounds, and it’s readily available as a by-product of the pulp and paper industry. It seems natural that chemists would want to take advantage of the material as a source of aromatics to reduce reliance on coal, oil, and natural gas. One problem is that lignin’s aromatics are locked up in complex insoluble polymeric chains. Gadi Rothenberg, Ron Wever, and coworkers of the University of Amsterdam have developed an enzymatic process to selectively add hydrophilic sulfate groups to lignin’s many phenol rings to make the material easier to dissolve for processing (ChemSusChem 2017, DOI: 10.1002/cssc.201700376). Wever’s group previously found that a bacterial aryl sulfotransferase enzyme can take sulfate groups from p-nitrophenylsulfate and add them to hydroxyl groups of various phenol compounds. The joint team has now extended the chemistry to various types of lignins. The researchers show that the process is selective for phenolic groups, leaving aliphatic hydroxyl groups in lignin side chains untouched. The resulting sulfated lignins dissolve easily in mildly alkaline solutions, with the increase in solubility visible to the naked eye and traceable by UV-Vis and NMR spectroscopy. The researchers note that the new method improves on prior lignin sulfating processes as well as current approaches that use caustic solutions, ionic liquids, or supercritical solvents, which are relatively costly and generate significant waste.

A reaction scheme shows a process to sulfate lignin to improve its solubility.
Credit: ChemSusChem

CORRECTION: The reaction scheme in this story was updated on May 19, 2017, to correct the sulfating agent structure. It was missing one oxygen in the sulfur group.

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