Method to Regioselectively Iodine-Tag Free-Phenolic Aromatic End-Groups in Lignin for 1H-13C-HSQC NMR Analysis

Phenolic end-groups on lignin molecules are important sites for continued lignification, their influence over lignin molecular properties, their ability to enhance lignin degradation, and their potential for derivatization to produce lignin materials. Here we use iodination to regioselectively tag, and HSQC-NMR to monitor, guaiacyl phenolic aromatic end-groups in lignins. Using dimeric lignin models, iodination at the 5-position of the phenolic ring was shown to be clean, and almost quantitative. Additionally, the 6-position C/H correlation peak shifted downfield in both carbon and proton dimensions in HSQC spectra. As phenolic aromatic rings could be selectively iodinated, and as their C6/H6 aromatic signals appear in a clear region of HSQC NMR spectra (different from the original position), we would have a new method for tagging and analyzing such phenolic aromatic rings in lignins if the method could be applied to the polymer. The new tagging method was therefore applied to a synthetic guaiacyl lignin polymer (DHP) and a milled wood lignin (MWL) isolated from Japanese cedar (Cryptomeria japonica). The derivatization proceeded directly in the NMR tube, without needing isolation or purification, from which HSQC-NMR revealed that the diagnostic C6/H6 correlations matched one from the dimeric model compound. This phenolic unit tagging method adds a useful method to the lignin structural analysis toolbox, helping to gain a better understanding of lignin's phenolic end-units.