NMR observation of a new lignin structure, a spiro-dienoneElectronic supplementary information (ESI) available: 13C, QUAT, HMBC and HSQC NMR spectra. See http://www.rsc.org/suppdata/cc/b1/b108285j/

Abstract

A spiro-dienone structure (b-1/a-O-a) has been observed as one of the important structures present in spruce and aspen lignins, with abundance as high as 3% in spruce lignin.Lignin macromolecules consist of phenylpropanoid units that are linked to each other by various types of ether and carboncarbon bonds. Opinions on the occurrence and frequency of the b-1 linkage in lignin have been quite divergent among wood chemists. The abundance of the b-1 structure has been estimated to range from 1% to 15% in spruce lignin, 1-3 depending on the analytical method applied. The b-1 structure has been observed to be one of the major type of lignin inter-unit linkages by studies using mild hydrolysis 4,5 and thioacidolysis. 6 On the other hand, efforts to detect the b-1 structure in milled wood lignin (MWL) by using 1 H NMR, 1 2D HMQC NMR 7,8 or ozonation 2 have not been very successful, suggesting that this structure might only be a minor constituent in milled wood lignin.To explain the discrepancies in the applied analytical methods, it has been suggested that a precursor of the b-1 structure, a dienone structure (I in Scheme 1) might be present in native lignin. 9 On acid hydrolysis, the dienone structure would be converted to the b-1 structure (II in Scheme 1).There have been several reports in the literature showing supporting evidence for the presence of such a precursor in lignin. We have observed previously that mild acid hydrolysis of spruce wood meal, pre-methylated with diazomethane, resulted in the release of lignin b-1 dimers predominantly carrying a free phenolic hydroxy group on ring B. 6 This observation suggested that the ring B originally was present in the form of a dienone structure. More evidence supporting the existence of the dienone structures in lignin includes the recent isolation of the sesquineolignan woorenol, 10 the synthesis of spirocyclohexadienone structure by radical coupling of ligninlike model compounds 11 and the isolation of an isochroman structure among the lignin degradation products after a DFRC treatment 12 as well as some 3D NMR evidence. 13 In the present communication, we present direct observations of the spirodienone structures of type I in spruce and aspen MWLs by NMR techniques.Two spruce MWL samples and one aspen MWL were prepared. One of the spruce lignin samples (MWL1) was prepared from freshly cut spruce wood by milling with steelballs for 3 days. 14 The other spruce lignin sample (MWL2) was prepared from an aged spruce wood log by milling with porcelain balls for 14 days. 15 The spiro-dienone structure was † Electronic supplementary information (ESI) available: 13 C, QUAT, HMBC and HSQC NMR spectra. See http://www.rsc.org/suppdata/cc/b1/ b108285j/ Scheme 1 Fig. 1 Signals for the spiro-dienone structure (b-1/a-O-a) were observed and assigned in the HSQC spectrum of an acetylated spruce milled wood lignin (MWL1 acetate in acetone-d 6 ). This journal is

Effects of Coumarate 3-Hydroxylase Down-regulation on Lignin Structure

“…Their 5HG analogs have recently been documented in COMTdeficient alfalfa lignins (36). Spirodienone structures S, ␤-1-coupled units only recently authenticated in lignin spectra (59,60), are readily seen in alfalfa (3). The diagnostic dibenzodioxocins D are also relatively newly discovered eight-membered ring structures resulting from radical coupling of a monolignol with a 5-5-coupled end unit (61).…”

Elucidation of Lignin Structure by Quantitative 2D NMR

“…[18,19] However, conventional 1D 1 H and 13 C NMR spectra of lignins do not allow detailed assignment and quantification of individual signals due to broad and overlapping NMR signals resulting in low-resolution spectra. The (2D) 13 C, 1 H-correlated (HSQC, HMQC) spectroscopic technique, which combines the sensitivity of 1 H NMR with the higher resolution of 13 C NMR, continues to be the best method to reveal the frequencies of the different lignin units and the inter-unit bonding patterns and has allowed their use as a valid analytical technique in the analysis of complex samples.…”

“…Such discrepancies have been explained by suggesting that the precursor of the b-1 structure, a spirodienone, might be present in native lignins and form b-1 moieties upon hydrolysis or thioacidolysis. [18] On the other hand HSQC studies have elucidated the presence of spirodienone besides b-1 units. Dibenzodioxocine moieties are key lignin components since they introduce branching points in lignins.…”

Characterization of nonderivatized plant cell walls using high‐resolution solution‐state NMR spectroscopy

“…[34] The cinnamyl alcohol endgroup correlation X1γ is shown at 4.14/61.7 ppm [34] and is visible in all the three species. Spirodienone/β -1 structures were detected in the aspen spectra only; however, because of the low quantity of such linkages in lignin (about 1.8% in aspen lignin), [38] the spectrum showed these only at low contour intensity. Spirodienone correlations Sα, Sβ, and Sβ′ (not shown) were found at 5.14/81.4 ppm, 2.81/60.3 ppm, and 4.18/79.8 ppm.…”

“…Spirodienone correlations Sα, Sβ, and Sβ′ (not shown) were found at 5.14/81.4 ppm, 2.81/60.3 ppm, and 4.18/79.8 ppm. [38,39] Lignin methoxyls, typically a strong and broad contour, show some separation between the guaiacyl and syringyl methoxyls in the aspen and kenaf spectra. [34] ii.…”