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Tricin was recently discovered in lignin preparations from wheat (Triticum aestivum) straw and subsequently in all monocot samples examined. To provide proof that tricin is involved in lignification and establish the mechanism by which it incorporates into the lignin polymer, the 49-Ob-coupling products of tricin with the monolignols (p-coumaryl, coniferyl, and sinapyl alcohols) were synthesized along with the trimer that would result from its 49-O-b-coupling with sinapyl alcohol and then coniferyl alcohol. Tricin was also found to cross couple with monolignols to form tricin-(49-O-b)-linked dimers in biomimetic oxidations using peroxidase/hydrogen peroxide or silver (I) oxide. Nuclear magnetic resonance characterization of gel permeation chromatography-fractionated acetylated maize (Zea mays) lignin revealed that the tricin moieties are found in even the highest molecular weight fractions, ether linked to lignin units, demonstrating that tricin is indeed incorporated into the lignin polymer. These findings suggest that tricin is fully compatible with lignification reactions, is an authentic lignin monomer, and, because it can only start a lignin chain, functions as a nucleation site for lignification in monocots. This initiation role helps resolve a long-standing dilemma that monocot lignin chains do not appear to be initiated by monolignol homodehydrodimerization as they are in dicots that have similar syringyl-guaiacyl compositions. The term flavonolignin is recommended for the racemic oligomers and polymers of monolignols that start from tricin (or incorporate other flavonoids) in the cell wall, in analogy with the existing term flavonolignan that is used for the lowmolecular mass compounds composed of flavonoid and lignan moieties.Lignin, a complex phenylpropanoid polymer in the plant cell wall, is predominantly deposited in the cell walls of secondary-thickened cells (Vanholme et al., 2010). It is synthesized via oxidative radical coupling reactions from three prototypical monolignols, p-coumaryl, coniferyl, and sinapyl alcohols, differentiated by their degree of methoxylation ortho to the phenolic hydroxyl group. Considered within the context of the entire polymer, the main structural features of lignin can be defined in terms of its p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S) units, derived respectively from these three monolignols (Ralph, 2010). Several novel monomers, all deriving from the monolignol biosynthetic pathway, have been found to incorporate into lignin in wild-type and transgenic plants. For example, monolignol acetate, p-hydroxybenzoate, and p-coumarate ester conjugates have all been shown to incorporate into lignin polymers and are the source of naturally acylated lignins (Ralph et al., 2004;Lu and Ralph, 2008); lignins derived solely from caffeyl alcohol were found in the seed coats of both monocot and dicot plants (Chen et al., 2012a(Chen et al., , 2012b; lignins derived solely from 5-hydroxyconiferyl alcohol were found in a cactus (for example, in a member of the genera Astrop...

Good-quality Raman spectra of most wood species can now be obtained by using near-infrared Fourier transform Raman spectroscopy. To make effective use of such spectroscopic information, one needs to interpret the data in terms of contributions from various wood components and, for each component polymer, in terms of vibrational modes of its substructural units/groups. In the present work, Raman spectral features of black spruce (Picea mariana) wood were associated with lignin and/or carbohydrate polymers. Lignin's spectral contributions were recognized in several ways. In addition to spectra of milled-wood and enzyme lignins, a spectrum of native lignin was obtained by subtracting the spectrum of acid chlorite delignified black spruce from the spectrum of an untreated wood sample. A comparison of lignin spectra indicated that the Raman features of the three lignins are very similar. Raman contributions of carbohydrate polymers, namely, those of cellulose and hemicellulose, were identified by using authentic and/or isolated samples and, in the case of cellulose, by using previously published spectra. Such an analysis showed that the hemicellulose present in black spruce did not give rise to any new, unique features that were not already present due to cellulose. Therefore, it was concluded that the hemicellulose contribution is broad and is hidden under the Raman contribution of cellulose. Also, peak positions of lignin contributions did not overlap with those of cellulose, and there were spectral regions where either lignin or cellulose contributed.

Lignin is an aromatic heteropolymer, abundantly present in the walls of secondary thickened cells. Although much research has been devoted to the structure and composition of the polymer to obtain insight into lignin polymerization, the lowmolecular weight oligolignol fraction has escaped a detailed characterization. This fraction, in contrast to the rather inaccessible polymer, is a simple and accessible model that reveals details about the coupling of monolignols, an issue that has raised considerable controversy over the past years. We have profiled the methanol-soluble oligolignol fraction of poplar (Populus spp.) xylem, a tissue with extensive lignification. Using liquid chromatography-mass spectrometry, chemical synthesis, and nuclear magnetic resonance, we have elucidated the structures of 38 compounds, most of which were dimers, trimers, and tetramers derived from coniferyl alcohol, sinapyl alcohol, their aldehyde analogs, or vanillin. All structures support the recently challenged random chemical coupling hypothesis for lignin polymerization. Importantly, the structures of two oligomers, each containing a g-p-hydroxybenzoylated syringyl unit, strongly suggest that sinapyl p-hydroxybenzoate is an authentic precursor for lignin polymerization in poplar.