Zhang, A., Lu, F., Sun, R., Ralph, J. Ferulate-Coniferyl Alcohol Cross-coupled Products Formed by Radical Coupling Reactions. Planta. 2009 Apr;229(5):1099-108. Epub 2009 Feb 21.

Abstract
Radical coupling reactions between ethyl ferulate (Et-FA), a simple model for feruloyl polysaccharides in planta, and coniferyl alcohol (CA), a monolignol, were studied in order to better understand the polymer cross-coupling interactions among polysaccharides and monolignols or lignin, mediated by ferulate (FA), in plant cell walls. Cross-coupled FA/CA dimers produced in an aqueous buVer (pH 5.0) containing peroxidase/hydrogen peroxide were isolated and characterized by NMR. The total coupling products were characterized by 2D 13C–1H correlation (HSQC) NMR spectroscopy and GC–MS. Results from this study showed that ferulate readily cross-couples with coniferyl alcohol through free radical coupling mechanisms producing a series of cross-coupled FA/CA dimers with β-O-4-, β-5-/8-5-, and 8-β-linkages; the syntheses and isolation of β-5- and 8-5-cross-coupled dimers are reported here. The transformation from 8-β-coupled FA/CA hydroxyl esters into lactones through intramolecular transesterification is demonstrated for the Wrst time and mechanisms behind these transformations are discussed. The Wnding of both β-5- and 8-5-cross-coupled dimers in this study suggests that analogs of both may be present in plant cell walls. Finally it is suggested that ferulates in plants indeed react with monolignols through free radical mechanisms producing a more diverse array of cross-coupled dimers than previously reported.

Stewart, J.J., Akiyama, T., Chapple, C.C.S., Ralph, J., Mansfield, S.D. The Effects on lignin structure of over-expression of ferulate 5-hydroxylase in hybrid poplar. Plant Physiol. April 22, 2009, as DOI:10.1104/pp.109.137059.

Abstract
Poplar lignins with exceedingly high syringyl monomer levels are produced by over-expression of the ferulate 5-hydroxlase (F5H) gene driven by a cinnamate 4-hydroxylase (C4H) promoter. Compositional data derived from both standard degradative methods and NMR analyses of the entire lignin component (as well as
isolated lignin fraction) indicated that the C4H::F5H transgenic lignin was comprised of as much as 97.5% syringyl units (derived from sinapyl alcohol), the remainder being guaiacyl units (derived from coniferyl alcohol); the syringyl level in the wild-type control was 68%. The resultant transgenic lignins are more linear, and display a lower degree of polymerization. Although the crucial β-ether content is similar, the distribution of other inter-unit linkages in the lignin polymer is markedly different, with higher resinol (β–β) and spirodienone (β–1) contents, but with virtually no phenylcoumarans (β–5, which can only be formed from guaiacyl units). p-Hydroxybenzoates, acylating the γ-positions of lignin sidechains, were reduced by over 50%, suggesting consequent impacts on related pathways. A model depicting the putative structure of the transgenic lignin resulting from the over-expression of F5H is presented. The altered structural features in the transgenic lignin polymer, as revealed here, support the contention that there are
significant opportunities to improve biomass utilization by exploiting the malleability of plant lignification processes.