Coupling and reactions of 5- hydroxyconiferyl alcohol in lignin formation

Abstract

The catechol alcohols, caffeyl and 5-hydroxyconiferyl alcohol, may be incorporated into lignin either naturally or through genetic manipulation. Due to the presence of o-OH groups, these compounds form benzodioxanes, a departure from the interunit connections found in lignins derived from the cinnamyl alcohols. In nature, lignins composed of caffeyl and 5- hydroxyconiferyl alcohol are linear homopolymers and, as such, may have properties that make them amenable for use in valueadded products, such as lignin-based carbon fibers. In the current work, results from density functional theory calculations for the reactions of 5-hydroxyconiferyl alcohol, taking stereochemistry into account, are reported. Dehydrogenation and quinone methide formation are found to be thermodynamically favored for 5-hydroxyconiferyl alcohol, over coniferyl alcohol. The comparative energetics of the rearomatization reactions suggest that the formation of the benzodioxane linkage is under kinetic control. Ring-opening reactions of the benzodioxane groups show that the bond dissociation enthalpy of the α-O cleavage reaction is lower than that of the β-O reaction. The catechol lignins represent a novel form of the polymer that may offer new opportunities for bioproducts and genetic targets.

Citation

Elder, Thomas; Berstis, Laura; Beckham, Gregg T.; Crowley, Michael F. 2016. Coupling and reactions of 5- hydroxyconiferyl alcohol in lignin formation. Journal of Agricultural and Food Chemistry, Vol. 64(23): 9 pages.: 4742-4750.  10.1021/acs.jafc.6b02234