title: 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 value-added 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.
authors.affiliation: Southern Research Station, USDA-Forest Service , 521 Devall Drive, Auburn, Alabama 36849, United States.
authors.forename: Thomas
authors.initials: T
authors.lastname: Elder
authors.affiliation: National Bioenergy Center, National Renewable Energy Laboratory , 15013 Denver West Parkway, Golden, Colorado 80401, United States.
authors.forename: Laura
authors.initials: L
authors.lastname: Berstis
authors.affiliation: National Bioenergy Center, National Renewable Energy Laboratory , 15013 Denver West Parkway, Golden, Colorado 80401, United States.
authors.forename: Gregg T
authors.initials: GT
authors.lastname: Beckham
authors.affiliation: Biosciences Center, National Renewable Energy Laboratory , 15013 Denver West Parkway, Golden, Colorado 80401, United States.
authors.forename: Michael F
authors.initials: MF
authors.lastname: Crowley
mesh_terms: D008031:Lignin / Q000737:chemistry*; D015394:Molecular Structure; D010636:Phenols / Q000737:chemistry*; D020005:Propanols / Q000737:chemistry; D013816:Thermodynamics
chemical_list: C000616851:5-hydroxyconiferyl alcohol; D010636:Phenols; D020005:Propanols; D008031:Lignin; C010559:coniferyl alcohol; C020722:cinnamyl alcohol
keywords: 5-hydroxyconiferyl alcohol; benzodioxane; coniferyl alcohol; lignin; stereochemistry