Formation of β-o-4 lignin models -: A theoretical study

The formation of two different beta-O-4 lignin models is investigated by means of density functional calculations. It is found that the coupling of two coniferyl alcohol radicals forming a quinone methide proceeds by an energy barrier of similar to2-5 kcal/mol, and that the associated reaction energy is negative by more than 20 kcal/mol. On the basis of the corresponding results obtained for the coupling of a coniferyl alcohol radical to a coniferyl alcohol, it is argued that the resulting radical, albeit being formed in an energetically less favourable process, might play an important role in lignin polymerisation. Finally, two different reaction mechanisms for the conversion of a quinone methide into a guaiacylglycerol-beta-coniferyl ether dilignol through the addition of water are explored.