Theoretical Study of the Remarkably Diverse Linkages in Lignin

Lignin in plant cell walls is a potential renewable source of biofuels, chemicals, and value-added products. It consists of various aryl ethers, irregularly connected by a variety of linkages creating a complex structural network; hence, it is difficult to identify selective bond breaking events. In this study, we predict dissociation tendencies of a diverse set of lignin linkages encompassing 65 lignin model compounds using the density functional theoretical (DFT) approach. The chosen 65 lignin model compounds represent the most prevalent carbon-oxygen (ether) and carbon-carbon (C-C) bond linkages. Results from our systematic study identify the weakest and strongest linkages connecting arene rings in different classes of lignin model compounds. Also, the dissociating linkages can have different adjacent substituents, such as the methoxy group on the arene ring and hydrocarbon, methyl, and hydroxyl group substitutions on aliphatic carbon atoms. These substituents affect the ease of dissociation of lignin linkages and can be used to develop predictive models for delignification.