Structure, flexibility and hydration properties of lignin dimers studied with Molecular Dynamics simulations

Lignin is an abundant polymer found in wood and grasses, but due to its heterogeneity and complex macromolecular structure it has been less utilized than cellulose. While the building blocks are known, the way they are linked is less understood. Here, Molecular Dynamics simulations were used to systematically characterize seven linkages found in native lignin. Their influence on lignin and water structure, and their interactions were analyzed. The study is limited to guaiacyl (G-G) dimers connected by the following common softwood linkages; 5-5′, 4-O-5′, α-O-4′, β-1′, β-5′, β-O-4′ and β-β′. The simulations show that the linkage has a significant effect on conformational preference and lignin-water interaction. Especially, the behavior of the β-O-4′ shows unique properties, both in terms of conformational freedom and interaction with water. Within the β-O-4′ dimer, π–π stacking between the aromatic rings is possible. The molecule has two distinct common conformations, one compressed and one extended. These preferences also lead to a different effect of β-O-4′ dimer on the surrounding water, where water is found close to the linkage itself but expelled from the aromatic rings to a larger extent than the other linkages. These findings are important for lignin solubility as well as its depolymerization mechanisms.