Lignin biosynthesis and degradation
Aim To understand at a detailed molecular level the fundamental interactions and reactions leading to lignin formation in the cellulose/hemicellulose matrix, and to describe the key reactions for lignin removal in pulping processes. Alternative methods for removal of lignin (new bleaching chemicals, photochemistry, and enzymatic degradation) will also be investigated.
Background Lignin is the second most abundant biopolymer on earth (after cellulose), and is generally found associated with hemicelluloses in the spaces between cellulose microfibrils in the primary and secondary cell walls of vascular plants. The randomly distributed lignin polymer is crucial for the plants in providing mechanical support and as a physiochemical barrier against pathogens.
Lignin is also of significant economic importance, from a number of perspectives. The key factor is the ability to easily and efficiently remove lignin in pulping processes, in order to generate high quality paper. At the same time, the high variability in the molecular structure of the lignin biopolymer makes lignin degradation in alkaline pulping and chemical bleaching a difficult task. In addition a number of questions still remain unresolved regarding the exact mechanisms for polymer formation, and the rate determining steps in pulping/bleaching processes.
Project description Theoretical chemical methodologies have during the past decade evolved into a highly accurate ‘toolbox’, able to reproduce experimental data, with very high predictive power. In the present project, we will set up and investigate different types of models for lignin formation and, primarily, for lignin degradation. However, in order to understand how to best attack lignin at the molecular level, a fundamental understanding of the mechanisms for formation is also vital. We have thus initiated the project by exploring different aspects of monolignols and the first steps in the polymerization process. To this end we are investigating using classical and hybrid quantum/classical mechanics molecular dynamics simulations, lignin monomers (closed shell and radical forms) in water and the energy barriers required for lignin transport across a lipid bilayer (cell membrane); see Fig 1. These investigations will provide insight into the interactions between lignin and its surrounding, and the diffusion mechanism inside the cellulose/hemicellulose matrix. On the more detailed quantum level, we have investigated the mechanisms for dimerisation of lignin momomers, leading to the 7 different cross linkages observed. The relative stabilities of these were also determined (see Fig 2).
In addition, the reaction mechanisms of existing bleaching chemicals with lignin will be explored, in order to determine rate determining steps, and possibly provide suggestions for alternative chemicals or degradation routes. We are also investigating the mechanisms of photochemical bleaching through excited state calculations and comparison of computed and experimental data for radical fragments resulting from these processes (EPR spectroscopy). Finally, the reaction mechanisms of lignin degrading enzymes will be explored using a combination of quantum chemistry and molecular dynamics simulations.
Fig. 1. Model used in MD simulations of lignin monomer diffusion through lipid a membrane.
Fig. 2. Optimized structure of Guaiacyl-glycerol-b-coniferyl ether b-O4 linkage.
Yanni Wang, postdoc, UU
Bo Durbeej, Ph.D. student, UU