The primary goal of the proposed work is to understand the dissolution and depolymerization mechanism of lignin in ionic liquids (ILs) using both experimental and computational approaches. Lignin is a complex biopolymer, holding together cellulose and hemicelluloses and forming a recalcitrant matrix. The ability of certain ILs to dissolve cellulose and/or lignin enables the possibility of commercial lignin upgrading and valorization to improve overall biorefinery economics. However, to date depolymerization mechanisms of lignin in ILs is not well understood. Lignin model compounds will be used at the first stage in order to establish a performance benchmark. We will leverage EMSL's expertise in LC/MS and NMR and develop new lignin characterization protocols. Based on the information gained from lignin model compounds, we will synthesize lignin model compounds at the Joint BioEnergy Institute (JBEI) and investigate the lignin dissolution and depolymerization reactions in selected ILs treated at specific temperatures and times. Native lignin derived from lignocellulosic biomass (switchgrass, eucalyptus and pine) will be investigated in the IL system. In parallel with the experimental study, the proposal also aims at a theoretical investigation on the structural properties and lignin interactions with the IL ions and ion pairs using ab initio quantum mechanics (QM) and combined QM/MM (molecular mechanics) methodologies. We will investigate computationally challenging lignin-IL systems in close collaboration with the EMSL computational team using the molecular science supercomputing facility located there. Proposed elementary steps for lignin breakdown in IL from the experiments have already been constructed and tested with QM calculations at JBEI. The proposed work will establish atomic potentials for ILs, lignin monomers and polymers, as well as advance simulation methods, algorithms and optimization techniques. It is anticipated that the data and modeling tools developed by this proposal will establish a mechanistic understanding of how lignin behaves in these complex environments that will enable the design of novel ILs and inform the genetic engineering of lignin biosynthesis pathways to produce feedstocks that are more amenable to deconstruction.