Lignin

Collaborative Research: The chemistry of lignin’s photochemical transformation in the environment: implications for global carbon cycling

The complex biopolymer lignin makes up approximately 25% of the organic carbon in the biosphere and is a major constituent in the formation of soil organic matter, natural organic matter in fresh and marine waters, and coastal marine sedimentary organic matter. Lignin plays a key role in the global C cycle that sustains the Earth by being an integral component of modern organic matter and in the past by its involvement in the formation of coal and other fossil fuels. In this project, we propose to employ a multidisciplinary approach to study the chemical fate of lignin as it becomes associated with modern environmental organic matter through abiological photochemical transformations involving reactive oxygen species (ROS). These ROS have also been implicated in biological oxidation of lignin. Recent and past studies suggest that lignin undergoes oxidative transformations in the environment that lead to both mineralization (to CO2 and H2O) as well as chemical rearrangement to new molecules which contribute to previously uncharacterized natural organic matter (e.g., humic substances). While mineralization is well understood chemically and biologically, the structural reordering by ROS to new molecules that have mainly been observed by ultrahigh resolution mass spectrometry and synchrotron X-ray absorption spectroscopy is a process that has only recently been noted but not adequately described chemically. It also appears to be catalyzed by transition metals (mainly Fe and Mn). The proposed work will use a plethora of advanced analytical methods in combination with computational studies to determine and establish the chemical transformation pathways for the molecular rearrangement of lignin in the environment and to test a proposed chemical transformation mechanism that is novel. The approach is to synthesize both labeled and unlabeled lignin as well as employ natural lignin concentrates that can be subjected to oxidative decomposition by ROS, mimicking the manner in which lignin is naturally degraded in the environment by mainly photochemical and biological oxidation.