Structural Elucidation of Deaminated Peptides in Anoxic Sediments

Marine sediments are globally a significant source of dissolved organic matter (DOM) to the water column. However, their role in the oceanic DOM cycle is unclear, because the molecular composition and reactivity of DOM that is produced in sediments remains largely unexplored. One of the possible mechanisms that could explain DOM accumulation in anoxic sediment porewaters is deamination of dissolved peptides. In this study we used Ultra-High-Performance Liquid Chromatography (UHPLC) coupled to an Orbitrap Fusion Tribrid Mass Spectrometer (OT-FTMS) to elucidate the chemical structure of deaminated peptides in porewaters collected from sediments in Santa Barbara Basin and Santa Catalina Basin. Preliminary results show that peptide deamination in these sediments is a selective process. The precursors of the deaminated peptides we detected have a high abundance of the amino acids: phenylalanine, proline, and tryptophan. We attribute this to limitations of extracellular proteinase enzymes to completely hydrolyze certain peptides to free amino acids, which results in residual small peptides that are rich in these three amino acids. We further hypothesize that these peptides are then used by microbes during fermentation processes, producing the deaminated peptides we observe. This work highlights the important connections between the oceanic and sedimentary carbon and nitrogen cycles as they relate to the formation of refractory DOM, and advances our understanding of the molecular composition of the proposed set of model refractory DOM compounds.