Amino Acids (AAs) and AA-based molecules are key sources of nitrogen in natural waters, providing building blocks for protein synthesis and energy for microbial growth. Dissolved free (DFAAs) and combined AAs (DCAAs) account for 20-65% of bacterial nitrogen demand in aquatic environments. In sunlit waters, abiotic transformation such as photochemical oxidation plays an important role for the fate of DFAAs. Photochemical oxidation occurs via direct and indirect photolysis by photochemically produced reactive intermediates (PPRIs), such as excited triplet state chromophoric dissolved organic matter (³CDOM*), singlet oxygen (¹O₂), and hydroxyl radicals (HO^●). Due to the complex nature of CDOM and subsequent radical-involved reaction mechanisms, little is known about the fate of DFAAs in natural waters. Thus, there is a need to understand and predict the fate and transformation of DFAAs and their role in the global nitrogen cycle. The goal of this study is to understand the photochemical fate of photo-viable DFAAs in freshwater aquatic environment. In this talk, we present the initial reactivities of three structurally unique DFAAs: tyrosine, histidine, and methionine in the presence of three surrogate CDOM. We use density functional theory quantum mechanical methods to predict initial elementary reactions. Theoretical aqueous-phase free energies were used to evaluate kinetics and thermodynamic feasibility of PPRI reaction pathways, validated by benchtop experiments. Theoretical and experimental results demonstrated that methionine and histidine were most susceptible to photochemical degradation by ¹O₂ and ³CDOM*while tyrosine only degraded slightly. Histidine and methionine both showed high dependence on solution temperature for degradation. Initial product screening via liquid chromatography coupled with mass spectrometry was employed for methionine and 1,4-napthoquinone to identify key oxidation products. Total carbon and nitrogen was measured to complete the mass balance on key elements. Lastly, a kinetic model was developed to predict the initial photochemical degradation and by-product formation.

Paul Doskey

Appearances