Dissolved free and combined amino acids are major sources of nitrogen and carbon in freshwater systems. Combined amino acids account for approximately 10-20% of wastewater DON. In the aquatic environment, DON is a limiting nutrient making it a valuable source for the environmental microbiome.  Although the amino acids comprise a few percent of dissolved free amino acids, dissolved free amino acids are considered ‘rare’ for the abundance and ‘expensive’ for the higher free energies of formation required. Consequently, understanding the fate of free amino acids in natural aquatic environment is a critical component of global nitrogen or sulfur cycling.<br/>Abiotic photochemical oxidation is a major transformation pathway of free amino acids in sunlit waters. Of the 20 proteinogenic free amino acids, only five (i.e., tryptophan, tyrosine, histidine, methionine, and cysteine) are susceptible to photochemical degradation. Photolysis-induced excitation of a complex mixture of CDOM that has numerous chromophore functional groups and short-lived reactive photochemically produced reactive intermediates (PPRI) makes it challenging to understand the photochemical reaction pathways and kinetics.<br/>In this study, we investigate the initial reactivities of three photo-viable amino acids (i.e., histidine, tyrosine, and methionine) in the presence of three surrogate CDOM (i.e., 1,4-napthquinone, 2-naphthaldehyde, and umbelliferone) and standard DOM isolates under a laboratory-scale visible light irradiation at pH 7 with 3 different solution temperatures. We use both density functional theory quantum mechanical calculations for the excited state reactions and bench-scale laboratory experiment for the quantification of initial fate of three free amino acids and probes of photochemically produced reactive intermediates as well as the transformation of surrogate CDOM. We develop a mechanistic kinetic model to predict the initial fate of free amino acids in the presence of surrogate CDOM.

Initial transformation of photo-viable free amino acids in the presence of surrogate and standard dissolved organic matters under sunlit irradiation

ACS Spring 2022

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