Book Description

Abstract : Sulfur (S)-containing amino acids are key sources of carbon, nitrogen, and sulfur involved in protein synthesis, protein function, and providing energy for microbial growth. Dissolved free and combined methionine is one of two S-containing amino acids incorporated into proteins and has been attributed to their stability and function. The oxidation of methionine has received considerable attention given its ubiquitous presence in most biological systems and has been associated with losses in protein function and pathological disorders. In natural waters, methionine is rapidly and selectively taken up by microorganisms to achieve cellular requirements of carbon, nitrogen, and sulfur. The abiotic transformation of methionine is ultimately a sink of key macronutrients and attributed to cycling across environmental compartments. In particular, the photochemical transformation of methionine in the presence of dissolved organic matter (DOM) is an important component of cycling in sunlit surface waters globally, yet knowledge is lacking on the fate and transformation of methionine in the environment. In this study, we investigated the photo-transformation products involved in the photochemical fate of dissolved free methionine in the presence of surrogate and standard isolate dissolved organic matter (DOM). Temperature-dependent, bench-top photolysis experiments under simulated sunlight at 10, 20, and 30 oC were conducted and a wide array of analytical analyses were employed to elucidate transformation products and provide insights into reaction mechanisms. Two surrogate DOM compounds structurally unique and relevant to complex mixtures of DOM were employed, including 1,4-naphthoquinone and 2-naphthaldehyde. The two surrogate DOM have common base structures and critical functional groups known to be important photosensitizers in the natural environment generating photochemically-produced reactive intermediates including excited triplet-state chromophoric DOM, singlet oxygen, and hydroxyl radicals. The quinone and carbonyl functionalities in 1,4-naphthoquinone and aldehyde and naphthalene functionalities in 2-naphthaldehyde generated unique transformation pathways for methionine and novel photo-transformation products were identified, providing key insights into the mechanisms of transformation. Photolysis experiments were expanded to two unique standard isolate DOM (Suwannee River Humic Acid, Elliott Soil Humic Acid) and previously identified transformation products were quantified to validate results in environmentally-relevant solutions of DOM. Mass balance analyses were performed to assess the transformation of key macronutrients including carbon, nitrogen, and sulfur present in methionine.