The photooxidation processes of tryptophan (Trp) in the presence of dissolved organic matter (DOM) were identified and quantified by steady-state photolysis experiments, laser spectroscopy and kinetic modeling. In sunlight, Trp photooxidation is dominated by the reaction with excited triplet DOM ((3)DOM), accounting for approximately 50-70% of the total degradation, depending on the DOM concentration and source. Reaction with singlet oxygen and direct photolysis are secondary processes that are both still more important than the reaction with hydroxyl radical. Both direct photolysis and reaction with (3)DOM form Trp radical cation (Trp(•+)) via Trp photoionization and direct oxidation, respectively. The Trp(•+) can be converted back to Trp by suitable electron or hydrogen atom donors. Transient absorption spectroscopy shows that DOM itself and low-molecular-weight analogues of redox-active moieties can reduce the lifetime of photochemically produced Trp(•+) and thus quench Trp degradation. This study demonstrates that DOM plays dual roles in the photodegradation of Trp acting as a sensitizer and quencher. The photochemistry of Trp and the participation of DOM have direct implications for photochemical reactions in extracellular proteins as well as for organic compounds in aquatic systems with similar photoionization processes.

The rate constant for the reaction between furfuryl alcohol (FFA) and singlet oxygen (O) in aqueous solution was measured as a function of temperature, pH and salt content employing both steady-state photolysis (β value determination) and time-resolved singlet oxygen phosphorescence methods. The latter provided more precise and reproducible data. The reaction rate constant, k, had a relatively small temperature dependence, no pH dependence and showed a small increase in the presence of high salt concentrations (+19% with 1 M NaCl). A critical review of the available literature suggested that the widely used value of 1.2 × 10 M s is likely overestimated. Therefore, we recommend the use of 1.00 × 10 M s for reactions performed in low ionic strength aqueous solutions (freshwater) at 22 °C. Furthermore, corrections are provided that should be applied when working at higher or lower temperatures, and/or at high salt concentrations (seawater).

The optical properties of dissolved organic matter influence chemical and biological processes in all aquatic ecosystems. Dissolved organic matter optical properties have been attributed to a charge-transfer model in which donor-acceptor complexes play a primary role. This model was evaluated by measuring the absorbance and fluorescence response of organic matter isolates to changes in solvent temperature, viscosity, and polarity, which affect the position and intensity of spectra for known donor-acceptor complexes of organic molecules. Absorbance and fluorescence spectral shape were largely unaffected by these changes, indicating that the distribution of absorbing and emitting species was unchanged. Overall, these results call into question the wide applicability of the charge-transfer model for explaining organic matter optical properties and suggest that future research should explore other models for dissolved organic matter photophysics.

Triplet-state chromophoric dissolved organic matter (CDOM*) plays an important role in aquatic photochemistry, yet much remains unknown about the reactivity of these intermediates. To better understand the kinetic behavior and reactivity of CDOM*, we have developed an indirect observation method based on monitoring time-resolved singlet oxygen (O) phosphorescence kinetics. The underpinning principle of our approach relies on the fact that O quenches almost all triplets with near diffusion limited rate constants, resulting in the formation of O, which is kinetically linked to the precursors. A kinetic model relating O phosphorescence kinetics to triplet excited states produced from isolated humic substances and in whole natural-water samples (hereafter referred to as CDOM*) was developed and used to determine rate constants governingCDOM* natural lifetimes and quenching by oxygen and 2,4,6-trimethylphenol (TMP), a common triplet probe molecule. CDOM* was found to exhibit smaller O and TMP quenching rate constants, ∼9 × 10 and ∼8 × 10 M s, respectively, compared with model sensitizers, such as aromatic ketones. Findings from this report shed light on the fundamental photochemical properties of CDOM in organic matter isolates and whole waters and will help refine photochemical models to more accurately predict pollutant fate in the environment.

Cysteine (Cys) plays numerous key roles in the biogeochemistry of natural waters. Despite its importance, a full assessment of Cys abiotic transformation kinetics, products and pathways under environmental conditions has not been conducted. This study is a mechanistic evaluation of the photochemical and nonphotochemical (dark) transformations of Cys in solutions containing chromophoric dissolved organic matter (CDOM). The results show that Cys underwent abiotic transformations under both dark and irradiated conditions. Under dark conditions, the transformation rates of Cys were moderate and were highly pH- and temperature-dependent. Under UVA or natural sunlight irradiations, Cys transformation rates were enhanced by up to two orders of magnitude compared to rates under dark conditions. Product analysis indicated cystine and cysteine sulfinic acid were the major photooxidation products. In addition, this study provides an assessment of the contributions of singlet oxygen, hydroxyl radical, hydrogen peroxide, and triplet dissolved organic matter to the CDOM-sensitized photochemical oxidation of Cys. The results suggest that another unknown pathway was dominant in the CDOM-sensitized photodegradation of Cys, which will require further study to identify.