The photochemical fate of five sulfa drugs with varying five-membered heterocyclic substituents (sulfamethoxazole, sulfisoxazole, sulfamethizole, sulfathiazole, and sulfamoxole) was investigated in aqueous solution. The rate of direct photolysis of these compounds is dependent upon the identity of the heterocyclic R group as well as the pH of the solution. Matrix deconvolution methods were employed to determine the absorption spectrum and photolysis rate of each protonation state (cationic, neutral, and anionic). From these data, quantum yields for direct photodegradation were calculated for each protonation state of the sulfa drugs. The quantum yields calculated range from <0.005 for the neutral state of sulfamethizole to 0.7 +/- 0.3 for the protonated state of sulfisoxazole. The protonation state that is most photoreactive varies among the sulfa drugs and cannot be attributed to the rate of photon absorption. Products arising from the direct photolysis of the sulfa drugs were also investigated. For all the compounds, sulfanilic acid was observed as a common product. The singlet oxygen quenching rates of the sulfa drugs were determined by laser flash photolysis and range from (2 +/- 1) x 10(4) M(-1) s(-1) for sulfamethoxazole to (3.0 +/- 0.7) x 10(8) M(-1) s(-1) for sulfamoxole. Reaction of the sulfa drugs with hydroxyl radical is not modulated by the R group, and the rate constants are all near the bimolecular diffusion-controlled limit of 10(10) M(-1) s(-1). The photodegradation of the sulfa drugs in natural water samples of Lake Josephine (St. Paul, MN) and Lake Superior was attributed solely to direct photolysis. This study indicates thatthese similarly structured antibiotics will be subject to a wide range of photodegradation rates with sulfathiazole degrading relatively quickly, sulfisoxazole and sulfamethizole degrading moderately, and sulfamethoxazole degrading much more slowly.

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The aquatic photochemical behavior of a class of sulfa drugs containing six-membered heterocyclic substituents (sulfamethazine, sulfamerazine, sulfadiazine, sulfachloropyridazine, and sulfadimethoxine) was investigated. Photodegradation of the sulfa drugs in a natural water sample was significantly enhanced relative to the degradation in deionized water, with the exception of sulfadimethoxine. This indicated an indirect photochemical process that was identified through the use of quenchers to be attributable to interaction with triplet excited-state dissolved organic matter (3DOM). The direct photolysis rate constant and quantum yield for both the neutral and anionic species of each sulfa drug were calculated using matrix deconvolution methods. The quantum yield values range from 0.01 x 10(-3) for the neutral form of sulfadimethoxine to 5 x 10(-3) for the anionic form of sulfamethazine and are significantly lower than those observed in a previous study for sulfa drugs containing five-membered heterocyclic substituents, although the rate constants are of similar magnitude. The primary product formed in both direct and indirect photodegradation for all five compounds was identified as a sulfur dioxide extrusion product. The predicted environmental half-lives solely attributable to direct photolysis range from 8.6 h in midsummer at 30 degrees latitude in pH 7 surface water for sulfachloropyridazine to 420 h in midwinter at 45 degrees in pH 7 surface water for sulfadimethoxine. These half-lives, except for sulfadimethoxine, will be decreased by interaction with 3DOM.

The role of photochemically generated singlet oxygen (1O2) in the DOM-sensitized degradation of eighteen dissolved free amino acids was investigated. The fraction of total sensitized degradation due to reaction with 1O2 was determined through a kinetic analysis based on a measured reaction rate constant for each amino acid coupled with measured 1O2 concentrations and was confirmed through quenching experiments. Only four of the eighteen free amino acid residues examined were found to be photolabile under environmentally relevant conditions: histidine, methionine, tyrosine, and tryptophan. The fraction of Suwannee River Humic Acid (SRHA)-sensitized degradation due to reaction with 1O2 ranged from an upper value of 110 +/- 10% for histidine to 8 +/- 1% for tryptophan, with 26 +/- 3% contribution for methionine and 33 +/- 4% for tyrosine. In addition to degradation through reaction with 1O2, other reactive intermediates involved in the SRHA-photosensitized degradation of these amino acids were identified. Methionine was thought to be additionally degraded through reaction with H2O2 and triplet excited-state DOM, and 67% of tyrosine's indirect photodegradation was assigned to an oxygen-dependent type I photooxidation reaction. The majority of tryptophan indirect degradation was due to reaction with 3DOM. Photodegradation experiments with various DOM sources including Pony Lake (Antarctica) fulvic acid and a synthetic estuarine sample, as well as Minnesota freshwater samples (lakes Itasca, Superior, Josephine, and the St Louis River), demonstrated distinct reactivity patterns, indicating that DOM's 1O2-generation efficiency is not strongly coupled to its ability to promote other photooxidation pathways. These four amino acids highlightthe differential photoreactivity of DOM from various sources.

An intuitive laboratory experiment designed to study the phenomenon of collisional ͑or pressure͒ broadening is presented. Molecular collisions are modeled by striking a tuning fork with a mallet. The response of the tuning fork is recorded via microphone by a computer. These time domain data can be transformed numerically to reveal the frequency spectrum. Results corresponding to a range of pressures from 0 to 111 kPA are presented.