title: Temperature study of indole, tryptophan and N-acetyl-L-tryptophanamide (NATA) triplet-state quenching by iodide in aqueous solution.
abstract: In this study, the temperature dependence of the measured phosphorescence lifetimes of aqueous indole, tryptophan and N-acetyl-L-tryptophanamide (NATA) between 6 and 55 °C in the absence and in the presence of iodide, a suitable intersystem crossing enhancer, has been determined. The obtained results suggest the existence of one process for the temperature-dependent, non-radiative deactivation of triplet states of the aqueous indoles in the absence of iodide. This process may be associated with the high sensitivity of indole triplet state lifetime to the subtle changes in the local viscosity of the surrounding aqueous environment or may be attributed to diffusional quenching by solvent molecules and/or by possible impurities present in water. However, the steep decrease in the measured phosphorescence lifetimes of indole and tryptophan with temperature suggests that diffusion-mediated quenching processes are not prevailing. Upon increasing concentration of iodide (up to 0.1 M), the obtained Arrhenius plots for the deactivation rate (1/τph) of the triplet states of the studied indoles were linear, which provided strong support for the hypothesis of the existence of one temperature dependent non-radiative process for the de-excitation of indoles triplet state. Our results showed that this process is attributed to the diffusion-controlled solute-quenching by iodide and, most probably, proceeds via reversibly formed exciplex. At concentration of iodide higher than 0.1M highly curved Arrhenius plots were obtained, which may indicate a change in the rate determining step with a change in temperature. This change most probably is associated with a transition from diffusion-controlled exciplex formation followed by rate-determining exciplex deactivation at high temperature.
authors.affiliation: Institute of General Food Chemistry, Faculty of Biotechnology and Food Sciences, Technical University of Lodz, ul. Stefanowskiego 4/10, 90-924 Lodz, Poland. agnieszka.kowalska-baron@p.lodz.pl
authors.forename: Agnieszka
authors.initials: A
authors.lastname: Kowalska-Baron
authors.forename: Krystian
authors.initials: K
authors.lastname: Gałęcki
authors.forename: Stanisław
authors.initials: S
authors.lastname: Wysocki
mesh_terms: D007211:Indoles / Q000737:chemistry*; D007454:Iodides / Q000737:chemistry*; D007700:Kinetics; D049449:Luminescence; D008968:Molecular Conformation; D012996:Solutions; D013696:Temperature; D013997:Time Factors; D014364:Tryptophan / Q000031:analogs & derivatives* / Q000737:chemistry*; D014867:Water / Q000737:chemistry
chemical_list: D007211:Indoles; D007454:Iodides; D012996:Solutions; D014867:Water; C016952:N-acetyltryptophanamide; C030374:indole; D014364:Tryptophan