1-Nitropyrene (1NPy) is the most abundant nitropolycyclic aromatic contaminant encountered in diesel exhausts. Understanding its photochemistry is important because of its carcinogenic and mutagenic properties, and potential phototransformations into biologically active products. We have studied the photophysics and photochemistry of 1NPy in solvents that could mimic the microenvironments in which it can be found in the atmospheric aerosol, using nanosecond laser flash photolysis, and conventional absorption and fluorescence techniques. Significant interactions between 1NPy and solvent molecules are demonstrated from the changes in the magnitude of the molar absorption coefficient, bandwidth at half-peak, oscillator strengths, absorption maxima, Stokes shifts, and fluorescence yield. The latter are very low (10 (-4)), increasing slightly with solvent polarity. Low temperature phosphorescence and room temperature transient absorption spectra demonstrate the presence of a low energy (3)(pi,pi*) triplet state, which decays with rate constants on the order of 10 (4)-10 (5) s (-1). This state is effectively quenched by known triplet quenchers at diffusion control rates. Intersystem crossing yields of 0.40-0.60 were determined. A long-lived absorption, which grows within the laser pulse, and simultaneously with the triplet state, presents a maximum absorption in the wavelength region of 420-440 nm. Its initial yield and lifetime depend on the solvent polarity. This species is assigned to the pyrenoxy radical that decays following a pseudo-first-order process by abstracting a hydrogen atom from the solvent to form one the major photoproducts, 1-hydroxypyrene. The (3)(pi,pi*) state reacts readily ( k approximately 10 (7)-10 (9) M (-1) s (-1)) with substances with hydrogen donor abilities encountered in the aerosol, forming a protonated radical that presents an absorption band with maximum at 420 nm.

Ground state absorption, first excited-singlet state, and properties of reactive intermediates of mononitropyrene isomers encountered in the atmospheric aerosol have been studied under different conditions that could mimic the environment. The nitro group can present different orientation relative to the pyrene ring depending on its geometric location and could induce differences in the photochemistry of the isomers. The 2-NO 2 Py isomer has the largest red shift and lowest oscillator strength in the UV-visible band associated with the nitrogroup. The isomers show very low fluorescence yields (10 −3 -10 −4 ). Only 1-NO 2 Py and 4-NO 2 Py have phosphorescence emission (Φ p ≈ 10 −4 ), indicating that the lowest triplet state decays mainly through effective radiationless channels. Laser photolysis produces a low lying triplet state (τ T = 10 −5 -10 −6 s), a long-lived pyrenoxy radical, and a PyNO 2 H radical in solvents in which the triplet can abstract a hydrogen atom. Similar triplet yields were calculated (0.1-0.6) for the isomers while significant differences in the relative yield of the long-lived species were determined. Differences in the quenching rate constants of the triplet by water and phenols suggest a strong hydrogen bond interaction with the nitro group in the C-2 position, which provides for radiationless deactivation routes.

The capacity of hydroxycinnamic acid derivatives to trap peroxyl radicals was evaluated by competitive kinetics and oxygen radical absorbance capacity (ORAC) indexes, using c-phycocyanin and pyranine as target molecules. The pattern of results is similar in all the systems, with the reactivity of the compound determined by the bond dissociation energy (BDE) of the hydrogen atom of the phenolic moiety. However, differences in the relative reactivity are observed depending upon the employed methodology (initial rate of consumption or ORAC-type methodology) and target molecule employed. These differences are explained in terms of the role played by secondary reactions of the initially formed phenoxyl radicals. This emphasizes the need for performing a complete kinetic analysis of the results in order to obtain meaningful evaluations of the relative reactivity of the tested compounds.

Gold nanoparticles capped with cucurbituril[7] have been prepared in the absence of metallic cations and organic ligands. Remarkably, these nanohybrids encapsulate dissolved oxygen and are highly active in electrochemical reduction. The effect of the presence of sodium and ammonium salts on this catalysed process is also analysed.

The oxidation of tryptophan (Trp) residues, mediated by peroxyl radicals (ROO.), follows a complex mechanism involving free radical intermediates, and short chain reactions. The reactivity of Trp towards ROO. should be strongly affected by its inclusion in peptides and proteins. To examine the latter, we investigated (by fluorescence) the kinetic of the consumption of free, peptide-and protein-Trp residues towards AAPH (2,2'-azobis(2-amidinopropane) dihydrochloride)-derived free radicals. Interestingly, the initial consumption rates (R-i) were only slightly influenced by the inclusion of Trp in small peptides and proteins (human serum albumin and human superoxide dismutase). Depending on the Trp concentration, the R-i versus Trp concentration ([Trp]) plots showed three regions. At low Trp concentrations (1-10 mu M), a linear dependence was observed between R-i and [Trp]; at intermediate Trp concentrations (10-50 mu M), the values of R-i were nearly constant; and at high Trp concentrations (50 mu M to 1 mM), a slower increase of R-i than expected for chain reactions. Similar behavior was detected for all three systems (free Trp, and Trp in peptides and proteins). For the first time we are showing that alkoxyl radicals, formed from self-reaction of ROO., are responsible of the Trp oxidation at low concentrations, while at high Trp concentrations, a mixture of peroxyl and alkoxyl radicals are involved in the oxidation of Trp residuesFondecyt 1141142 3140307 Novo Nordisk Foundation NNF13OC000429