Mechanism of photodegradation of aqueous organic polluants. I, EPR spin-trapping technique for the determination of .OH radical rate constants in the photooxidation of chlorophenols following the photolysis of H2O2 (original) (raw)

Mechanism of photodegradation of aqueous organic pollutants. 1. EPR spin-trapping technique for the determination of hydroxyl radical rate constants in the photooxidation of chlorophenols following the photolysis of hydrogen peroxide

The Journal of Physical Chemistry, 1991

A technique for the determination of the rate constants for the reaction of 'OH radicals with organic substrate is described. This technique is based on the use of spin trapping with electron paramagnetic resonance (EPR) detection of spin adducts. The desired rate constants are obtained by measuring either the initial rate of production of the EPR signal of the spin adduct or its amplitude after a fixed time, as a function of the concentration of the substrate. By using the known rate constant for the reaction of 'OH radicals with the spin trap (DMPO), rate constants for the reaction of *OH radicals with a given substrate can be obtained within a competition kinetic scheme. The method has been validated by determining the rate constants for the reaction of 'OH radicals with phenol and with formate, two reactions well studied by using pulse radiolysis. Our results agree with literature values within experimental error. The method is then applied to several chlorophenols. The rate constants for 3-chloro-substituted phenols are significantly less than for 4-chloro or 2-chloro-substituted phenols. Some rate constants are significantly larger than the diffusionantrolled rate constant. This is explained by proposing a Grotthus-type mechanism for the movement of 'OH radicals through water. (16) Bolton, J. R.; Clayton, R. K.; Red, D. W. Phorochem. Phorobiol. 1969, 9, 209. (17) Moa, P. N.; Fytianos, K.; Samanidou, V.; Korte, F. Bull. Enuiron. (18) Neta, P.; Steenken, S.; Janzen, E. G.; Shetty, R. V.

Abstracts, Division of Biological Chemistry, 190th National Meeting of the American Chemical Society, September 8-13, 1985

Biochemistry Usa, 1985

Flavodoxins, the FMN-containing electron carriers present in a variety of rriicroorganisms, have lower oxidation-reduction potentials than other members of the flavoprotein family. Crystal strucfure analyses of these proteins, with FMN in the oxidized, semiquinone, and reduced forms, have been carried out in an attempt to understand how the interaction with proteins modulates the oxidation-reduction potential of flavins. Initial studies with flavodoxin from Clostridium MP have suggested that the oxidized/semiquinone potential depends on formatign of a hydrogen bond between the polypeptide backbone and the "(5) of the flavin semiquinone, while the semiquinone/reduced potential may depend on constraints on the bending and motion of the reduced isoalloxazine ring. These'proposals have been explored by structural comparisons with other species of flavodoxins, especially the protein from Anacystis nidulans, which has a higher potential for the oxidized/semiquinqne equilibrium than most other flavodoxins.

31P nuclear magnetic resonance studies of effects of some chlorophenols on Escherichia coli and a pentachlorophenol-degrading bacterium

Journal of Bacteriology

A Flavobacterium sp. that mineralizes pentachlorophenol degrades some, but not all, of the other chlorinated phenols. Whole-cell 31P nuclear magnetic resonance was used to compare and observe transmembrane pH gradients and nucleotide pools in the Flavobacterium sp. and Escherichia coli after pentachlorophenol and 3,4,5-trichlorophenol were added to the cell suspensions. The data suggest that those chlorinated phenols which are not degraded by the Flavobacterium sp. may be resistant to degradation because they act as proton dissipators.

Journal of Photochemistry & Photobiology, B: Biology

Benzotriazole UV-stabilizers (BUVSs) are commonly used in industry as solar filters, due to their strong UV light absorption. Because of their extended usage, environmental contamination of waters due to BUVSs constitutes a growing concern. Direct photodegradation of BUVSs is not efficient due to their intrinsic thermal pathways to release the absorbed light. Nevertheless, their abatement in natural environments could be assisted by chromophoric dissolved organic matter. Among the BUVSs, three representative candidates were selected, UV-326, UV-327 and UV-328, to demonstrate the potential of Riboflavin (RF) as a natural visible-light absorbing photocatalyst for the abatement of these recalcitrant pollutants under reductive conditions. The use of visible light and DABCO, as a model sacrificial electron donor, generates the radical anion RFTA .-. This key species reacts with the solar filters, achieving their reductive abatement from the medium. Moreover, the participation of every potential reactive species has been investigated by photophysical techniques, together with determination of the quenching rate constant for every reaction pathway. Consequently, evidence supported the main role of the reductive photodegradation pathway, being RFTA .the key species in the abatement of BUVSs.

Formation and decay of monodehydroascorbate radicals in illuminated thylakoids as determined by EPR spectroscopy

Biochimica et Biophysica Acta (BBA) - Bioenergetics, 1995

H20 2 is reduced by an ascorbate-specific peroxidase (APX) in chloroplasts, generating the monodehydroascorbate (MDA) radical as the primary oxidation product. Using EPR spectroscopy we have measured the light-driven formation and decay of this species in thylakoids containing active APX. Illumination caused a rapid exponential rise in the steady-state MDA radical concentration in the absence of added electron acceptors other than 0 2. This increase was sensitive to KCN and catalase and was prevented by anaerobic conditions, demonstrating the requirement for APX activity and endogenously generated H202, i.e., the Mehler reaction. When the illumination was removed, a second, transient increase in the radical signal was observed, indicating that photoreduction of the MDA radical and 0 2 were occurring simultaneously in the light. This interpretation is also supported by the sigmoidal behavior of the chlorophyll dependence of MDA radical formation in illuminated thylakoids. Ferredoxin lowered the light-induced, steady-state MDA radical concentration, and is thus implicated as the physiological photoreductant for this Hill acceptor. In the absence of uncoupler, NADP + prevented formation of the MDA radical by lowering the flux to molecular O 2. However, in the presence of uncoupler (5 mM NH4C1) this constraint was apparently overcome, i.e., net formation of the radical occurred. The EPR method represents a novel approach to investigating the interaction of 0 2 and ascorbate metabolism in chloroplasts under a variety of physiologically relevant conditions, to be applied in future studies of plant response to environmental stress.