Laura S. Villata

Photoinduced cleavage of plasmid DNA in the presence of pterin

Summary The photoinduced cleavage of plasmid DNA by UV -A light in the presence of pterin was investigated. Electrophoretic analysis of the irradiated plasmid pUCI8 in the presence of pterin showed that UV light of 350 nm induced the transformation of a significant proportion of the supercoiled plasmid to its relaxed form. A minor proportion of plasmid forms are also converted to the linear plasmid isomer at the longer irradiation times. All these transformations during irradiation can be observed in the absorption spectrum of DNA as function of time. Such spectral modifications correlated with the extent and the kinetics of plasmid relaxation, but not with the appearance of the linear plasmid. None of the exchanges were operative without the irradiation with UV-A light. Control experiments with pterin or plasmid DNA irradiated separately, showed no photochemical changes. Results taken together suggest that the observed changes in the supercoiled plasmid as well as the spectral modifications both derive from the generation of single-strand break in the DNA.

Solvent quenching of the 5D0 → 7F2 emission of Eu(6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedionate)3

Abstract Quenching of luminescence of Eu(fod) 3 (fod = 6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5 octanedionate) has been studied in different solvents (i.e., acetonitrile, ethanol, methanol, n -butanol, 2-propanol, bencylic alcohol, chloroform, dichloromethane, 1,1-dichloroethane, 1,2 dichloroethane, carbon tetrachloride, benzene, toluene, m -and p -xilene, acetone, ciclohexanone, methylethylketone, methylpropylketone (iso), fluorobenzene, chlorobenzene, bromobenzene, ethyl ether, dioxane and tetrahydrofurane, and deuterated solvents acetonitrile (d 3 ), ethanol (d 1 ), methanol (d 4 ), benzene (d 6 ), acetone (d 6 ) and chloroform (d)). Absorption and emission spectra of the complex in the different solvents, their luminescence lifetimes at room temperature and at the boiling temperature of liquid air, so as in solid phase were measured. Evidences of different mechanisms for quenching of the luminescence is presented and discussed.

Photochemical and catalytic properties of dimeric species of molybdenum(V)

Abstract The chemistry and photochemistry of molybdenum coordination compounds in its several oxidation states is of great importance from basic as well as applied points of view. Molybdenum is a relevant element for the synthesis of many homogeneous and heterogeneous catalysts. This element is also essential in several enzymatic systems. One of the characteristics of the molybdenum chemistry is related to the easy conversion between its oxidation states and to the changes of coordination number, observed particularly between Mo III , Mo V and Mo VI . Many aspects of the chemistry of Mo V have been reviewed. However, the thermal and photochemical reactivity of its complexes was not completely well understood until few years ago. Our studies have shown that aqueous solutions of Mo 2 O 4 (H 2 O) 6−x L x n+ complexes (where L is a monodentate ligand, such as H 2 O, Cl − , NCS − , Br − or bidentate such as cysteine), present dual reactivity; ligand–metal charge transfer (LMCT) and photolabilization of the ligand. The excitation induces photolabilization resulting in the formation of a single bridged μ-oxo-dimer that undergoes dissociative disproportionation into monomeric Mo IV and Mo VI species. Mo IV reacts with coordinated water leading to H 2 and Mo VI . Photoreduction of the complexes to Mo IV Mo V dimers which can be characterized as an LMCT process has also been reported. The main objective of the present review is to present an update of data obtained from studies performed on these systems and the contribution of our laboratory to the understanding of the photochemical and catalytic properties of the di μ-oxo-dimers of molybdenum(V).

Photoinduced reduction of divalent mercury by quinones in the presence of formic acid under anaerobic conditions

The laser flash photolysis technique (λ(exc)=355 nm) was used to investigate the mechanism of the HgCl(2) reduction mediated by CO(2)(-) radicals generated from quenching of the triplet states of 1,4-naphthoquinone (NQ) by formic acid. Kinetic simulations of the experimental signals support the proposed reaction mechanism. This system is of potential interest in the development of UV-A photoinduced photolytic procedures for the treatment of Hg(II) contaminated waters. The successful replacement of NQ with a commercial fulvic acid, as a model compound of dissolved organic matter, showed that the method is applicable to organic matter-containing waters without the addition of quinones.

The flash photolysis of di-μ-oxo-bis(oxo-molybdate(V)) acid aqueous solutions in the presence of excess chloride

Abstract The flash photolysis of di-μ-oxo bis(oxo-molybdenum(V)) complexes in aqueous solutions of perchloric acid in the presence of excess chloride induced both photolabilization and photo-oxidation of Cl − and/or H 2 O ligands. Photolabilization produces the formation of reaction intermediates identified as μ-oxo-bis (oxo-molybdenum(V)) species: Mo 2 O 2 (μ-O)(H 2 O) 8 4+ ( A ) and Mo 2 O 2 (μ-O)(Cl)(H 2 O) 7 3+ ( B ). A rapid equilibria between the transient species is established before they decay through a first order reaction. Intermediate A is the only species effective in promoting the observed decay leading to Mo(VI) and H 2 as end products. The relative absorption spectrum of species B is reported, as well as its stability constant in the temperature range 25–45 °C. The effect of ligands on the overall mechanism is discussed.

Temperature effects on the quenching of the 5D0→7F2 emission of Eu(6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedionate)3 by a Cu(II) macrocycle

Luminescence quenching of Eu(fod)3(fod = 6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedionate) by a Cu(II) macrocycle was studied at 25, 35 and 45 degrees C by steady-state and flash luminescence techniques, varying the Cu(II) concentration between 0.2 and 20 mM. Experimental variation of the observed rate constant with the quencher concentration is rationalized in terms of a mechanism involving the quenching of two unequilibrated species by the Cu(II) macrocycle.

The catalytic effect of MoV on the oxidation of iodide by chlorate

Abstract The kinetics of the catalytic effect of Mo2O4(H2O)2+6 on the iodide oxidation by chlorate was studied at different temperatures (25–45°C) in Ar-saturated acid aqueous solutions under conditions of excess iodide ion by monitoring the formation of I−3 at 352 nm. The initial reaction rate at 30°C is represented by the following equation: v = 1 3 d[I−3]/d t = kMo[H+]2[I−][ClO−3] with kMo = 9 × 10−5 s−1 · M−3 + 0.73 s−1 · M−3 [H+]−1[Mo2O2+4] where the brackets indicate molar concentrations and time is expressed in seconds. A mechanism, which includes the formation of a dimeric molybdenum complex as rate determining step, is proposed to account for the experimental results.

Oxidation of bromide by chlorate catalysed by MoV

Abstract The kinetics of the catalytic effect of Mo2O4(H2O)2+6 on the bromide oxidation by chlorate was studied at different temperatures (22–39°C) in acid aqueous solutions under conditions of excess bromide ion by monitoring the formation of Br3− at 265 nm. The initial rate of Br3− formation at 25°C is represented by the following equation: d [ Br 3 − ] d t ={1.1×10 −5 [ H + ] 2 }+(0.012+0.04[ H + ])}[ Br − ][ ClO 3 − ] where the brackets indicate molar concentrations and time is expressed in seconds. A mechanism, which includes the formation of a dimeric molybdenum complex as rate determining step, is proposed to account for the experimental results.

One-electron oxidation of antioxidants: A kinetic-thermodynamic correlation

Abstract The values of the bimolecular rate constants for the reactions of 2,2′-azino-bis(3-ethylbenz-thiazoline-6-sulfonic acid) radical cation with epicatechin (((2.4 ± 0.2)) s−1 M−1), and epigallocatechingallate ((29 ± 5) s−1 M−1) were obtained by spectrophotometric measurements. We propose a correlation between the Gibbs energy ΔGo for the one-electron charge-transfer reactions from several antioxidants to radical species and the rate constants of the corresponding bimolecular reactions. This correlation can be used to predict rate constants of reactions of known ΔGo values.

Physicochemical and toxicological studies on 4-chloro-3,5-dinitrobenzoic acid in aqueous solutions

Physicochemical characterization of hazardous compounds often is required for the development of structure-reactivity correlations. Physical, chemical, and toxicological properties of target pollutants require determination for an efficient application of wastewater treatments. In the present work, we chose a chloro-nitro-aromatic derivative (4-chloro-3,5-dinitrobenzoic acid [CDNBA]), as a model compound on which to perform physicochemical and toxicological studies. Several properties of CDNBA are not available in the literature, although many aromatic nitro-compounds are considered hazardous materials. Measurements of solubility in water, acid dissociation constant, and kinetic parameters for the nucleophilic substitution of chlorine atom in alkaline media are reported. We also performed cytotoxicity studies of CDNBA and ultraviolet-irradiated CDNBA solutions. From the analysis of CDNBA solubility in water at different temperatures, an enthalpy of solution of 23.2 +/- 2.5 kJ/mol was found. The study of the acid dissociation constant Kc by using conductivity measurements and the modified Grans method yielded values for the equilibrium constant Ka of 2.36 x 10(-3) and 2.26 x 10(-3), respectively. The bimolecular rate constant for the reaction of CDNB- and hydroxyl ion (HO) measured at room temperature and 0.1 M of ionic strength was 5.92/M x s, and the activation energy for this process was 70.7 +/- 3.4 kJ/mol. Cytotoxicity assays with aqueous suspensions of Tetrahymena pyriformis showed lethal effects due to the pH change induced by CDNBA. On the other hand, in buffered solutions, a value of 104.47 microM was observed for the median effective concentration, that is, the concentration of CDNBA at which the proliferation was restricted to one half of the blank. Irradiation of CDNBA solutions increased the toxicity, suggesting the formation of intermediate products with higher cytotoxicity effects.