Biomedical subjects

Alberto L Capparelli

Publications and source records attributed to Alberto L Capparelli.

12 recordsLinked to original sources

PubMed2004

Photooxidation of pterin in aqueous solutions: biological and biomedical implications.

Studies of the photochemical reactivity of pterin (= 2-aminopteridin-4(3H)-one; PT) in acidic (pH 5.0-6.0) and alkaline (pH 10.2-10.8) aqueous solutions have been performed. The photochemical reactions were followed by UV/VIS spectrophotometry, thin layer chromatography (TLC), high-performance liquid chromatography (HPLC), and an enzymatic method for H2O2 determination. PT is not light-sensitive in the absence of molecular oxygen, but it undergoes photooxidation in the presence of O2, yielding several nonpteridinic products. The quantum yields for PT disappearance were found to be 8.2 (+/-0.6) x 10(-4) and 1.2 (+/-0.2) x 10(-3) in acidic and alkaline media, respectively. H2O2 was detected and quantified in irradiated solutions of PT; and its importance from a biomedical point of view is discussed. The rate constant of the chemical reaction between singlet oxygen ((1)O2) and PT was determined to be 2.5 (+/-0.2) x 10(5) l mol(-1) s(-1) in alkaline medium, and the role of (1)O2 in the photooxidation of pterin was evaluated.

Oxidation-Reduction↗

PubMed2004

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 Gran's 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.

Acid-Base Equilibrium↗

PubMed2003

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.

Journal Article↗

PubMed2003

Quenching of the fluorescence of pterin derivatives by anions.

Steady-state and time-resolved studies of the fluorescence of pterins in aqueous solutions in the presence of different anions have been performed using the single-photon counting technique. In the pH range between 3 and 13, most pterins exist in a protonated and a deprotonated form. Results obtained for both acid and basic forms of five compounds belonging to the pterin family (pterin, 6-carboxypterin, 6-formylpterin, biopterin and neopterin) show that the fluorescence of the acid forms is dynamically quenched by phosphate and acetate, and the corresponding bimolecular rate constants of fluorescence quenching (k(q)) are reported. These results are of importance from the technical and analytical points of view because measurements of the fluorescence of pterin derivatives for a variety of purposes are often performed in the presence of salts, especially buffers, and significant quenching of the pterin fluorescence by the buffer might lead to errors in interpretation and erroneous conclusions. No quenching of the fluorescence of the acid forms by chloride, sulfate or nitrate was detected. The fluorescence of the basic forms was either not quenched by anions or any such quenching was negligible in comparison with that observed for the acid forms.

Anions↗

PubMed2003

Competition kinetics using the UV/H2O2 process: a structure reactivity correlation for the rate constants of hydroxyl radicals toward nitroaromatic compounds.

The rate constants for hydroxyl radical reaction toward a set of nitroaromatic substrates kS, have been measured at 25 degrees C using competition experiments in the UV/H2O2 process. For a given pair of substrates S1 and S2, the relative reactivity beta (defined as kS1/kS2) was calculated from the slope of the corresponding double logarithmic plot, i.e., of ln[S1] vs. ln[S2]. This method is more accurate and remained linear for larger conversions in comparison with the plots of ln[S1] and ln[S2] against time. The rate constants measured ranged from 0.33 to 8.6 x 10(9) M(-1)s(-1). A quantitative structure-reactivity relationship was found using the Hammett equation. Assuming sigma values to be additive, a value of -0.60 was obtained for the reaction constant rho. This value agrees with the high reactivity and the electrophilic nature of HO* radical.

Environmental Pollutants↗

PubMed2003

Singlet oxygen (1deltag) production by pterin derivatives in aqueous solutions.

Six compounds of the pterin family (pterin, 6-carboxypterin, 6-formylpterin, folic acid, biopterin and neopterin) have been investigated for their efficiencies of singlet oxygen (O2(1deltag)) production and quenching in aqueous solutions. The quantum yields of 1O2 production (phidelta) have been determined by measurements of the 1O2 luminescence in the near-infrared (1270 nm) upon continuous excitation of the sensitizer. Under our experimental conditions, all studied compounds (except folic acid) are relatively efficient 1O2 sensitizers with phidelta values of up to 0.47. Results show that the nature of the substituent at position 6 on the pterin moiety, as well as the pH, affect considerably the capacity of pterins to produce 1O2. All compounds investigated are efficient 1O2 quenchers: the rate constant of 1O2 total quenching (kt) by folic acid (3.0(+/- 0.3) x 10(7) M(-1) s(-1)) is one order of magnitude larger than those for the other pterin derivatives investigated (1.4(+/- 0.1) x 10(6) M(-1) s(-1) to 2.9(+/- 0.3) x 10(6) M(-1) s(-1)).

Hydrogen-Ion Concentration↗

PubMed2002

Evaluation of the efficiency of photodegradation of nitroaromatics applying the UV/H2O2 technique.

Photolysis of nitroaromatic compounds in aqueous solution is a very slow and inefficient process. As already observed for a variety of organic pollutants, considerably faster degradation rates of nitrobenzene (NBE), 1-chloro-2,4-dinitrobenzene (CDNB), 2,4-dinitrophenol (DNP), and 4-nitrophenol (PNP) could be achieved, when the oxidative degradation of these compounds was initiated by hydroxyl radicals produced by UV-C photolysis of H2O2. Analysis of intermediate products formed during irradiation by HPLC and IC showed that cleavage of the aromatic ring should occur at an early stage of the oxidation process and that organic nitrogen was almost completely converted to nitrate. The optimal initial concentration of hydrogen peroxide ([H2O2]OPT) leading to the fastest oxidation rate, which depends on the initial substrate concentration ([S]0), could be evaluated using a simplified expression based on the main reactions involved in the first stages of the degradation process. Using only a minimum of kinetic and analytical information, this expression shows that the ratio R(OPT) (= [H2O2]OPT/[S]0) is related to the bimolecular rate constants for the reactions of hydroxyl radicals with substrate (kS) and H2O2 (kHP) and to the corresponding molar absorption coefficients (epsilonS, epsilonHP). Competition experiments between selected pairs of the substrates showed that their relative reactivity toward hydroxyl radicals could be correctly predicted using the same simplified approach. The results of our investigations as well as literature data support the general validity of the proposed procedure for optimizing oxidation rates of the UV/H2O2 process.

2,4-Dinitrophenol↗

PubMed2002

Fluorescence of pterin, 6-formylpterin, 6-carboxypterin and folic acid in aqueous solution: pH effects.

Steady-state and time-resolved studies have been performed on four compounds of the pterin family (pterin, 6-carboxypterin, 6-formylpterin and folic acid) in aqueous solution, using the single photon counting technique. The fluorescence characteristics (spectra, quantum yields, lifetimes) of these compounds and their dependence on the pH have been investigated. Most pterins can exist in two acid-base forms over the pH range between 3 and 13. Emission spectra and excitation spectra were obtained for both forms of each compound studied. Fluorescence quantum yields (phi(F)) in acidic and basic media were measured. The phi(F) of folic acid (< 0.005 in both media) is very low compared to those of pterin (0.27 in basic media and 0.33 in acidic media), 6-carboxypterin (0.18 in basic media and 0.28 in acidic media) and 6-formylpterin (0.07 in basic media and 0.12 in acidic media). The variation in integrated fluorescence intensity and fluorescence lifetimes (tau(F)) was analysed as a function of pH. Dynamic quenching by OH- was observed and the corresponding bimolecular rate constants for quenching of fluorescence (k(q)) were calculated. The reported values for k(q) (M(-1) s(-1)) are 3.6 x 10(9), 1.9 x 10(9) and 1.1 x 10(10) M(-1) s(-1) for pterin, 6-carboxypterin and 6-formylpterin, respectively.

Fluorescence↗

PubMed2002

Degradation of nitroaromatic compounds by the UV-H2O2 process using polychromatic radiation sources.

The UV-H2O2 process, a standard advanced oxidation process (AOP) for water treatment, has been applied to the degradation of a series of nitroaromatic compounds (nitrobenzene, 1-chloro-2,4-dinitrobenzene, 2,4-dinitrophenol, 3-nitrophenol, 4-nitrophenol and 4-chloro-3,5-dinitrobenzoic acid) using polychromatic radiation sources. The optimal concentration of hydrogen peroxide ([H2O2]OPT) leading to the fastest degradation rate of a given substrate (S) was determined experimentally and estimated using a simplified kinetic model based on the main reactions involved in the first stages of the oxidation. We have shown that, under conditions of monochromatic irradiation, the ratio ROPT (= [H2O2]OPT/[S]0) is given by a simple mathematical expression containing only a few parameters, whereas, under conditions of polychromatic irradiation, ROPT is expressed by a complex mathematical equation (involving the spectral distribution of the lamp emission and the absorption spectra of H2O2 and the substrate). Two numerical analysis procedures are proposed for obtaining the bimolecular rate constants for the reaction of hydroxyl radicals with a substrate (kS) from this equation. The rate constants, kS, determined for the substrates investigated in this work are in agreement with the expected reactivity trend, taking into account the effects of substituents on the distribution of electron density in the aromatic ring. The methods proposed in this work offer a double advantage: i) a standard AOP may be used to evaluate the rate constants of reaction of substrates with hydroxyl radicals under polychromatic as well as under monochromatic irradiation, ii) optimal amounts of additive may be obtained using only a few parameters as predictive tools.

Journal Article↗

PubMedDate not supplied

Photochemical behavior of 6-methylpterin in aqueous solutions: generation of reactive oxygen species.

Pterins are a family of heterocyclic compounds present in a wide range of living systems that participate in relevant biological functions and are involved in different photobiological processes. 6-Methylpterin (MPT) was investigated for its efficiency of singlet-oxygen (1O2) production and quenching in aqueous solution. The quantum yields of 1O2 production (phi(delta)) was determined by measurements of the 1O2 luminescence in the near-infrared upon continuous excitation of the sensitizer. Values of phi(delta) were found to be 0.10 +/- 0.02 and 0.14 +/- 0.02 in acidic and alkaline media, respectively. Studies of the photooxidation of MPT in acidic (pH = 5.0-6.0) and alkaline (pH = 10.2-10.8) aqueous solutions at 350 nm and room temperature have been performed. The photochemical reactions were followed by UV-visible spectrophotometry, high-performance liquid chromatography and an enzymatic method for H2O2 determination. MPT is not light sensitive in the absence of oxygen, but it undergoes a photooxidation reaction in the presence of oxygen, yielding several nonpteridinic products. The quantum yields of MPT disappearance were determined and values of 2.4 (+/-0.5) x 10(-4) and 8.1 (+/-0.8) x 10(-4) were obtained in acidic and alkaline media, respectively. H2O2 was detected and quantified in irradiated solutions of MPT. The rate constant of the chemical reaction between 1O2 and MPT (k(r)) was determined to be 4.9 x 10(6) M(-1) s(-1) in alkaline medium and the role of 1O2 in the photooxidation of MPT is discussed.

Pterins↗

PubMedDate not supplied

Substituent effects on the photophysical properties of pterin derivatives in acidic and alkaline aqueous solutions.

Pterins are heterocyclic compounds with important biological functions, and most of them may exist in two acid-base forms in the pH range between 3 and 13 in aqueous solution. In this work, the photophysical properties of acid and basic forms of six compounds of the pterin family (6-hydroxymethylpterin [HPT], 6-methylpterin [MPT], 6,7-dimethylpterin [DPT], rhamnopterin [RPT], N-methylfolic acid [MFA], and pteroic acid [PA]) have been studied. The effects of the chemical nature of the substituents at position 6 of the pterin moiety and the effects of the pH on the absorption and emission properties are analyzed. The fluorescence characteristics (spectra, quantum yields, lifetimes) of these compounds have been investigated using the single-photon-counting technique. Results obtained for pterin derivatives containing small substituents with 1 carbon atom (HPT, MPT, DPT) and short hydrocarbon chain (4 carbon atoms) (RPT) are different from those found for pterin derivatives containing a p-aminobenzoic acid (PABA) moiety in the substituent (MFA and PA). Fluorescence quantum yields (Phi(F)) of the first group of compounds are relatively high (>/=0.4), whereas MFA and PA exhibit very small Phi(F) values (</=0.01).

Hydrogen-Ion Concentration↗

PubMedDate not supplied

Reactivity of conjugated and unconjugated pterins with singlet oxygen (O2(1Deltag)): physical quenching and chemical reaction.

Pterins (PTs) belong to a class of heterocyclic compounds present in a wide range of living systems. They participate in relevant biological functions and are involved in different photobiological processes. We have investigated the reactivity of conjugated PTs (folic acid [FA], 10-methylfolic acid [MFA], pteroic acid [PA]) and unconjugated PTs (PT, 6-hydroxymethylpterin [HPT], 6-methylpterin [MPT], 6,7-dimethylpterin [DPT], rhamnopterin [RPT]) with singlet oxygen (1O2) in aqueous solutions, and compared the efficiencies of chemical reaction and physical quenching. The chemical reactions between 1O2, produced by photosensitization, and PT derivatives were followed by UV-visible spectrophotometry and high-performance liquid chromatography, and corresponding rate constants (k(r)) were evaluated. Whenever possible, products were identified and quantified. Rate constants of 1O2 total quenching by the PT derivatives investigated were obtained from steady-state 1O2 luminescence measurements. Results show that the behavior of conjugated PTs differs considerably from that of unconjugated derivatives, and the mechanisms of 1O2 physical quenching by these compounds and of their chemical reaction with 1O2 are discussed in relation to their structural features.

Kinetics↗