Norman A. Garcı́a

Universidad Nacional de San Luis, San Luis, San Luis, Argentina

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Publications (11)25.36 Total impact

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    ABSTRACT: A kinetic study of the processes involved in the vitamin B2 (riboflavin, Rf)-sensitized photooxidation of selected flavonoids (F) was carried out in methanolic solution. Under aerobic visible-light-irradiation conditions a complex picture of competitive interactions takes place: the singlet and triplet excited states of Rf are quenched by three F, namely 5,7-dihydroxy-3′,4′-dimethoxyflavanone (FNN); 5,7-dihydroxy-4′,6,8-trimethoxyflavone (FLV) and 2′,4′-dihydroxychalcone (CHL). Concomitantly, the species singlet molecular oxygen [O2(1Δg)] and superoxide anion radical (O2•−) are generated and interact with F and with Rf. CHN and FNN are photooxidised, probably by means of O2•−, whereas FVN, the poorest quencher of excited triplet Rf, allows the generation of O2(1Δg), which oxidises the very flavonoid. The photodegradation of the vitamin is delayed due to an electron transfer process to ground state oxygen.
    Dyes and Pigments. 01/2003;
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    ABSTRACT: A kinetic study on the effect of the therapeutic anti-oxidative drugs 4- and 5-aminosalicylic acids (ASA) in a Vitamin B2 (riboflavin (Rf))-photosensitised process was performed. Employing Rf 0.01–0.02 mM and ASA 0.5–1 mM both in the excited singlet and triplet states of the pigment are quenched by ASA with rate constant values slightly lower than the diffusional ones. From the latter states singlet molecular oxygen and Rf− are generated and subsequently scavenged with the concomitant degradation of ASA, mainly by means of superoxide radical anion. As a consequence, the photodegradation of the vitamin is impeded due to the electron transfer process from Rf− to ground state dissolved oxygen.
    Journal of Photochemistry and Photobiology A Chemistry 01/2003; · 2.42 Impact Factor
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    ABSTRACT: The kinetics of the Rose Bengal (RB)-sensitized photooxidation of 2,3-dihydroxypyridine (2,3-DHP) and 2,4-dihydroxypyridine (2,4-DHP), two compounds profusely employed in multiple fields such as pesticides design, pharmacology, and clinical therapeutics have been studied in water at pH 5, 11 and 14, and in the mixture MeCN–water 4:1 (v/v) with and without 0.01M KOH. These compounds present different ionization states, depending on the pH of the medium. Rate constants values in the range of 8×105–6.80×108M−1s−1 for both overall (kt) and reactive (kr) singlet molecular oxygen, O2(1Δg), quenching processes were determined by time-resolved O2(1Δg) phosphorescence detection and by spectrophotometric, spectrofluorimetric and polarographic methods. The experimental evidence suggests a photooxidative process through a charge-transfer mediated mechanism involving an excited encounter complex. The ratios kr/kt indicate that the O2(1Δg) oxidation of both dihydroxypyridines is a relatively efficient pathway over an extended pH range, with respect to the same oxidation of the monohydroxypyridines. In a general sense, the increase of pH favours the overall interaction dihydroxypyridine–O2(1Δg), although this interaction does not correlate with the importance of the photooxidation reaction. In pH 5 medium or in the mixture MeCN–water, solvents where the respective un-ionized 2-pyridone forms predominate, only 2,3-DHP is photooxidized, whereas 2,4-DHP slightly deactivates O2(1Δg) only in a physical fashion. In pH 11 aqueous solution or in alkalinized MeCN–water, where the respective mono-ionized pyridone forms predominate, the highest kr/kt ratios are reached. In addition, in pH 14 water solution both di-ionized dihydroxypyridines show the highest kt values, but lower relative reactivity than in the pH 11 solutions. All these results indicate that in aquatic environments containing these dihydroxypyridines or related aquatic pollutants, a simple change in the pH conditions of the medium could allow the switch between reactive and non-reactive O2(1Δg)-mediated processes.
    Journal of Photochemistry and Photobiology A Chemistry 01/2002; 153(1):101-107. · 2.42 Impact Factor
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    ABSTRACT: Kinetics and mechanism of the oxidation of tyrosine (Tyr) and valine (Val) di- and tripeptides (Tyr–Val, Val–Tyr and Val–Tyr–Val) mediated by singlet molecular oxygen [O2(1Δg)], phosphate (HPO4⋅− and PO4⋅2−) and sulfate (SO4⋅−) radicals was studied, employing time-resolved O2(1Δg) phosphorescence detection, polarographic determination of dissolved oxygen and flash photolysis. All the substrates were highly photooxidizable through a O2(1Δg)-mediated mechanism. Calculated quotients between the overall and reactive rate constants for the quenching of O2(1Δg) by Tyr-derivatives (kt/kr values, accounting for the efficiency of the effective photooxidation) were 1.3 for Tyr, 1 for Tyr–Val, 2.8 for Val–Tyr and 1.5 for Val–Tyr–Val. The effect of pH on the kinetics of the photooxidative process confirms that the presence of the dissociated phenolate group of Tyr clearly dominates the O2(1Δg) quenching process. Products analysis by LC–MS indicates that the photooxidation of Tyr di- and tripeptides proceeds with the breakage of peptide bonds. The information obtained from the evolution of primary amino groups upon photosensitized irradiation is in concordance with these results. Absolute rate constants for the reactions of phosphate radicals (HPO4⋅− and PO4⋅2−, generated by photolysis of the P2O84− at different pH) and sulfate radicals (SO4⋅−, produced by photolysis of the S2O82−) with Tyr peptides indicate that for all the substrates, the observed tendency in the rate constants is: SO4⋅−≥HPO4⋅−≥PO4⋅2−. Formation of the phenoxyl radical of tyrosine was detected as an intermediate involved in the oxidation of tyrosine by HPO4⋅−.
    Journal of Photochemistry and Photobiology B Biology 01/2002; · 3.11 Impact Factor
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    ABSTRACT: The aerobic riboflavin (Rf)-sensitized visible-light irradiation of Trolox (TX), a polar-solvent-soluble model for vitamin E, has been studied employing stationary photolysis, polarographic detection of oxygen uptake, stationary and time-resolved fluorescence spectroscopy, and laser flash photolysis. Results indicate that in methanolic solution, no dark complexation exists between Rf and TX. The latter quenches singlet and triplet states of Rf, with rate constants of 6.2×109 M−1 s−1 and 4.7×109 M−1 s−1, respectively. The photodecomposition of Rf, a known process taking place from triplet Rf, has been found to depend on the concentration of dissolved TX: at ≥30 mM very slight Rf photodecomposition occurs due to the massive quenching of excited singlet Rf, while at TX concentrations ≤1 mM triplet Rf is photogenerated and subsequently quenched either by oxygen, giving rise to O2(1Δg), or by TX, yielding semireduced Rf through an electron transfer process. Complementary experiments performed in pure water employing superoxide dismutase and sodium azide inhibition of the oxygen uptake, in coincidence with flash photolysis data, indicate that superoxide anion and singlet molecular oxygen are generated, likely by the reaction of the anion radical from Rf with dissolved oxygen, also yielding neutral, ground state Rf or by energy transfer from triplet Rf to ground-state oxygen, respectively. The final result is that both TX and Rf are photodegraded, likely through oxidation with activated oxygen species. In the absence of oxygen no degradation of TX can be detected, but Rf photodegradation is favoured because Rf regeneration is avoided.
    Journal of Photochemistry and Photobiology B Biology 09/2001; · 3.11 Impact Factor
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    ABSTRACT: Within our current studies on the natural photodegradation of representative aquatic environmental contaminants, the visible light irradiation of the model compound 4-hydroxypyridine (4-OHP), in air-equilibrated aqueous solution and in the presence of riboflavin (Rf), has been studied employing stationary photolysis, polarographic detection of oxygen uptake, stationary and time-resolved fluorescence spectroscopy, and laser flash photolysis. The results indicate that 4-OHP — a compound inert towards the attack of singlet molecular oxygen (O2(1Δg)) generated by Rose Bengal sensitization — quenches excited singlet and triplet states of Rf, with rate constants of and , respectively. The photodecomposition of Rf, a known process taking place from triplet Rf, has been found to depend on the concentration of dissolved 4-OHP: at ≥20 mM limited photodecomposition occurs due to the quenching of excited singlet Rf, while at ca. 5 mM triplet Rf is photogenerated and subsequently quenched either by oxygen, giving rise to O2(1Δg), or by 4-OHP, yielding semireduced Rf through an electron transfer process. Superoxide dismutase inhibition of the oxygen uptake and flash photolysis data indicate that superoxide anion is generated, likely by the reaction of the anion radical from Rf with dissolved oxygen, also yielding neutral, ground state Rf. The final result is that both 4-OHP and Rf are photodegraded, probably through oxidation with superoxide anion.
    Journal of Photochemistry and Photobiology A Chemistry 03/2001; · 2.42 Impact Factor
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    ABSTRACT: Within our current studies on the natural photodegradation of representative aquatic environmental contaminants, the visible light irradiation of the model compound 4-hydroxypyridine (4-OHP), in air-equilibrated aqueous solution and in the presence of riboflavin (Rf), has been studied employing stationary photolysis, polarographic detection of oxygen uptake, stationary and time-resolved fluorescence spectroscopy, and laser flash photolysis. The results indicate that 4-OHP — a compound inert towards the attack of singlet molecular oxygen (O2(1Δg)) generated by Rose Bengal sensitization — quenches excited singlet and triplet states of Rf, with rate constants of 2.4×109M−1s−1 and 1.2×107M−1s−1, respectively. The photodecomposition of Rf, a known process taking place from triplet Rf, has been found to depend on the concentration of dissolved 4-OHP: at ≥20mM limited photodecomposition occurs due to the quenching of excited singlet Rf, while at ca. 5mM triplet Rf is photogenerated and subsequently quenched either by oxygen, giving rise to O2(1Δg), or by 4-OHP, yielding semireduced Rf through an electron transfer process. Superoxide dismutase inhibition of the oxygen uptake and flash photolysis data indicate that superoxide anion is generated, likely by the reaction of the anion radical from Rf with dissolved oxygen, also yielding neutral, ground state Rf. The final result is that both 4-OHP and Rf are photodegraded, probably through oxidation with superoxide anion.
    Journal of Photochemistry and Photobiology A-chemistry - J PHOTOCHEM PHOTOBIOL A-CHEM. 01/2001; 139(2):199-204.
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    ABSTRACT: Visible light-promoted interactions between the natural pigment Riboflavin (Rf) and 3hydroxypyridine (3-OHP) have been studied in air equilibrated aqueous solution, as a model system for the natural photodegradation of some aquatic contaminants. Results from static and dynamic fluorescence, laser flash photolysis, and polarography of oxygen consumption support the existence of competitive oxidation processes that involve singlet molecular oxygen and/or superoxide anion, with their relative importance depending on the 3-OHP concentration. Both the excited singlet and triplet states of Rf are quenched by 3-OHP, with respective rate constants of 5.9×109 M−1 s−1—close to the diffusion rate constant—and 8.9×108 M−1 s−1. The final result of the interactions is the photooxidation of 3-OHP and the partial photostabilization of Rf.
    Dyes and Pigments 03/1999; 41(3):233-239. · 3.53 Impact Factor
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    ABSTRACT: The monosubstituted naphthalenes (MSN) 1-hydroxynaphthalene (1-OHN), 2-hydroxynaphthalene (2-OHN), 1-methoxynaphthalene, 2-methoxynaphthalene, 1-aminonaphthalene (1-AN), 2-aminonaphthalene (2-AN), 1-methylnaphthalene, 1-fluoronaphthalene and 1-cyanonaphthalene, and the parent compound naphthalene have been evaluated as physical and chemical quenchers and as generators of singlet molecular oxygen, [O2(1Δg)], in water and in organic solvents. Rate constant values for the overall (kt) and chemical (kr) quenching of O2(1Δg), respectively, determined by time-resolved phosphorescence detection and by substrate or oxygen consumption- were from less than 104–3.92×108M−1s−1, depending on the solvent polarity and the pH of the solution. The derivatives 1-OHN, 2-OHN, 1-AN and 2-AN react with O2(1Δg), generated with external sensitisers, with quantum efficiencies in the range 0.03–0.67. The photooxidation mechanism must include the formation of an encounter complex with partial charge-transfer character of the type [O2(1Δg)δ−…MSNδ+], in accordance with the observed variation of kt with the global free energy change. All the herein studied MSN generate O2(1Δg) upon direct irradiation, with fair to good quantum yields.The structure of the photooxidation products from 1-OHN and 2-OHN, two compounds with environmental interest, have been determined by combined GC/MS analysis. Benzoic acid, phthalic acid, 1,4-naphthoquinone and a dimer were identified in the reaction mixture from 1-OHN, while 2-OHN yielded phthalic acid and the dimer 2,2′-dihydroxy-1,1′-binaphthyl.
    Journal of Photochemistry and Photobiology A Chemistry 01/1999; 129(1):25-32. · 2.42 Impact Factor
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    ABSTRACT: The intense colouring antibiotic Sodium Rifamicyn SV (Rfc) is decomposed by direct and sensitized photoirradiation, through a complex mechanism. This influences the microbiological activity of Rfc. It reacts from its electronic excited states upon light absorption and by a singlet molecular oxygen- and superoxide ion-mediated mechanism upon sensitized photoirradiation, employing Rose Bengal as a dye sensitizer. The rate constant for the reaction with singlet molecular oxygen was determined as 1.2×108 and 5.1×108 M−1s−1 at pH 6 and 12, respectively, by means of comparative methods. Both an increase in the decomposition rate as the solvent polarity decreases and a clear dependence of the decomposition kinetics on the pH of the medium can be observed. The latter factor suggests that the hydroquinonic structure of Rfc is the molecular moiety responsible for the photorreaction. The decrease in the antimicrobial power parallels the progress of the photodegradation. This indicates the lack of antibiotic activity by the reaction products.
    Dyes and Pigments 01/1998; 37(2):93-102. · 3.53 Impact Factor
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    ABSTRACT: The kinetics of the dye-sensitized photooxygenation of the monohydroxylated N-heteroaromatic compounds, 2-hydroxypyridine, 3- hydroxypyridine, 4-hydroxypyridine, 4-hydroxyquinoline, 8-hydroxyquinoline, 2-hydroxypyrimidine and 4-hydroxypyrimidine, with the basic structures of some pesticides, has been studied. Rate constants in the range 106–108M−1s−1 for both the overall and the reactive singlet molecular oxygen [O2(1Δg)] quenching were determined by time-resolved O2(1Δg) phosphorescence detection and polarographic methods. Photooxidation quantum yields were in the range of 0.01–0.50 in the cases where reaction was observed, with values significantly higher in aqueous media. The hydroxy substituent plays a key role in the photooxidative process. In 2- and 4-hydroxypyridines, 4-hydroxyquinoline and 4-hydroxypyrimidines the predominant, and almost exclusive, presence of the tautomeric oxo form drastically suppresses the interaction with O2(1Δg). In the case of 3-hydroxypyridine and 8-hydroxyquinoline, compounds with hydroxy form, the OH-ionization greatly enhances the photooxidative process. Experimental evidence suggests the participation of a charge-transfer mediated mechanism, involving an initial excited encounter complex.
    Journal of Photochemistry and Photobiology A Chemistry 01/1998; 119(1):9-14. · 2.42 Impact Factor