Oxidation of di- and tripeptides of tyrosine and valine mediated by singlet molecular oxygen, phosphate radicals and sulfate radicals
Universidad Nacional de Río Cuarto, Río Cuarto, Cordoba, ArgentinaJournal of Photochemistry and Photobiology B Biology (Impact Factor: 2.96). 12/2002; 65(1):74-84. DOI: 10.1016/S1011-1344(01)00239-1
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⋅−.
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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.
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ABSTRACT: Kinetic aspects of the sensitized photooxidation of alpha- and beta-chymotrypsins have been studied at pH 6 and 8. The sensitization, employing classical O2(1Deltag)-photogenerators, such as xanthene dyes, is a kinetically intricate process because of the presence of ground state dye-protein associations and to the simultaneous participation of superoxide ion and singlet molecular oxygen [O2(1Deltag)]. Both proteins, that possess the same distribution pattern of photooxidizable amino acids, suffer a pure O2(1Deltag)-mediated photodynamic attack, using the carbonylic sensitizer Perinaphthenone. Overall and reactive rate constants for the O2(1Deltag)-quenching (in the order of 108 and 107/M/s, respectively), and rates of oxygen consumption determined by time-resolved, spectroscopic and polarographic methods indicate that alpha- and beta-chymotrypsins are less photooxidizable at pH 6, as a result of an enhancement of the O2(1Deltag)-physical quenching component. In general terms, beta-chymotrypsin exhibits the greater overall proclivity to interact with O2(1Deltag), whereas structural factors, possibly evidenced by a higher exposure of the reactive tryptophan residues, impart an increased photooxidation degree to the proteins at pH 8, specially to the alpha-chymotrypsin.
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ABSTRACT: The oxidation kinetics and mechanism of the phenolic derivatives of α,α,α-trifluorotoluene, 2-trifluoromethylphenol, 3-trifluoromethylphenol (3-TFMP), 4-trifluoromethylphenol and 3,5-bis(trifluoromethyl)phenol, mediated by singlet molecular oxygen, O2(1Δg), and hydrogen phosphate radicals were studied, employing time-resolved O2(1Δg) phosphorescence detection, polarographic determination of dissolved oxygen and flash photolysis. All the substrates are highly photo-oxidizable through a O2(1Δg)-mediated mechanism. The phenols show overall quenching constants for O2(1Δg) of the order of 106 M−1 s−1 in D2O, while the values for the phenoxide ions in water range from 1.2 × 108 to 3.6 × 108 M−1 s−1. The effects of the pH and polarity of the medium on the kinetics of the photo-oxidative process suggest a charge-transfer mechanism. 2-Trifluoromethyl-1,4-benzoquinone is suspected to be the main photo-oxidation product for the substrate 3-TFMP. The absolute rate constants for the reactions of HPO4˙− with the substrates range from 4 × 108 to 1 × 109 M−1 s−1. The 3-trifluoromethylphenoxyl radical was observed as the organic intermediate formed after reaction of 3-TFMP with HPO4˙−, yielding 2,2′-bis(fluorohydroxymethyl)biphenyl-4,4′-diol as the end product. The observed results indicate that singlet molecular oxygen and hydrogen phosphate radicals not only react at different rates with the phenols of α,α,α-trifluorotoluene, but the reactions also proceed through different reaction channels.
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