Oxidation of di- and tripeptides of tyrosine and valine mediated by singlet molecular oxygen, phosphate radicals and sulfate radicals
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⋅−.
- [Show abstract] [Hide abstract]
ABSTRACT: The kinetics of rose bengal-sensitized photooxidation of tyrosine and several tyrosine-derivatives (tyr-D) named tyrosine methyl ester, tyrosine ethyl ester and tyrosine benzyl ester was studied in buffered pH 11 water, and buffered pH 11 micellar aqueous solutions of 0.01 M cetyltrimethylammonium chloride (CTAC) and 0.01 M-octylphenoxypolyethoxyethanol [triton X100 (TX100)]. Through time-resolved phosphorescence detection of singlet molecular oxygen (O(2)((1)Delta(g))) and polarographic determination of oxygen consumption, the respective bimolecular rate constants for reactive (k(r)) and overall (k(t)) quenching of the oxidative species by tyr-D were evaluated. Both rate constants behave in different fashion depending on the particular reaction medium. k(r)/k(t) values, increase in the sense CTAC<TX100 < water, indicating, for the tyr-D family studied, an excellent degree of self-protection against O(2)((1)Delta(g))-attack in the CTAC micellar system and a high photooxidability level in water. Results were interpreted in terms of a competition between solvent polarity effects, local substrate concentration and electron donating capabilities of the substrates in the different media that can contribute to predict the extent of photodynamic damage in biological environments.Amino Acids 07/2008; 35(1):201-8. · 3.91 Impact Factor
- [Show abstract] [Hide abstract]
ABSTRACT: Kinetics and mechanism of the aerobic Riboflavin (Rf, vitamin B2) sensitized photodegradation of Phenylephrine (Phen), a phenolamine belonging to the sympathomimetic drugs family, has been studied in water, employing continuous photolysis, polarographic detection of oxygen uptake, steady-state and time-resolved fluorescence spectroscopy, time-resolved IR-phosphorescence and laser flash photolysis. Results indicate the formation of a weak dark complex Rf-Phen, with an apparent association constant of 5.5+/-0.5M(-1), only detectable at Phen concentrations much higher than those employed in the photochemical experiments. Under irradiation, an intricate mechanism of competitive reactions operates. Phen quenches excited singlet and triplet states of Rf, with rate constants of 3.33+/-0.08 and 1.60+/-0.03x10(9)M(-1)s(-1), respectively. With the sympathomimetic drug in a concentration similar to that of dissolved molecular oxygen in water, Phen and oxygen competitively quench triplet excited Rf, generating superoxide radical anion and singlet molecular oxygen (O2((1)Deltag)) by processes initiated by electron- and energy-transfer mechanisms respectively. As a global result, the photodegradation of the vitamin, a known process taking place from its excited triplet state, is retarded, whereas the phenolamine, practically unreactive towards these oxidative species, behaves as a highly efficient physical deactivator of O2((1)Deltag). The phenolamine structure in Phen appears as an excellent scavenger of activated oxygen species, comparatively superior, in kinetic terms, to some commercial phenolic antioxidants.Journal of Photochemistry and Photobiology B Biology 09/2005; 80(2):130-8. · 3.11 Impact Factor
- [Show abstract] [Hide abstract]
ABSTRACT: Thyronine derivatives are essential indicators of thyroid gland diseases in clinical diagnosis and are currently used as standards for developing ordinary biochemical assays. Photooxidation of gland hormones of the thyronine (TN) family and structurally related compounds (TN, 3,5-diiodo-thyronine,3,3′,5-triiodothyronine and 3,3′,5,5′-tetraiodothyronine or thyroxine) was studied using rose bengal, eosin and perinaphthenone (PN) as dye sensitizers. Tyrosine (Tyr) and two iodinated derivatives (3-iodotyrosine and 3,5-diiodotyrosine) were also included in the study for comparative purposes. Irradiation of aqueous solutions of substrates containing xanthene dyes with visible light triggers a complex series of competitive interactions, which include the triplet excited state of the dye (3Xdye*) and singlet molecular oxygen [O2(1Δg)]-mediated and superoxide ion-mediated reactions. Rate constants for interaction with the 3Xdye*, attributed to an electron transfer process, are in the order of 108-109M−1 s−1 depending on the dye and the particular substrate. The photosensitization using PN follows a pure Type-II (O2(1Δg) mediated) mechanism. The presence of the phenolic group in Tyr, TN and iodinated derivatives dominates the kinetics of photooxidation of these compounds. The reactive rate constants, kr, and the quotient between reactive and overall rate constants (krlkt values, in the range of 0.7–0.06) behave in an opposite fashion compared with the overall rate constants and oxidation potentials. This apparent inconsistency was interpreted on the basis of an internal heavy atom effect, favoring the intersystem-crossing deactivation route within the encounter complex with the concomitant reduction of effective photooxidation.Photochemistry and Photobiology 02/2005; 81(2):325 - 332. · 2.29 Impact Factor