A reevaluation of the photolytic properties of 2-hydroxybenzophenone-based UV sunscreens: are chemical sunscreens inoffensive?

The excited states of a set of popular sunscreen agents (2-hydroxybenzophenone, oxybenzone, and sulisobenzone) are studied by using femto- and nanosecond time-resolved spectroscopy. Upon excitation, the compounds undergo an ultrafast excited-state intramolecular proton transfer (ESIPT) reaction as the major energy-wasting process and the rate constant of this reaction is k=2×10(12) s(-1) . The ESIPT yields a keto conformer that undergoes a fast, picosecond internal conversion decay. However, a photodegradative pathway is a monophotonic HO bond breakage that subsequently leads to trace yields of phenoxyl radicals. Because potentially harmful phenoxyl radicals are formed upon irradiation of sunscreen agents, care should be taken about their reactivity towards biologically relevant compounds.

Spectral and Kinetic Properties of Radical Cations Derived from Oxoisoaporphines: Relevance to Electron-Transfer Processes Involving Phytoalexins.

The thermally induced intermolecular electron transfer reaction in acetonitrile between the tetracyanoethylene (TCNE), a π-electron acceptor with a large electron affinity, and six oxoisoaporphines (2,3-dihydro-7H-dibenzo[de,h]quinolin-7-one, 5-methoxy-2,3-dihydro-7H-dibenzo[de,h]quinolin-7-one, 1-azabenzo[de]anthracen-7-one, 5-methoxy-1-azabenzo[de]anthracen-7-one, 7H-benzo[e]perimidin-7-one, and 2-methyl-7h-benzo[e]perimidin-7-one) is reported. Spectral and kinetic characteristics are presented for radical cations derived from these six oxoisoaporphines either generated by a thermal reaction or generated radiolytically in argon-saturated 1,2-dichloroethane, oxygen-saturated acetone, and acetonitrile. The radical cations of oxoisoaporphines are insensitive to oxygen and are mostly characterized by absorption maxima of their most intense bands located at λmax = 400-410 nm, except of the radical cations derived from 2,3-dihydrooxoisoaporphines. For the latter compounds, the absorption maxima of the most intense absorption bands are located at λmax = 290-295 nm. Their locations are independent of the presence of functional groups and the solvents used. They are formed in bimolecular processes with pseudo-first-order rate constants ranging from 2.1 × 105 to 1.5 × 106 s-1 (in solutions containing 10-4 M of the substrate), depending on the derivative and the solvent used. They are stable either when formed via the electron-transfer reaction with TCNE or when generated in isolation in pulse radiolysis of Ar-saturated 1,2-dichloroethane. In acetone and acetonitrile they decay predominantly by first-order kinetics with the first-order rate constants ranging from 2.3 × 104 to 5.1 × 104 s-1. Formation of dimeric radical cations for all of the oxoisoaporphines studied was observed in acetonitrile solutions, and for azaoxoisoaporphines also in acetone solutions. The experimental spectra show a reasonably good agreement with the ZINDO/S semiempirical quantum mechanical calculations of radical cation absorptions.

Kinetic studies on the formation of sulfonyl radicals and their addition to carbon-carbon multiple bonds.

The reactions of α-hydroxyl and α-alkoxyl alkyl radicals with methanesulfonyl chloride (MeSO(2)Cl) have been studied by pulse radiolysis at room temperature. The alkyl radicals were produced by ionizing radiation of N(2)O-saturated aqueous solution containing methanol, ethanol, isopropanol, or tetrahydrofuran. The transient optical absorption spectrum consisted of a broad band in the region 280-380 nm with a maximum at 320 nm typical of the MeSO(2)(•) radical. The rate constants in the interval of 1.7 × 10(7)-2.2 × 10(8) M(-1) s(-1) were assigned to an electron-transfer process that leads to MeSO(2)Cl(•-), subsequently decaying into MeSO(2)(•) radical and Cl(-). The rate constants for the addition of CH(3)SO(2)(•) to acrolein and propiolic acid were found to be 4.9 × 10(9) M(-1) s(-1) and 5.9 × 10(7) M(-1) s(-1), respectively, in aqueous solutions and reversible. The reactivity of tosyl radical (p-CH(3)C(6)H(4)SO(2)(•)) toward a series of alkenes bearing various functional groups was also determined by competition kinetics in benzene. The rate constants for the addition of tosyl radical to alkenes vary in a much narrower range than the rate constants for the reverse reaction. The stabilization of the adduct radical substantially contributes to the increase of the rate constant for the addition of tosyl radical to alkenes and, conversely, retards the ß-elimination of tosyl radical.

Analytical and toxicological studies of decomposition of insecticide parathion after gamma-irradiation and ozonation.

The decomposition of the widely used organophosphorus pesticide parathion was carried out in aqueous solutions by the use of gamma-irradiation from a 60Co source or ozonation by means of an ozone generator, and by combined processes of ozonation and radiolysis. Factors affecting the parathion decomposition as well formation and decomposition of the main by-products, including irradiation dose, length of ozonation time, and presence of common scavengers, were investigated. The most efficient was found to be the gamma-irradiation process combined with a short ozonation period; about 1 kGy irradiation dose was sufficient to decompose the pesticide in 15 mg/L solutions. Chemical studies of the decomposition of parathion were accompanied by monitoring of toxicity changes of irradiated solutions with the Microtox test.

New insights into the reaction paths of hydroxyl radicals with 2'-deoxyguanosine.

The reaction of HO(•) radical with 2'-deoxyguanosine is intensively studied as a model for DNA damage. Several aspects related to the reaction paths responsible for the most relevant lesions are not well understood. We have reinvestigated the reaction of HO(•) with 2'-deoxyguanosine by pulse radiolysis and extended our studies to a variety of substituted derivatives. The main path of hydrogen abstraction was confirmed to be from the exocyclic NH(2) group, followed by a water-assisted tautomerization. The rate constant (k = 2.3 × 10(4) s(-1)) obtained from the spectral changes at 620 nm is influenced by the substituent at the C8 position. When N1-H is replaced by N1-CH(3), the tautomerization does not occur. The spectral changes at 370 nm that correspond to a rate constant of 6.9 × 10(5) s(-1) were assigned to the cyclization of 2'-deoxyguanosin-5'-yl radical with formation of 5',8-cyclo-2'-deoxyguanosine as the product. When NEt(2) replaces the exocyclic NH(2), the spectral changes at all wavelengths follow second-order kinetics, suggesting a "slow" ring-opening of the 8-hydroxyl adduct of 2'-deoxyguanosine.

Photoreduction of oxoisoaporphines by amines: laser flash and steady-state photolysis, pulse radiolysis, and TD-DFT studies.

Photoreduction of oxoisoaporphine (OIA) (1-aza-benzo-[de]anthracen-7-one) and its 5-methoxy (5-MeO-OIA) derivative by selected amines (two non-alpha-hydrogen-donating amines (1,4-diaza[2.2.2]-bicyclooctane (DABCO) and 2,2,6,6-tetramethylpiperidine (TMP)) and three alpha-hydrogen-donating amines (triethylamine (TEA), diethylmethylamine (DEMA), and dimethylethylamine (DMEA))) has been studied in deaerated neat acetonitrile solutions using laser flash and steady-state photolysis. The triplet excited states of OIA and 5-MeO-OIA are characterized by intense absorption maxima located at lambda(max) = 450 nm and lifetimes of 34.7 +/- 0.5 and 44.6 +/- 0.4 micros, respectively. In the presence of tertiary amines, both triplets are quenched with a rate constant that varies from the near diffusion limit (>10(9) M(-1) s(-1)) to a rather low value (approximately 10(7) M(-1) s(-1)) and shows the expected dependence on the reduction potential for one-electron-transfer reactions. The transient absorption spectra observed after quenching of the respective triplet states are characterized by distinct absorption maxima located at lambda(max) = 480 and 490 nm (for OIA and 5-MeO-OIA, respectively) and accompanied by broad shoulders in the range of 510-560 nm. They were assigned to either solvent-separated radical ion pairs and/or isolated radical anions. In the presence of alpha-hydrogen-donating amines these species undergo protonation that leads to the formation of neutral hydrogenated radicals A1H(*)/A2H(*) with two possible sites of protonation, N and O atoms. Pulse radiolysis and molecular modeling together with TD-DFT calculations were used to support the conclusions about the origin of transients.

Transient phenomena in the pulse radiolysis of oxoisoaporphine derivatives in acetonitrile.

The absorption-spectral and kinetic behavior of radical ions and triplet states of two oxoisoaporphine derivatives, 2,3-dihydrooxoisoaporphine (2,3-DHOA) and 5-methoxy-2,3-dihydrooxoisoaporphine (5-MeO-2,3-DHOA), have been studied by UV-vis spectrophotometric pulse radiolysis in a neat acetonitrile saturated with argon and oxygen at room temperature. The radical anions of 2,3-DHOA and 5-MeO-2,3-DHOA are characterized by intense absorption maxima located at lambda max = 605 and 590 nm, with molar absorption coefficients 605 = 5600 M (-1) cm (-1) and 590 = 4900 M (-1) cm (-1), respectively. Both radical anions decay via first-order kinetics with the rate constants in the range (1.5-2.6) x 10 (5) s (-1), predominantly through protonation by adventitious water forming neutral-hydrogenated radicals. Oxygen insensitive, the radical cations of 2,3-DHOA are characterized by a strong nondescript absorption band with no distinct lambda max in the range 350-450 nm. On the other hand, the radical cations of 5-MeO-2,3-DHOA are characterized by the distinctive absorption band with lambda max = 420 nm. The experimental spectra of the neutral-hydrogenated radicals and the triplet excited states derived from 2,3-DHOA and 5-MeO-2,3-DHOA are in accordance with the spectra observed previously during laser flash photolysis ( De la Fuente, J. R. ; et al. J. Phys. Chem. 2005, 109, 5897 ). Most of the transient spectra generated radiolytically are adequately reproduced by quantum mechanical semiempirical PM3 and ZINDO/S methods.

Reactivity pattern of bromonucleosides induced by 2-hydroxypropyl radicals: photochemical, radiation chemical, and computational studies.

The bromonucleosides (BrdX's) 5-bromo-2'-deoxyuridine (BrdU), 5-bromo-2'-deoxycytidine (BrdC), 8-bromo-2'-deoxyadenosine (BrdA), and 5-bromo-2'-deoxyguanosine (BrdG) may substitute for ordinary nucleosides in DNA. As indicated by electron-stimulated desorption experiments, such a modified biopolymer is greater than 2-3-fold more sensitive to damage induced by excess electrons. The other major product of water radiolysis, the (•)OH radical, may form a number of other radicals in chemical reactions with the complex content of the cell. Thus, the well-proved BrdU-labeled DNA radiosensitivity may be, at least in part, related to secondary organic radicals. Therefore, in the current study, the propensity of BrdX's to damage induced by 2-hydroxypropyl radical (OHisop(•))-a prototype radical species-was investigated. The HPLC and LC-MS analyses revealed the formation of two major products from the brominated pyrimidine nucleosides, a native nucleoside and an adduct of BrdX and OHisop(•) , and only an adduct of BrdX from the bromopurine nucleosides. Quantum chemical calculations ascribed this evident difference between purines and pyrimidines to the electron transfer from OHisop(•) to BrdX that is especially favorable in pyrimidines.

Photoreduction of azaoxoisoaporphines by amines: laser flash and steady-state photolysis and pulse radiolysis studies.

Photoreduction of 7H-benzo[e]perimidin-7-one (3-AOIA, A1) and its 2-methyl derivative (2-Me-3-AOIA, A2) by non-H-donating amines (1,4-diazabicyclo[2.2.2]octane [DABCO]; 2,2,6,6-tetramethylpiperidine [TMP]), and a hydrogen-donating amine (triethylamine [TEA]), has been studied in deaerated neat acetonitrile solutions using laser flash photolysis (LFP) and steady-state photolysis. The triplet excited states of A1 and A2 were characterized by a strong absorption band with λmax = 440 nm and lifetimes of 20 and 27 µs respectively. In the presence of tertiary amines, both triplet excited states were quenched with rate constants close to the diffusional limit (kq ranged between 10(9) and 10(10) M(-1) s(-1)). The transient absorption spectra observed after quenching with DABCO and TMP were characterized by maxima located at 460 nm and broad shoulders in the range of 500-600 nm. These transient species are attributed to solvent-separated radical ion pairs and/or to isolated radical anions. In the presence of TEA, these transients undergo proton transfer, leading to the neutral hydrogenated radicals, protonated over the N1- and O-atoms. Transient absorption spectra of these transients were characterized by maxima located at 400 and 520 nm and 430 nm respectively. Additional support for these spectral assignments was provided by pulse radiolysis (PR) experiments in acetonitrile and 2-propanol solutions.

Methionine residue acts as a prooxidant in the •OH-induced oxidation of enkephalins.

Enkephalins are bioactive pentapeptides (Tyr-Gly-Gly-Phe-Leu (Leu-enk) and Tyr-Gly-Gly-Phe-Met (Met-enk)) produced while an organism is under mental and/or physical stress. In the course of their biological action they are exposed to reactive oxygen and nitrogen species. We have reinvestigated the reactions of (•)OH radicals toward these peptides in order to elucidate the oxidation mechanisms and the final products. Nanosecond pulse radiolysis was used to obtain the spectra of the reaction intermediates and their kinetics. Additional insight into details of the oxidation mechanism was gained by identification of main final products by means of UV-vis spectrophotometry, HPLC coupled with fluorescence spectroscopy, and mass spectrometry. The key processes are different in both peptides. In Leu-enk, the first step is an (•)OH radical addition to the aromatic rings of Tyr and Phe residues that leads to hydroxylated residues, dihydroxyphenylalanine (DOPA) from Tyr and tyrosine isomers from Phe, respectively. In Met-enk, these processes are less important, an additional target being the sulfur atom of the methionine residue. Depending on pH either an OH-adduct (hydroxysulfuranyl radical) or a sulfur radical cation undergo intramolecular electron transfer with Tyr residue resulting in a repair of Met and oxidation of Tyr to tyrosyl radicals and a final formation of dityrosine. At low pH, the OH-adducts to Tyr residue are precursors of tyrosyl radicals and dityrosine. Thus, the final products coming from oxidation of the Tyr residue depend strongly on the neighboring residues and the pH.