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At least 919 records · Page 51Linked to original sources
PubMed2006

Enhancement of two-photon absorption cross-section and singlet-oxygen generation in porphyrins upon beta-functionalization with donor-acceptor substituents.

The microwave-enhanced synthesis, comparative singlet oxygen sensitization efficiency, and nonlinear optical characterization of a new beta-functionalized porphyrin and its copper complex are described. We show that the introduction of a donor-acceptor push-pull conjugated fragment in the beta position strongly perturbs the porphyrin electronic structure leading to a remarkable one- and two-photon NIR absorption enhancement.

Journal Article↗
PubMed2001

Repair and mutagenic potential of oxaluric acid, a major product of singlet oxygen-mediated oxidation of 8-oxo-7,8-dihydroguanine.

Oxidative reactions within DNA commonly result in base modifications. Among the four DNA bases, guanine is the most susceptible to various oxidants, and its related oxidized form, 8-oxo-7,8-dihydroguanine, has been extensively studied in terms of repair and mutagenicity. However, 8-oxo-7,8-dihydroguanine is readily subjected to further oxidation, and this has become a point of interest. We recently found that singlet oxygen oxidation of 8-oxo-7,8-dihydroguanine led to the predominant formation of oxaluric acid as the final product. We report herein on the biological features of oxaluric acid dealing with in vitro DNA synthesis and its removal from DNA by repair enzymes. Nucleotide insertion opposite oxaluric acid, catalyzed by Kf exo(-) and Taq indicates, that oxaluric acid induces G to T and G to C transversions. On the other hand, oxaluric acid represents a block when synthesis is performed with pol beta. Interestingly, DNA repair experiments carried out with formamidopyrimidine DNA N-glycosylase (Fpg) and endonuclease III (endo III) show that oxaluric acid is a substrate for both enzymes. Values of k(cat)/K(m) for the Fpg-mediated removal of oxidative guanine lesions revealed that 8-oxoGua is only a slightly better substrate than oxaluric acid. Interestingly, the results obtained with endo III suggest that oxaluric acid is a much better substrate than is 5-hydroxycytosine (5-OHC), an oxidized pyrimidine base.

Animals↗
PubMedDate not supplied

Antioxidant activity of the pyridoindole stobadine. Pulse radiolytic characterization of one-electron-oxidized stobadine and quenching of singlet molecular oxygen.

Antioxidant properties of stobadine, a pyridoindole derivative described to exhibit cardioprotective properties, were characterized. The radical scavenging potential of stobadine was evaluated using pulse radiolysis with optical detection, by which it is shown that one-electron oxidation of stobadine with radicals such as C6H5O., CCl3O2., Br2.-, and HO. (reaction rate constants approximately 5 x 10(8)-10(10) M-1 s-1) leads to the radical cation (absorbance maxima at 280 and 445 nm) which deprotonates from the indolic nitrogen (pKa = 5.0) to give a nitrogen-centered radical (absorbance maxima at 275, 335, and 410 nm), probably bearing a positive charge at the pyrido nitrogen. The radical of stobadine reacts with Trolox (i.e., 6-hydroxy-2,5,7,8-tetramethyl-chromane-2-carboxylic acid) with a rate constant of 1.2 x 10(7) M-1 s-1 at pH 7.0 by one-electron oxidation to yield the phenoxyl-type radical of Trolox. This reaction is reversible (k = 2 x 10(5) M-1 s-1). The redox potential of stobadine at pH 7 is 0.58 V/NHE. Stobadine is also a quencher of singlet molecular oxygen (1O2) with an overall quenching rate constant of 1.3 x 10(8) M-1 s-1, determined with the endoperoxide of 3,3'-(1,4-naphthylene)dipropionate (NDPO2) as 1O2 source and by monitoring 1O2 photoemission with a germanium diode.

Antioxidants↗
PubMed1998

Mutations induced in the supF gene of pSP189 by hydroxyl radical and singlet oxygen: relevance to peroxynitrite mutagenesis.

We previously showed that the oxidant peroxynitrite (ONOO-) was strongly mutagenic in the supF shuttle vector pSP189 replicated in bacteria or human cells. Qualitative characteristics of the mutational spectra induced by ONOO- differed significantly from those reportedly caused by hydroxyl radical (OH.) in other experimental systems but showed similarities to spectra reportedly produced by singlet oxygen (1O2). The molecular mechanisms of ONOO--mediated DNA damage are unknown. The objective of the present set of experiments was to characterize mutational effects induced in the supF gene of pSP189 by OH* and 1O2 to permit direct comparison with mutational spectra induced by ONOO- in this system. Base substitutions were the major form of mutation induced in plasmids replicated in human (AD293) cells by ONOO- (84%) and 1O2 (71%), whereas OH* induced fewer of them (49%). In plasmids replicated in bacteria (Escherichia coli MBL50), frequencies of base substitutions induced by the three treatments were similar. G:C-to-T:A transversions were the most common form of base substitution induced by ONOO- (75% and 67%, respectively, in AD293- and MBL50-replicated plasmids) and 1O2 (68% and 71%); they were induced at lower frequencies by OH. (51% and 47%). G:C-to-C:G transversions or G:C-to-A:T transitions were induced at almost equal frequencies by both ONOO- and 1O2, whereas OH* induced these mutations at different frequencies in the AD293 system. Collectively, our results confirm that in several important respects mutational spectra induced by ONOO- have greater similarity to spectra induced by 1O2 than to those induced by OH* and suggest that genotoxic derivatives of ONOO- are likely to include species that have DNA-damaging properties resembling those of 1O2 in selectivity for guanine but not identical in sequence specificity.

Amino Acid Sequence↗
PubMed2002

Diels-Alder and ene reactions of singlet oxygen, nitroso compounds and triazolinediones: transition states and mechanisms from contemporary theory.

Singlet oxygen, nitroso compounds and triazolinediones have similar electronic structures: they share a low lying LUMO, making them powerful electrophiles, and a high lying HOMO, orthogonal to the LUMO and consisting of an antibonding combination of lone paris. This bestows some nucleophilic character on these species. We describe a number of studies employing the best levels of theory currently available for systems of this size and demonstrate that the Diels-Alder and ene reactions of these three species are calculated to show subtle changes in mechanism. The calculations have been calibrated, wherever possible, by comparison to experimental observations including measured activation and reaction energies, regio- and stereo-selectivities, intermediates observed either spectroscopically or by trapping, and kinetic isotope effects.

Journal Article↗
PubMed2002

Sensitization of singlet oxygen via encounter complexes and via exciplexes of pipi* triplet excited sensitizers and oxygen.

Both excited singlet states, 1sigma(g)+ and 1delta(g), and the triplet ground state, 3sigma(g)-, of molecular oxygen are competitively formed during the quenching by O2 of triplet (T1) excited sensitizers of sufficient energy. The corresponding overall rate constants kT(1sigma), kT(1delta), and kT(3sigma) as well as the T1 state energies E(T) and the oxidation potentials E(ox), have been determined for a series of six fluorene derivatives. Graduated and in part strong charge transfer (CT) effects on kT(1sigma), kT(1delta), and kT(3sigma) are observed. These and literature data strongly indicate that quenching occurs in two different channels each capable of producing O2(1sigma(g)-), O2(1delta(g)), and O2(3sigma(g)-). One proceeds via internal conversion (IC) of excited 1,3(T1 x 3sigma) complexes with no CT character (nCT), which cannot be distinguished from encounter complexes, the other via IC of 1,3(T1 x 3sigma) exciplexes with partial CT character (pCT). The contributions of nCT and pCT deactivation channels to the overall formation of O2(1sigma(g)+), O2(1delta(g)). and O2(3sigma(g)-) depend on E(T) and E(ox). The rate constants of the nCT channel are controlled by the excess energies of the respective IC processes by an energy gap law. The rate constants of the pCT channel depend on the change of free energy deltaG(CET) for complete electron transfer from T1 excited sensitizer to O2. Equations are presented which show the functional form of the dependence of the oxygen quenching rate constants on E(T) and E(ox). Particular emphasis is laid on the question of whether these relations could generally be valid for pipi* triplet sensitizers.

Journal Article↗
PubMed2002

Photophysics of trioxatriangulenium ion. electrophilic reactivity in the ground state and excited singlet state.

Trioxatriangulenium (TOTA+. 4,8,12-trioxa-4,8,12,12c-tetrahydro-dibenzo[cd,mn]-pyrenylium) is a closed shell carbenium ion, which is stable in non-nucleophilic polar solvents at ambient temperatures In alcohols, small quantities of the leuco ether are formed in a reversible reaction. The physical and chemical properties of the excited singlet state of the trioxatriangulenium (TOTA+) carbenium ion are investigated by experimental and computational means The degeneracy of the lowest excited states is counteracted by Jahn-Teller-type distortion, which leads to vibronic broadening of the long wavelength absorption band. A strong fluorescence is observed at 520 nm (tau(fl) = 14.6 ns, phi(fl) = 0.12 in deaerated acetonitrile). The fluorescence is quenched by 10 aromatic electron donors predominantly via a dynamic charge transfer mechanism, but ground state complexation is shown to contribute in varying degrees Quenching is also observed in the presence of halide ions Quenching rate constants are derived from lifetime measurements while charge transfer (CT) complex formation constants follow from the steady-state Stern-Volmer plots. CT-complex formation with three discogenic triphenylenes is studied separately. Phosphorescence spectra, triplet lifetimes. and triplet-triplet absorption spectra are provided. In the discussion, TOTA+ is compared to the unsubstituted xanthenium ion and its 9-phenyl derivative with respect to the excited state properties.

Journal Article↗
PubMed2003

Singlet-energy transfer in quadruple hydrogen-bonded oligo(p-phenylenevinylene)perylene-diimide dyads.

The photophysical properties of a supramolecular donor-acceptor dyad consisting of an oligo(p-phenylenevinylene) unit and a perylene-diimide unit are described. The dyad is created by functionalising the two chromophores with quadruple hydrogen bonding 2-ureido-4[1H]-pyrimidinone units, which provide a high association constant (K approximately 10(8) M-1 in toluene). This feature enabled us to study the time-resolved photoinduced singlet-energy transfer reaction between the two chromophores in dilute solution with transient pump-probe spectroscopy. This energy transfer occurs with a time constant of 5.1 ps.

Journal Article↗
PubMed2003

Singlet-oxygen generation in the catalytic reaction of dioxiranes with nucleophilic anions.

Nucleophilic anions such as Cl(-), I(-), Br(-), t-BuO(-), O(2)(-) and OH(-) efficiently catalyze the decomposition of dimethyldioxirane (DMD) and methyl(trifluoromethyl)dioxirane (TFD). Singlet oxygen ((1)O(2)) is formed in these catalytic reactions, as demonstrated by the characteristic infrared chemiluminescence (IR-CL) at 1268 nm. The yield of (1)O(2) generation, measured by the IR-CL method, lies in the range between 2 and 98%, which depends on the particular anion employed. For the bromide ion, the catalytic decomposition of the dioxirane is in competition with the oxidation of Br(-) to elemental bromine.

Journal Article↗
PubMed2004

Photogeneration of singlet oxygen by humic substances: comparison of humic substances of aquatic and terrestrial origin.

The singlet oxygen (1(O2)) luminescence of 27 isolated humic substances (HS), natural organic matter, ultrafiltrates, and the synthetic fulvic acid HS1500 has been investigated by time-resolved spectroscopy in buffered D(2)O. The samples include both reverse osmosis isolates from lakes in Scandinavia, Canada, and Germany, and IHSS fulvic and humic acids of aquatic and terrestrial origin. The quantum yields of 1(O2) formation (PhiDelta) obtained on laser excitation at 480 nm ranged between 0.06 (HS1500) and 2.7%(fulvic acid from soil, IHSS). In our study, a general trend towards higher PhiDelta in terrestrial HS was observed. The comparison of reverse osmosis isolates from surface waters collected during fall 1999 and spring 2000 from five Scandinavian sites yielded, in all cases, higher PhiDelta for the spring samples. For the aquatic sampling sites Hietajarvi and Birkenes, PhiDelta even exceeded values of 0.6%, which were found to be typical for terrestrial or soil water material. Investigation of the excitation wavelength dependence of PhiDelta in the spectral range 355-550 nm yielded different spectral shapes for aquatic HS and "non-aquatic" HS, respectively. On the basis of these excitation spectra, 1(O2) production rates were calculated for eight representative HS.

Journal Article↗
PubMed2004

Strickler-Berg analysis of excited singlet state dynamics in DNA and RNA nucleosides.

The excited singlet state lifetime of the ribonucleoside uridine was found to be 210+/-30 fs by femtosecond transient absorption spectroscopy. This value is considerably shorter than all previous time-domain measurements. This result and our previous lifetime measurements [see J.-M. L. Pecourt, J. Peon and B. Kohler, J. Am. Chem. Soc., 2001, 123, 10 370] for the other common nucleosides are compared with lifetimes calculated from available photophysical data using the Strickler-Berg (SB) equation. The calculated lifetimes for pyrimidine nucleosides are 10-25% lower than the lifetimes measured in femtosecond transient absorption experiments. For the purine nucleosides, guanosine and adenosine, consideration of just the lowest 1pi --> pi* transition led to predicted lifetimes that are three times greater than experimental ones. On the other hand, inclusion of both of the lowest energy 1pi --> pi* absorption bands in the SB equation resulted in much better agreement with the experimental values. This suggests that both 1pi pi* states of the purine nucleosides contribute to their emission. Decay by the bright 1pi pi* state (or states, in the case of the purines) is believed to be responsible for the experimentally observed lifetimes.

DNA↗
PubMed2004

Characterisation of the lowest singlet and triplet excited states of S-flurbiprofen.

The photophysical properties of S-flurbiprofen [S-2-fluoro-alpha-methyl-4-biphenylacetic acid], a nonsteroidal anti-inflammatory drug, have been examined using steady-state and time-resolved spectroscopic techniques. The energy of its first singlet excited state is 99 kcal mol(-1). The fluorescence quantum yields and lifetimes (at 300 nm) have been determined in acetonitrile, methanol, hexane and PBS; they are in the range 0.15<phi(F)< 0.33 and 0.7<tau(F)<2.0 ns. The intersystem crossing quantum yields are between 0.45 and 0.71; the lambda(max) of the T-T absorption is 360 nm, and the triplets live from 15 to 106 micros. Steady state photolysis in aqueous medium leads to S-2-hydroxy-alpha-methyl-4-biphenylacetic acid via photonucleophilic aromatic substitution, in addition to the photodecarboxylation products observed in organic solvents.

Anti-Inflammatory Agents, Non-Steroidal↗