C Atoms Research Articles

Rh(III)-Catalyzed Coupling of N-Chloroimines with α-Diazo-α-phosphonoacetates for the Synthesis of 2H-Isoindoles.

We report herein the first use of N-chloroimines as effective synthons for directed C-H functionalization. Rh(III)-catalyzed coupling of N-chloroimines with α-diazo-α-phosphonoacetates allows for efficient dechlorinative/dephosphonative access to 2H-isoindoles. Further deesterification under Ni(II) catalysis enables the complete elimination of reactivity-assisting groups and full exposure of reactivity of C3 and N2 ring atoms for attaching structurally distinct appendages.

Theoretical Study on Asymmetric [2 + 2] Cycloaddition of an Alkynone with a Cyclic Enol Silyl Ether Catalyzed by a Chiral N,N′-Dioxide-Zn(II) Complex

The reaction mechanism and enantioselectivity of the asymmetric [2 + 2] cycloaddition between an alkynone (R1) and a cyclic enol silyl ether (R2) were studied theoretically by the DFT method at the B3LYP-D3(BJ)/6-311G**(CH2Cl2,SMD)//B3LYP-D3(BJ)/def2-SVP(CH2Cl2,SMD) theoretical level. The noncatalytic reaction occurred via a stepwise mechanism. The first C–C bond was constructed by coupling two pseudo radical centers generated at the most nucleophilic C2 atom in the cyclic enol silyl ether and the most electrophilic terminal Cβ atom in the alkynone, which was responsible for the regioselectivity of the reaction. The counterion NTf2– could stabilize the Zn(II) complex by coordinating to the center metal, forming a high-reactivity hexacoordinate Zn(II)-complex intermediate. The bulky CF3 group in the NTf2– ion adjusted the blocking effect of o-iPr in aniline of the ligand toward the reactive site (that is, the Cβ atom in the alkynone) and induced the si face of the cyclic enol silyl ether to approach the alkynone from its less hindered re face, achieving a high enanotioselectivity of products. The Pauli repulsion between the Zn(II)-associated moiety and cyclic enol silyl ether fragment was the main contributor to the stereodifference of the two competing pathways in chiral N,N′-dioxide-Zn(II)-catalyzed [2 + 2] cycloaddition. The unfavorable steric repulsion between the o-iPr group of aniline in the ligand and tert-butyldimethylsilyl (TBS) in the cyclic enol silyl ether along the re face path translated into a more destabilizing ΔEPauli value, leading to the predominant cycloaddition product (P-RR) observed in experiments. Variation of the linkage and chiral backbone could affect the repulsion among the o-iPr in the ligand, the counterion NTf2–, and substrates, leading to different stereochemical outcomes. These results are in good agreement with experimental observations.

Sydnone-alkyne cycloaddition: Which factors are responsible for reaction rate ?

In this work we report extensive DFT study of sydnone-alkyne cyclization which included investigation of the reaction mechanism, analysis of different factors affecting sydnone and alkyne reactivity as well as attempt to reproduce quantitatively experimental activation free energy. The calculations were performed for a set of 18 sydnone-alkyne reactions with a help of a semi-automatized workflow involving reagent preparation and generation of starting structures for a plausible transition state. Reconstructed reaction path supported two-step mechanism: cycloaddition followed by retro-Diels-Alder reaction. Since the latter had a tiny barrier, the cycloaddition step was predicted to be the rate-limiting. For the ensemble of reactions, calculations reproduce activation free energies extracted from experimental reaction rates (k) with the accuracy of 2 kcal/mol. Accounting for solvation effects didn't change the overall trend of activation free energies as a function of substituents. A series of statistical model linking logk and sydnones structure was built using Support Vector Regression and Multiple Linear Regression machine-learning methods coupled with different types of molecular descriptors; none of them demonstrated a good performance at cross-validation stage. Detailed analysis of different factors affecting reaction rate variation as a function of substituents revealed particular role of the charge on C3 atom in the sydnone moiety as well as of the size of the substituent at C3.

Degradation of sulfamethoxazole by UV/persulfate in different water samples: Influential factors, transformation products and toxicity

Sulfate radical (SO4−) based advanced oxidation processes are effective for the removal of antibiotics including sulfonamides from freshwater. However, few studies were focused on the degradation of sulfonamides in marine aquaculture water and seawater, as well as the impact of transformation products on seafood such as fishes. In this work, sulfamethoxazole (SMX) was treated with UV/persulfate (UV/PS) system in different water samples. Chloride ions (Cl−) had an inhibitory effect possibly due to the consumption and mutual quenching of reactive radicals. A slight promotion by bromide ions (Br−) was observed and should be related not only to the transformation of Br, but also to the properties of the SMX itself. Bicarbonate ions (HCO3–) significantly inhibited degradation as it is a scavenger of SO4− and HO in UV/PS. Sulfate ions (SO42−) showed a negligible impact on SMX degradation. The active sites of SMX were calculated to be the N1 and C7 atoms of the isoxazole ring, as well as the C9, C13 and N3 atoms of the benzene ring. Seven organic transformation products were identified by LC-MS/MS. Bromate (BrO3−) and chloromethane as the potential human carcinogens were not detected from the reaction system of UV/PS with SMX in marine aquaculture water and seawater. Preliminary analysis of toxicity by the EPI Suite model, absorbable organic halogen (AOX) analysis and growth inhibition of Chlorella vulgaris proved that the UV/PS process could be used as a new disinfection method and it is relatively safer than traditional UV/NaClO disinfection.

Effect of solvent polarity on the regioselective hydroxyalkylation of indole with trifluoroacetaldehyde hemiacetals

The effect of solvents has been found as a crucial factor in determining the regioselectivity of the hydroxyalkylation of indole with trifluoroacetaldehyde hemiacetals. The appropriate selection of the solvent ensured to achieve absolute N1 or C3 regio/chemoselectivity of the reaction depending on the polarity and dielectric constant of the medium. Reaction conditions were optimized considering the effect of solvent, including temperature, time, and molar ratio of reactants to base. In order to rationalize this effect, density functional theory has been employed in which implicit as well as explicit role of solvent was studied, which were further validated with in situ 1H NMR spectroscopy experiments. The comparison of transition states derived from the implicit calculations revealed that the lowest energy path of the reaction in dimethyl sulfoxide (DMSO) leads to product formation with N-selectivity. Further DFT calculations on explicit interactions of indole with DMSO indicated enhanced nucleophilicity on the N atom compared to that of C3 atom, which is evident from the calculated electrostatic potential (ESP) fit charges of these complexes. These findings appear to support the experimental data and explain the N-selectivity in DMSO.

Interdigitated lamellar phases in the frozen state: Spin-label CW- and FT-EPR

Interdigitated lamellar phases composed of dipalmitoylphosphatidylcholine (DPPC) and equimolar content of lyso-palmitoylphosphatidylcholine (Lyso-PPC) or DPPC hydrated in ethanol containing water (60% v/v) have been studied in the frozen state. Electron paramagnetic resonance spectra of labeled lipids at C5 or C16 carbon atom positions along the chain are indicative of segmental librational motion over the temperature range 120–260 K. For any dispersion, the mean-square-angular amplitudes of the librations are comparable for both label positions but are larger in DPPC/etOH than in DPPC/Lyso-PPC interdigitated sample. The temperature dependences of the librational amplitudes of the labels in the lipid matrices show a rapid increase at the dynamical transition at Td ≈ 220 K with an activation energy of 20–30 kJ/mol. Three-pulse electron spin echo envelope modulation by D2O revealed comparable solvent accessibility and fractions of singly and doubly hydrogen-bonded nitroxides to deuterons for both positional isomers in the interdigitated lamellae at 77 K. The overall EPR results indicate that the interdigitated DPPC/etOH sample is more loosened packed compared to DPPC/Lyso-PPC sample. The findings of the present work obtained at cryogenic temperatures point out dynamic and molecular properties of interdigitated lamellae that contribute to the biophysical characterization of membrane model systems.

Order-disorder Twinning of Malachite

Abstract Malachite has a layered structure composed of triangular CO3 groups and CuO2(OH)2 coordination squares of two types. In 2D projection, each layer can be decomposed into two OD strips: the A strip, containing CO3 groups and square co-ordinations of Cu2, with idealized symmetry pgm2, and the B strip, without CO3 groups, with symmetry p2 (the b axis of malachite is the infinite strip direction). In the 3D structure, these strips form the ‘A' OD-layers with symmetry reduced by coordination requirements to approximately p2212, which alternate with the ‘B' OD-layers with layer symmetry . The B-layer can assume two positions, which are related by a 180° rotation axis running through the C1 and O1 atoms of the CO3 group in the A-layer. This axis is oriented close to [100]*. The fit of the A-layer and of the coplanar rotated B-layer is hindered by the presence of important partial gaps and overlaps at the boundary. Inclining these two structure portions towards one another, together with small positional adjustments, appears to alleviate the misfit problems; the two layer portions so related form an angle of about 124°, i.e., they form a twin relationship. This desymmetrization of the malachite structure away from the ideal model results in occasional twinning instead of the fully developed monoclinic polytype structure which is derived in this paper. In the structure of the Cu-Zn analogue, rosasite, the above-described match problem is solved in a different way, by shifting a Zn coordination octahedron (which replaces Cu2 in malachite) into the interlayer space. Malachite polytypes 1 and 2 and rosasite illustrate three different orientations of P21/a symmetry elements with respect to the structural layers, resulting in three different structure configurations.

Structure, Vibrations, Molecular Orbitals, Reactivity Properties of 3-Trifluoromethylphenylchloroformate by FT-IR, FT-Raman, FT-NMR and DFT Studies

The geometry of 3-trifluoromethylphenylchloroformate (FMPCF) was optimized with B3LYP method using 6–311++G** and cc–pVTZ basis sets. The molecular structural parameters and thermodynamic properties of the compound have been determined. The vibrational frequencies of the fundamental modes of the compound have been precisely assigned, analyzed and the theoretical results were compared with the experimental data. The energies of important molecular orbitals of the compound are also evaluated from DFT method. The Frontier orbital energy gap (ELUMO–EHOMO) is found to be 6.2143 eV. The extreme limits of the electrostatic potential is +8.301e × 10–3 to –8.301e × 10–3 while the total electron density spreads between +3.835e × 10–2 to –3.835e × 10–2. 1H NMR and 13C NMR chemical shifts are measured and compared with their gauge independent atomic orbital (GIAO) calculated values. The n(O7) →π*(C13–O14) and π(C1–C6) →π*(C2–C3) transitions are best stablized with 48.40 and 21.03 kcal mol–1, respectively. In 3-trifluoromethylphenylchloroformate, the atoms C13 is favourable for electrophilic attack. The atoms C2 and C8 are more favourable for nucleophilic attack. The dual descriptors (Δfk, Δsk and Δωk) revealed that the order of nucleophilic attack is C1 > C4 > C2 > C8 > C5. Thus, the present investigation provides complete structure, vibrations and reactivity characteristics of the compound.

Hydroxyl Radical Addition to Thymine and Cytosine and Photochemistry of the Adducts at the C6 Position

AbstractOxidative damage can compromise the biological function of DNA/RNA nucleobases. Among the several agents that can cause oxidative damage in cells, the hydroxyl radical (.OH) is considered one of the most reactive species able to add at either the C5 or C6 positions of the C5=C6 double bond of pyrimidine nucleobases. The present work tackles, firstly, the ground state reactivity of the thymine and cytosine nucleobases with .OH using the DFT method. We find that the most favorable attack occurs at the C5 atom in both cases, although C6 is also probable, as observed in previous experimental studies. Secondly, the photochemistry of the C6OH. and T6OH. adducts (which can absorb visible light) is studied at the CASPT2//CASSCF level of theory. In C6OH., the most probable decay corresponds to the return to the initial geometry, whereas in T6OH., distinct processes can compete: (i) return to the Franck‐Condon region, (ii) deactivation by fluorescence emission, (iii) reversion of the oxidation lesion and (iv) the formation of an unstable ketene photoproduct reported herein for the first time.

Formation of methyl radicals derived from cumene hydroperoxide in reconstructed human epidermis: an EPR spin trapping confirmation by using 13C-substitution

Dermal exposure to cumene hydroperoxide (CumOOH) during manufacturing processes is a toxicological issue for the industry. Its genotoxicity, mutagenic action, ability to promote skin tumour, capacity to induce epidermal hyperplasia, and aptitude to induce allergic and irritant skin contact dermatitis are well known. These toxic effects appear to be mediated through the activation to free radical species such as hydroxyl, alkoxyl, and alkyl radicals characterised basically by electron paramagnetic resonance (EPR) and spin-trapping (ST) techniques. To be a skin sensitiser CumOOH needs to covalently bind to skin proteins in the epidermis to form the antigenic entity triggering the immunotoxic reaction. Cleavage of the O–O bond allows formation of unstable CumO•/CumOO• radicals rearranging to longer half-life specific carbon-centred radicals R• proposed to be at the origin of the antigen formation. Nevertheless, it is not still clear which R• is precisely formed in the epidermis and thus involved in the sensitisation process. The aim of this work was to elucidate in conditions closer to real-life sensitisation which specific R• are formed in a 3D reconstructed human epidermis (RHE) model by using 13C-substituted CumOOH at carbon positions precursors of potentially reactive radicals and EPR-ST. We demonstrated that most probably methyl radicals derived from β-scission of CumO• radicals occur in RHE through a one-electron reductive pathway suggesting that these could be involved in the antigen formation inducing skin sensitisation. We also describe a coupling between nitroxide radicals and β position 13C atoms that could be of an added value to the very few examples existing for the coupling of radicals with 13C atoms.

6-Substituted purines as ROCK inhibitors with anti-metastatic activity

Rho-associated serine/threonine kinases (ROCKs) are principal regulators of the actin cytoskeleton that regulate the contractility, shape, motility, and invasion of cells. We explored the relationships between structure and anti-ROCK2 activity in a group of purine derivatives substituted at the C6 atom by piperidin-1-yl or azepan-1-yl groups. Structure-activity relationship (SAR) analyses suggested that anti-ROCK activity is retained, and may be further increased, by substitution of the parent compounds at the C2 atom or by expansion of the C6 side chain. These inhibitors of ROCK can reach effective concentrations within cells, as demonstrated by a decrease in phosphorylation of the ROCK target MLC, and by inhibition of the ROCK-dependent invasion of melanoma cells in the collagen matrix. Our study may be useful for further optimization of C6-substituted purine inhibitors of ROCKs and of other sensitive kinases identified by the screening of a broad panel of protein kinases.

Reassimilation of Leaf Internal CO2 Contributes to Isoprene Emission in the Neotropical Species Inga edulis Mart.

Isoprene (C5H8) is a hydrocarbon gas emitted by many tree species and has been shown to protect photosynthesis under abiotic stress. Under optimal conditions for photosynthesis, ~70%–90% of carbon used for isoprene biosynthesis is produced from recently assimilated atmospheric CO2. While the contribution of alternative carbon sources that increase with leaf temperature and other stresses have been demonstrated, uncertainties remain regarding the biochemical source(s) of isoprene carbon. In this study, we investigated leaf isoprene emissions (Is) from neotropical species Inga edulis Mart. as a function of light and temperature under ambient (450 µmol m−2 s−1) and CO2-free (0 µmol m−2 s−1) atmosphere. Is under CO2-free atmosphere showed light-dependent emission patterns similar to those observed under ambient CO2, but with lower light saturation point. Leaves treated with the photosynthesis inhibitor DCMU (3-(3,4-dichlorophenyl)-1,1-dimethylurea) failed to produce detectable Is in normal light under a CO2-free atmosphere. While strong temperature-dependent Is were observed under CO2-free atmosphere in the light, dark conditions failed to produce detectable Is even at the highest temperatures studied (40 °C). Treatment of leaves with 13C-labeled sodium bicarbonate under CO2-free atmosphere resulted in Is with over 50% containing at least one 13C atom. Is under CO2-free atmosphere and standard conditions of light and leaf temperature represented 19% ± 7% of emissions under ambient CO2. The results show that the reassimilation of leaf internal CO2 contributes to Is in the neotropical species I. edulis. Through the consumption of excess photosynthetic energy, our results support a role of isoprene biosynthesis, together with photorespiration, as a key tolerance mechanism against high temperature and high light in the tropics.

Two Types of σ-Allenyl Complexes from Reactions of Silylenes and Germylenes with Chromium Fischer Alkynyl(alkoxy)carbenes.

The reactivity of amidinatotetrylenes of the type E(tBu2 bzm)R1 (E=Si, Ge; tBu2 bzm=N,N'-bis(tertbutyl)benzamidinate; R1 =alkyl or aryl) with the chromium Fischer alkynylcarbene complexes [Cr{C(OEt)C2 R2 }(CO)5 ] (R2 =Ph; ferrocenyl, Fc) has been studied. At room temperature, two different reaction pathways have been identified: (a) attack of the amidinatotetrylene to the alkynyl C2 atom (γ-attack), which leads to σ-allenyl complexes in which the original Ccarbene atom maintains its attachment to the Cr(CO)5 and OEt groups (compounds 3 ), and (b) attack of the amidinatotetrylene to the Ccarbene atom (α-attack), which ends in σ-allenyl complexes in which the original Ccarbene atom is not attached to the metal atom and has been inserted into an E-N bond of the amidinatotetrylene forming an E-C-N-C-N five-membered ring (compounds 4 ). It has been found that compounds 3 are thermodynamically less stable than their corresponding 4 isomers and that some of the former (E=Ge; R1 =CH2 SiMe3 ) can be transformed into the latter upon heating. At high temperatures (>70 °C) the reactions involving bulky amidinatotetrylenes (R1 =Mes, tBu) end in the carbene-substitution products [Cr{E(tBu2 bzm)R1 }(CO)5 ].

Structural Characterization of Phenoxy Radical with Mass-Correlated Broadband Microwave Spectroscopy.

A combination of broadband microwave spectroscopy and VUV photoionization time-of-flight mass spectra has been used to record rotational spectra of the prototypical phenoxy radical, its per-deuterated isotopomers, and the full set of singly 13C-substituted analogues. Rotational parameters associated with the fits to the full set of isotopomers produce a highly accurate r0 structure for the phenoxy radical. High-level ab initio calculations accurately reproduce the rotational constants and spin-rotation parameters. The structure of the phenoxy radical is distinctly quinoidal, with delocalization of the unpaired electron spin density on the oxygen and phenyl ring. The fitted Fermi contact terms for the 13C atoms reflect a weighting of resonance structures that is 27% on the O atom, 21.5% on each of the two ortho C's, and 30% on the para C, providing a quantitative measure of its sites for subsequent reactions that will control its abundances in combustion and atmospheric environments.

Cyclophosphamide and isophosphamide – DFT conformational studies in the gas phase and solution

Cyclophosphamide and isophosphamide have been subjected to comprehensive conformational studies in the vacuum and solution using the SMD solvation model. Vacuum calculations were conducted using the B3LYP, M05-2X, M06-2X and ωB97XD functionals. Natural bond orbital (NBO) analysis has been performed for selected geometries. A preference for a chair conformation with the axial P=O bond is shown (1C4). The 5S0 conformation is 1.25–2.31 kcal/mol and 1.72–2.92 kcal/mol higher in energy than the global minimum conformations of cyclophosphamide and isophosphamide, respectively. In the gas phase, the chair conformation with the equatorial P=O bond (4C1) is of comparable stability or less stable than the skew form, depending on the method used, while it is slightly more favored than the 5S0 conformation in solution. The stereoelectronic effects do not differentiate the ring conformer stability. The steric strains between N(EtCl)1–2 and the C4 and C6 carbon atoms mainly influence the stability of cyclophosphamide and isophosphamide conformers.

Structure-based drug design of novel carborane-containing nicotinamide phosphoribosyltransferase inhibitors.

A series of carborane-containing NAMPT inhibitors were designed and synthesized based on the structure of compounds 1 and the NAMPT inhibitory activity was evaluated using NAMPT Colorimetric Assay. Among the compounds synthesized, compounds 2b and 2c showed significant NAMPT inhibitory activity with IC50 values of 0.098 ± 0.008 and 0.057 ± 0.001 µM, respectively. Docking simulation of compound 2 toward NAMPT using the crystal structure of the FK866-NAMPT complex (PDB code: 2GVJ) with replacing the boron atom type by the C3 atom type of carboranes predicted that the NAMPT inhibitory activity of 2c was improved by the hydrogen bond formation between the carborane amide and H191 of NAMPT. Although dicarborane compounds 38, 50, 51, and 55 were synthesize aiming to two hydrophobic pockets present in the binding pocket of NAMPT, their inhibitory activity was moderate.

NanoSIMS reveals unusual enrichment of acetate and propionate by an anammox consortium dominated by Jettenia asiatica

Anaerobic ammonium-oxidizing (anammox) bacteria convert ammonium and nitrite into N2 in a chemolithoautotrophic way, meaning that they utilize CO2/HCO3 solely as their carbon sources. Such autotrophic behavior limits their competitiveness with heterotrophic microorganisms in both natural environments and engineered systems. Recently, environmental metagenomic results have indicated the capability of anammox bacteria to metabolize short-chain fatty acids, further confirmed by limited experimental evidence based on highly enriched cultures. However, clear evidence is difficult to get because of the limits of traditional methodologies which rely on the availability of a pure anammox culture. In this study, we identified and quantified the uptake of acetate and propionate, on a single-cell level, by an anammox consortium that was dominated by Candidatus Jettenia asiatica (relative abundance of 96%). The consortium, growing in granular form with an average relative abundance of anammox bacteria of 96.0%, was firstly incubated in a13C-labelled acetate or propionate medium; then microtome sections were scanned by a nanometer-scale secondary ion mass spectrometer (NanoSIMS). The NanoSIMS scannings revealed that the consortium enriched acetate and propionate at a >10 times higher efficiency than bicarbonate incorporation. Our results also suggest that acetate or propionate was likely not assimilated by J. asiatica directly, but firstly oxidized to CO2, which then served as carbon sources for the follow-up autotrophy in J. asiatica cells. Furthermore, more [15N]ammonium was enriched by the propionate-fed consortium than the acetate-fed consortium despite that exactly the same amount of 13C atoms were supplied. Our study strongly indicates an alternative lifestyle, namely organotrophy, in addition to chemolithoautotrophy of anammox bacteria, making it more versatile than often expected. It suggests that the niche of anammox bacteria in both natural and engineered ecosystems can be much broader than usual assumed. Recognising this is important for their role in wastewater treatment and the global nitrogen turn-over rates.

Confirmation of local water structure confined in ionic liquids using H/D exchange

We investigated the local structure of water molecules confined inside ionic liquid (IL) nanodomain structures of 1-butyl-3-methylimidazolium tetrafluoroborate, (abbreviated as [bmim][BF4]) using NMR and Raman spectroscopy to monitor H/D exchange reaction between H2O and D2O water molecules. The results showed that fewer than 5 water molecules were probably present in the water domain of the IL nanospace at less than ~60 water mol%, where the translational motion of water molecules was inhibited and proton exchange rate was highly restricted. These phenomena were attributed to the absence of bulk hydrogen-bonded network structures in the water domain. The H atoms of the HDO molecule tended to interact with BF4− anions, whereas the D atoms of HDO interacted with the C2 atoms of the bmim+ cation. As a result, water molecules became trapped inside the IL framework, which is likely to be the main cause of the reduction in H/D exchange rate among water molecules in the mixture.

Structural and functional insights into phosphomannose isomerase: the role of zinc and catalytic residues.

Phosphomannose isomerase (PMI) is a housekeeping enzyme that is found in organisms ranging from bacteria to fungi to mammals and is important for cell-wall synthesis, viability and signalling. PMI is a zinc-dependent enzyme that catalyses the reversible isomerization between mannose 6-phosphate (M6P) and fructose 6-phosphate (F6P), presumably via the formation of a cis-enediol intermediate. The reaction is hypothesized to involve ring opening of M6P, the transfer of a proton from the C2 atom to the C1 atom and between the O1 and O2 atoms of the substrate, followed by ring closure resulting in the product F6P. Several attempts have been made to decipher the role of zinc ions and various residues in the catalytic function of PMI. However, there is no consensus on the catalytic base and the mechanism of the reaction catalyzed by the enzyme. In the present study, based on the structure of PMI from Salmonella typhimurium, site-directed mutagenesis targeting residues close to the bound metal ion and activity studies on the mutants, zinc ions were shown to be crucial for substrate binding. These studies also suggest Lys86 as the most probable catalytic base abstracting the proton in the isomerization reaction. Plausible roles for the highly conserved residues Lys132 and Arg274 could also be discerned based on comparison of the crystal structures of wild-type and mutant PMIs. PMIs from prokaryotes possess a low sequence identity to the human enzyme, ranging between 30% and 40%. Since PMI is important for the virulence of many pathogenic organisms, the identification of catalytically important residues will facilitate its use as a potential antimicrobial drug target.

Reactivity of 4-thiothymidine with Fenton reagent investigated by UV-visible spectroscopy and electrospray ionization mass spectrometry.

The reactivity of the sulfur-containing nucleoside 4-thio-(2'-deoxy)-thymidine usually abbreviated as 4-thio-thymidine, (S4 -TdR) under Fenton conditions, ie, in the presence of H2 O2 and catalytic amounts of Fe(II), was investigated by UV-vis spectroscopy and electrospray ionization single and tandem mass spectrometry (ESI-MS and MS/MS). S4 -TdR hydroxylated on the S atom was found to be a key reaction intermediate, ultimately leading to (2'-deoxy)-thymidine usually abbreviated as thymidine, (TdR) as the main reaction product. This finding was in accordance with the outcome of the reaction between S4 -TdR and H2 O2 , previously investigated in our laboratory. On the other hand, the additional presence of •OH radicals, induced by the Fe(II)/H2 O2 combination, led to the increased generation of another interesting S4 -TdR product, already observed after its reaction with H2 O2 alone, ie, the covalent dimer including a SS bridge between two S4 -TdR molecules. More importantly, multihydroxylated derivatives of S4 -TdR and TdR were detected as peculiar products obtained under Fenton conditions. Among them, a product bearing an OH group both on the methyl group linked to the thymine ring and on the C5 atom of the ring was found to prevail. The results obtained during this study, integrated by those found previously in our laboratory, indicate 4-thiothymidine as a promising molecular probe for the recognition, through a careful characterization of its reaction products, of the prevailing species among reactive oxygen species (ROS) corresponding to singlet-state oxygen, hydrogen peroxide, and hydroxylic radical.