Crystal structures of NUDT15 variants enabled by a potent inhibitor reveal the structural basis for thiopurine sensitivity.

The enzyme NUDT15 efficiently hydrolyzes the active metabolites of thiopurine drugs, which are routinely used for treating cancer and inflammatory diseases. Loss-of-function variants in NUDT15 are strongly associated with thiopurine intolerance, such as leukopenia, and preemptive NUDT15 genotyping has been clinically implemented to personalize thiopurine dosing. However, understanding the molecular consequences of these variants has been difficult, as no structural information was available for NUDT15 proteins encoded by clinically actionable pharmacogenetic variants because of their inherent instability. Recently, the small molecule NUDT15 inhibitor TH1760 has been shown to sensitize cells to thiopurines, through enhanced accumulation of 6-thio-guanine in DNA. Building upon this, we herein report the development of the potent and specific NUDT15 inhibitor, TH7755. TH7755 demonstrates a greatly improved cellular target engagement and 6-thioguanine potentiation compared with TH1760, while showing no cytotoxicity on its own. This potent inhibitor also stabilized NUDT15, enabling analysis by X-ray crystallography. We have determined high-resolution structures of the clinically relevant NUDT15 variants Arg139Cys, Arg139His, Val18Ile, and V18_V19insGlyVal. These structures provide clear insights into the structural basis for the thiopurine intolerance phenotype observed in patients carrying these pharmacogenetic variants. These findings will aid in predicting the effects of new NUDT15 sequence variations yet to be discovered in the clinic.

Design, synthesis and biological evaluation studies of novel small molecule ENPP1 inhibitors for cancer immunotherapy.

Ecto-nucleotide pyrophosphatase/phosphodiesterases 1 (ENPP1 or NPP1), is an attractive therapeutic target for various diseases, primarily cancer and mineralization disorders. The ecto-enzyme is located on the cell surface and has been implicated in the control of extracellular levels of nucleotide, nucleoside and (di) phosphate. Recently, it has emerged as a critical phosphodiesterase that hydrolyzes cyclic 2'3'- cGAMP, the endogenous ligand for STING (STimulator of INterferon Genes). STING plays an important role in innate immunity by activating type I interferon in response to cytosolic 2'3'-cGAMP. ENPP1 negatively regulates the STING pathway and hence its inhibition makes it an attractive therapeutic target for cancer immunotherapy. Herein, we describe the design, optimization and biological evaluation studies of a series of novel non-nucleotidic thioguanine based small molecule inhibitors of ENPP1. The lead compound 43 has shown good in vitro potency, stability in SGF/SIF/PBS, selectivity, ADME properties and pharmacokinetic profile and finally potent anti-tumor response in vivo. These compounds are a good starting point for the development of potentially effective cancer immunotherapy agents.

Ultrafast Excited-State Decay Mechanisms of 6-Thioguanine Followed by Sub-20 fs UV Transient Absorption Spectroscopy.

Understanding the primary steps following UV photoexcitation in sulphur-substituted DNA bases (thiobases) is fundamental for developing new phototherapeutic drugs. However, the investigation of the excited-state dynamics in sub-100 fs time scales has been elusive until now due to technical challenges. Here, we track the ultrafast decay mechanisms that lead to the electron trapping in the triplet manifold for 6-thioguanine in an aqueous solution, using broadband transient absorption spectroscopy with a sub-20 fs temporal resolution. We obtain experimental evidence of the fast internal conversion from the S2(ππ*) to the S1(nπ*) states, which takes place in about 80 fs and demonstrates that the S1(nπ*) state acts as a doorway to the triplet population in 522 fs. Our results are supported by MS-CASPT2 calculations, predicting a planar S2(ππ*) pseudo-minimum in agreement with the stimulated emission signal observed in the experiment.

Thioguanine restoration through type I photosensitization-superoxide oxidation-glutathione reduction cycles.

UVA-induced deleterious effect of thiopurine prodrugs including azathioprine, 6-mercaptopurine and 6-thioguanine (6-TG) increases the risk of cancer development due to the incorporation of 6-TG in patients' DNA. The catalytic mechanism by which thiobases act as a sustained oxidant producer has yet to be explored, especially through the Type I electron transfer pathway that produces superoxide radicals (O2˙-). Under Fenton-like conditions O2˙- radicals convert to extremely reactive hydroxyl radicals (˙OH), thus carrying even higher risk of biological damage than that induced by the well-studied type II reaction. By monitoring 6-TG/UVA-induced photochemistry in mass spectra and superoxide radicals (O2˙-) via nitro blue tetrazolium (NBT) reduction, this work provides two new findings: (1) in the presence of reduced glutathione (GSH), the production of O2˙-via the type I reaction is enhanced 10-fold. 6-TG thiyl radicals are identified as the primary intermediate formed in the reaction of 6-TG with O2˙-. The restoration of 6-TG and concurrent generation of O2˙- occur via a 3-step-cycle: 6-TG type I photosensitization, O2˙- oxidation and GSH reduction. (2) In the absence of GSH, 6-TG thiyl radicals undergo oxygen addition and sulfur dioxide removal to form carbon radicals (C6) which further convert to thioether by reacting with 6-TG molecules. These findings help explain not only thiol-regulation in a biological system but chemoprevention of cancer.

Gene Expression of 4'-Thioguanine DNA via 4'-Thiocytosine RNA.

DNA and RNA nucleotides are ubiquitous molecules that store and transmit genetic information. The emergence of synthetic elements that fulfill the function of DNA and RNA provides an alternative gene expression system. Herein, we demonstrate the gene expression of 4'-thioguanine DNA (dSG DNA) via 4'-thiocytosine RNA (dSC RNA) to give green fluorescent protein (GFPuv) in a single test tube. In replication, transcription, and translation, DNA/RNA polymerases and Escherichia coli (E. coli) ribosome can tolerate the replacement of O4' with S4' in the nucleotide, despite the fact that sulfur has a larger atomic radius than oxygen. Additionally, dSG DNA and dSC RNA acted as alternative genetic polymers to natural DNA and RNA for protein synthesis in artificial cells comprising a reconstituted E. coli gene expression machinery. This work involved simple experiments that are widely used in molecular biology, but which underscore the feasibility of life control by substances other than DNA/RNA nucleotides.

Elucidating the binding mechanism of thione-containing mercaptopurine and thioguanine drugs to small gold clusters.

Density functional theory methods were employed to clarify the adsorption/desorption behaviors of the thione-containing mercaptopurine and thioguanine drugs on the gold surface using both small Au6 and Au8 clusters as model reactants. Structural features, thermodynamic parameters, bonding characteristics, and electronic properties of the resulting complexes were investigated using the Perdew-Burke-Ernzerhof (PBE) and LC-BLYP functionals along with correlation-consistent basis sets, namely cc-pVDZ-PP for gold and cc-pVTZ for non-metals. Computed results show that the drug molecules tend to anchor on the gold cluster at the S atom with binding energies around -34 to -40 kcal/mol (in vacuum) and - 28 to -32 kcal/mol (in aqueous solution). As compared to Au8 , Au6 undergoes a shorter recovery time and a larger change of energy gap that could be converted to an electrical signal for selective detection of the drugs. Furthermore, interactions between the drugs and gold clusters are reversible processes and a drug release mechanism was also proposed. Accordingly, the drugs are able to separate from the gold surface due to either a slight change of pH in tumor cells or the presence of cysteine residues in protein matrices.

Nucleobase derivatives induce in-source decay of oligonucleotides as new matrix-assisted laser desorption/ionization matrices.

RATIONALE: For quality control of oligonucleotide therapeutics, accurate and efficient structural characterization using mass spectrometry techniques, such as liquid chromatography/mass spectrometry (LC/MS) and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS), is essential. In MALDI MS analysis, matrix selection is critical and a new matrix could enable more efficient and rapid structural analysis. METHODS: We hypothesized that nucleobase derivatives could act as matrices more efficiently than the currently used matrices for oligonucleotides because of structural similarity, which leads to close contact with the analyte. To evaluate their suitability as matrices, 16 nucleobase derivatives were selected and tested as matrix candidates for oligonucleotide analysis. RESULTS: Six of the 16 nucleobase derivatives acted as matrices for oligonucleotides. Particularly, 6-thioguanine (TG) performed well and induced clear in-source decay fragmentation. When TG or 2-amino-6-chloropurine was used as the matrix, oligonucleotides were ionized, and mainly the w and d fragment ions were observed. CONCLUSIONS: Herein we demonstrate that a 10-mer RNA or DNA sequence can be successfully characterized using TG as matrix and suggest the possibility of using nucleobase derivatives as novel matrices in oligonucleotide sequencing.

Effective imprinting of an anticancer drug, 6-thioguanine, via mussel-inspired self-polymerization of dopamine over reduced graphene oxide.

Apart from being a vital catecholamine molecule responsible for the proper functioning of the central nervous system (CNS), hormonal and renal systems, dopamine (DA) has also been increasingly employed as a functional monomer in the fabrication of surface molecular imprinting polymers (MIPs) for valuable analytes. Herein, we demonstrate the effective imprinting of 6-thioguanine (6-TG), an anticancer drug, via mussel-inspired self-polymerization of dopamine conducted in a weakly alkaline solution over reduced graphene oxide (rGO). The polymerization of 6-TG resulted into a thin polydopamine (PDA) film of 8.4 nm thickness. Removal of 6-TG molecules from this imprinted PDA film created numerous cavities of 6-TG. The electrochemical investigation of MIP electrodes found an excellent electrocatalytic activity toward 6-TG with a significant decrease in the over-potential as compared to that of the bare glassy carbon electrode (GCE). This can be attributed to the graphene's distinct physical and chemical features such as subtle electronic characteristics, an attractive π-π interaction as well as the strong adsorptive capability of MIP films. This electrochemical sensor displayed a high selectivity owing to the specific imprinted cavities for adrenaline and worked well over a wide linear concentration range of adrenaline between 0.0015 and 50 µM with a detection limit (LOD) of 0.25 nM and good reproducibility and stability. Our system depicts excellent recoveries from 97.0% to 100.6% for two different samples of urine and thioguanine drugs. These results show the great potential of our system with multiple advantages, including convenient fabrication and optimization, high sensitivity and selectivity, high reproducibility and stability, and cost-effectiveness.

The adduct formation between the thioguanine-polyamine ligands and DNA with the AP site under UVA irradiated and non-irradiated conditions.

The AP sites are representative of DNA damage and known as an intermediate in the base excision repair (BER) pathway which is involved in the repair of damaged nucleobases by reactive oxygen species, UVA irradiation, and DNA alkylating agents. Therefore, it is expected that the inhibition or modulation of the AP site repair pathway may be a new type of anticancer drug. In this study, we investigated the effects of the thioguanine-polyamine ligands (SG-ligands) on the affinity and the reactivity for the AP site under UVA irradiated and non-irradiated conditions. The SG-ligands have a photo-reactivity with the A-F-C sequence where F represents a tetrahydrofuran AP site analogue. Interestingly, the SG-ligands promoted the ß-elimination of the AP site followed by the formation of a covalent bond with the ß-eliminated fragment without UVA irradiation.

In vivo modification of tRNA with an artificial nucleobase leads to full disease remission in an animal model of multiple sclerosis.

Queuine is a modified pyrrolopyrimidine nucleobase derived exclusively from bacteria. It post-transcriptionally replaces guanine 34 in transfer RNA isoacceptors for Asp, Asn, His and Tyr, in almost all eukaryotic organisms, through the activity of the ancient tRNA guanine transglycosylase (TGT) enzyme. tRNA hypomodification with queuine is a characteristic of rapidly-proliferating, non-differentiated cells. Autoimmune diseases, including multiple sclerosis, are characterised by the rapid expansion of T cells directed to self-antigens. Here, we demonstrate the potential medicinal relevance of targeting the modification of tRNA in the treatment of a chronic multiple sclerosis model­murine experimental autoimmune encephalomyelitis. Administration of a de novo designed eukaryotic TGT substrate (NPPDAG) led to an unprecedented complete reversal of clinical symptoms and a dramatic reduction of markers associated with immune hyperactivation and neuronal damage after five daily doses. TGT is essential for the therapeutic effect, since animals deficient in TGT activity were refractory to therapy. The data suggest that exploitation of the eukaryotic TGT enzyme is a promising approach for the treatment of multiple sclerosis.

Heavy-Atom-Substituted Nucleobases in Photodynamic Applications: Substitution of Sulfur with Selenium in 6-Thioguanine Induces a Remarkable Increase in the Rate of Triplet Decay in 6-Selenoguanine.

Sulfur substitution of carbonyl oxygen atoms of DNA/RNA nucleobases promotes ultrafast intersystem crossing and near-unity triplet yields that are being used for photodynamic therapy and structural-biology applications. Replacement of sulfur with selenium or tellurium should significantly red-shift the absorption spectra of the nucleobases without sacrificing the high triplet yields. Consequently, selenium/tellurium-substituted nucleobases are thought to facilitate treatment of deeper tissue carcinomas relative to the sulfur-substituted analogues, but their photodynamics are yet unexplored. In this contribution, the photochemical relaxation mechanism of 6-selenoguanine is elucidated and compared to that of the 6-thioguanine prodrug. Selenium substitution leads to a remarkable enhancement of the intersystem crossing lifetime both to and from the triplet manifold, resulting in an efficiently populated, yet short-lived triplet state. Surprisingly, the rate of triplet decay in 6-selenoguanine increases by 835-fold compared to that in 6-thioguanine. This appears to be an extreme manifestation of the classical heavy-atom effect in organic photochemistry, which challenges conventional wisdom.

Mechanism of enhanced triplet decay of thionucleobase by glycosylation and rate-modulating strategies.

The decay of the triplet state of photosensitizers is essential to their performance in singlet-oxygen generation. Experiments have shown that in thionucleosides, this decay is enhanced compared to that in the corresponding thionucleobases. In this work, we applied quantum-chemical methods and chemical-kinetic modeling to investigate the effects of the sugar substituent on the triplet decay of thionucleosides. The computed rates for the energetically favored conformers of thiothymidine, thiouridine, and thioguanosine (and the respective thionucleobases) show a remarkable quantitative agreement with the experimental results. We additionally show that the triplet decay enhancement is caused by the repulsion interaction between the sugar group and the sulfur atom, which reduces the activation energy for intersystem crossing by destabilizing the T1 minimum. In some instances, an intramolecular hydrogen bond stabilizes the energy of the T1/S0 crossing point, also reducing the activation energy. This molecular understanding of the mechanism of enhanced triplet decay provides a guideline to control the triplet decay rate, which was tested in new thiothymidine derivatives.

DNA-thioguanine nucleotide concentration and relapse-free survival during maintenance therapy of childhood acute lymphoblastic leukaemia (NOPHO ALL2008): a prospective substudy of a phase 3 trial.

BACKGROUND: Adjustment of mercaptopurine and methotrexate maintenance therapy of acute lymphoblastic leukaemia by leucocyte count is confounded by natural variations. Cytotoxicity is primarily mediated by DNA-incorporated thioguanine nucleotides (DNA-TGN). The aim of this study was to establish whether DNA-TGN concentrations in blood leucocytes during maintenance therapy are associated with relapse-free survival. METHODS: In this substudy of the NOPHO ALL2008 phase 3 trial done in 23 hospitals in seven European countries (Denmark, Estonia, Finland, Iceland, Lithuania, Norway, and Sweden), we analysed data from centralised and blinded analyses of 6-mercaptopurine and methotrexate metabolites in blood samples from patients with non-high-risk childhood acute lymphoblastic leukaemia. Eligible patients were aged 1·0-17·9 years; had been diagnosed with non-high-risk precursor B-cell or T-cell leukaemia; had been treated according to the Nordic Society of Pediatric Hematology and Oncology ALL2008 protocol; and had reached maintenance therapy in first remission. Maintenance therapy was (mercaptopurine 75 mg/m2 once per day and methotrexate 20 mg/m2 once per week, targeted to a leucocyte count of 1·5-3·0 × 109 cells per L). We measured DNA-TGN and erythrocyte concentrations of TGN nucleotides, methylated mercaptopurine metabolites, and methotrexate polyglutamates. The primary objective was the association of DNA-TGN concentrations and 6-mercaptopurine and methotrexate metabolites with relapse-free survival. The secondary endpoint was the assessment of DNA-TGN concentration and 6-mercaptopurine and methotrexate metabolites during maintenance therapy phase 2. FINDINGS: Between Nov 26, 2008 and June 14, 2016, 1509 patients from the NOPHO ALL2008 study were assessed for eligibility in the DNA-TGN substudy, of which 918 (89%) of 1026 eligible patients had at least one DNA-TGN measurement and were included in the analyses. Median follow-up was 4·6 years (IQR 3·1-6·1). Relapse-free survival was significantly associated with DNA-TGN concentration (adjusted hazard ratio 0·81 per 100 fmol/µg DNA increase, 95% CI 0·67-0·98; p=0·029). In patients with at least five blood samples, erythrocyte concentrations of TGN, methylated mercaptopurine metabolites, and methotrexate polyglutamates were associated with DNA-TGN concentration (all p<0·0001). INTERPRETATION: Our results suggest the need for intervention trials to identify clinically applicable strategies for individualised drug dosing to increase DNA-TGN concentration, and randomised studies to investigate whether such strategies improve cure rates compared with current dose adjustments based on white blood cell counts. FUNDING: Danish Cancer Society, Childhood Cancer Foundation (Denmark), Childhood Cancer Foundation (Sweden), Nordic Cancer Union, Otto Christensen Foundation, University Hospital Rigshospitalet, and Novo Nordic Foundation.

Comparison of 6-mercaptopurine with 6-thioguanine for the analysis of thiopurine S-methyltransferase activity in human erythrocyte by LC-MS/MS.

Thiopurines (TPDs) are first-line drugs in treating neuromyelitis optica spectrum disorders (NMOSD). Evaluation of thiopurine S-methyltransferase activity (TPMT), a major determinant of TPD toxicity, before TPD treatment using 6-mercaptopurine (6-MP) and 6-thioguanine (6-TG) as substrate was suggested. However, the equivalent of the two substrates in TPMT activity evaluation was unknown, and an alternative substrate was required in TPMT activity evaluation in patients who were already taking 6-MP or 6-TG. Before evaluating the agreement of 6-MP and 6-TG in TPMT activity measurement in patients with NMOSD, the affinity of the two substrates for the active center of TPMT should be established. A computer-based simulation indicated that 6-MP and 6-TG had similar affinities for the two active sites of TPMT. According to the guidelines, an LC-MS/MS method was developed and validated to evaluate the TPMT activity in human erythrocyte hemolysate using 6-MP or 6-TG as substrates via 1 h incubation at 37°C. The method was applied in 81 patients with NMOSD. Evaluated by Bland-Altman plot, 6-methylmercaptopurine and 6-methylthioguanine represented TPMT activities were in agreement with each other. Further studies are warranted to confirm the results.

Thiopurine Drugs Repositioned as Tyrosinase Inhibitors.

Drug repositioning is the application of the existing drugs to new uses and has the potential to reduce the time and cost required for the typical drug discovery process. In this study, we repositioned thiopurine drugs used for the treatment of acute leukaemia as new tyrosinase inhibitors. Tyrosinase catalyses two successive oxidations in melanin biosynthesis: the conversions of tyrosine to dihydroxyphenylalanine (DOPA) and DOPA to dopaquinone. Continuous efforts are underway to discover small molecule inhibitors of tyrosinase for therapeutic and cosmetic purposes. Structure-based virtual screening predicted inhibitor candidates from the US Food and Drug Administration (FDA)-approved drugs. Enzyme assays confirmed the thiopurine leukaemia drug, thioguanine, as a tyrosinase inhibitor with the inhibitory constant of 52 µM. Two other thiopurine drugs, mercaptopurine and azathioprine, were also evaluated for their tyrosinase inhibition; mercaptopurine caused stronger inhibition than thioguanine did, whereas azathioprine was a poor inhibitor. The inhibitory constant of mercaptopurine (16 µM) was comparable to that of the well-known inhibitor kojic acid (13 µM). The cell-based assay using B16F10 melanoma cells confirmed that the compounds inhibit mammalian tyrosinase. Particularly, 50 µM thioguanine reduced the melanin content by 57%, without apparent cytotoxicity. Cheminformatics showed that the thiopurine drugs shared little chemical similarity with the known tyrosinase inhibitors.

Efficient Thymidine-Selective DNA Interstrand Photo-activated Crosslinking by the 6-Thioguanine Connected via an Ethylene-Linker to the 2'-Deoxyribose Unit.

Cross-linking is a widely-used technology in the studies of DNA, RNA and their complexes with proteins. Intrinsically active alkylating moieties and photo-activated agents are chemically or enzymatically incorporated into nucleic acids. Thionucleobases resemble the corresponding natural bases, and form cross-links by UVA irradiation. They form cross-links only with a site in close contact, thereby allowing identification of the contacts within the nucleic acids and/or between the nucleic acids and proteins in complex nucleoprotein assemblies. On the other hand, the thionucleobase forms a cross-link less efficiently for the reaction with the opposite natural base in the DNA duplex. In this study, 6-thioguanine was connected to 2'-deoxyribose through an ethylene linker at the 1'-position (Et-thioG). The linker was expected to bring the 6-thio group close to the nucleobase in the opposite strand. In a duplex in which the 2'-deoxy-6-thioguanosine (6-thio-dG) did not form a crosslink, Et-thioG efficiently formed crosslink with a high selectivity for T by UVA irradiation, but with a much lower efficiency for dA, dG, dC, 5-methyl-dC or dU. Interestingly, the yield of the photo-crosslinked product with dT was effectively improved in the presence of dithiothreitol or sodium hydrosulfide (NaSH) at a low UVA irradiation dose. The efficient and selective cross-link formation at a low UVA dose may be beneficial for the biological application of Et-thioG.

Control of plant defense mechanisms and fire blight pathogenesis through the regulation of 6-thioguanine biosynthesis in Erwinia amylovora.

Fire blight is a devastating disease of Rosaceae plants, such as apple and pear trees. It is characterized by necrosis of plant tissue, caused by the phytopathogenic bacterium Erwinia amylovora. The plant pathogen produces the well-known antimetabolite 6-thioguanine (6TG), which plays a key role in fire blight pathogenesis. Here we report that YcfR, a member of the LTTR family, is a major regulator of 6TG biosynthesis in E. amylovora. Inactivation of the regulator gene (ycfR) led to dramatically decreased 6TG production. Infection assays with apple plants (Malus domestica cultivar Holsteiner Cox) and cell cultures of Sorbus aucuparia (mountain ash, rowan) revealed abortive fire blight pathogenesis and reduced plant response (biphenyl and dibenzofuran phytoalexin production). In the presence of the ΔycfR mutant, apple trees were capable of activating the abscission machinery to remove infected tissue. In addition to unveiling the regulation of 6TG biosynthesis in a major plant pathogen, we demonstrate for the first time that this antimetabolite plays a pivotal role in dysregulating the plant response to infection.

The identification, analysis and structure-based development of novel inhibitors of 6-hydroxymethyl-7,8-dihydropterin pyrophosphokinase.

6-Hydroxymethyl-7,8-dihydropterin pyrophosphokinase (HPPK) is an essential enzyme in the microbial folate biosynthetic pathway. This pathway has proven to be an excellent target for antimicrobial development, but widespread resistance to common therapeutics including the sulfa drugs has stimulated interest in HPPK as an alternative target in the pathway. A screen of a pterin-biased compound set identified several HPPK inhibitors that contain an aryl substituted 8-thioguanine scaffold, and structural analyses showed that these compounds engage the HPPK pterin-binding pocket and an induced cryptic pocket. A preliminary structure activity relationship profile was developed from biophysical and biochemical characterizations of derivative molecules. Also, a similarity search identified additional scaffolds that bind more tightly within the HPPK pterin pocket. These inhibitory scaffolds have the potential for rapid elaboration into novel lead antimicrobial agents.

Formation of guanine-6-sulfonate from 6-thioguanine and singlet oxygen: a combined theoretical and experimental study.

As an end metabolism product of the widely used thiopurine drugs, 6-thioguanine (6-TG) absorbs UVA and produces (1)O2 by photosensitization. This unusual photochemical property triggers a variety of DNA damage, among which the oxidation of 6-TG itself by (1)O2 to the promutagenic product guanine-6-sulfonate (G(SO3)) represents one of the major forms. It has been suspected that there exists an initial intermediate, G(SO), prior to its further oxidation to G(SO2) and G(SO3), but G(SO) has never been observed. Using density functional theory, we have explored the energetics and intermediates of 6-TG and (1)O2. A new mechanism via G(SOOH) → G(SO2) → G(SO4) → G(SO3) has been discovered to be the most feasible energetically, whereas the anticipated G(SO) mechanism is found to encounter an inaccessibly high barrier and thus is prevented. The mechanism through the G(SOOH) and G(SO4) intermediates can be validated further by joint experimental measurements, where the fast rate constant of 4.9 × 10(9) M(-1) s(-1) and the reaction stoichiometry of 0.58 supports this low-barrier new mechanism. In addition to the dominant pathway of G(SOOH) → G(SO2) → G(SO4) → G(SO3), a side pathway with higher barrier, G(SOOH) → G, has also been located, providing a rationalization for the observed product distributions of G(SO2) and G(SO3) as major products and G as minor product. From mechanistic and kinetics points of view, the present findings provide new chemical insights to understand the high phototoxicity of 6-TG in DNA and point to methods of using 6-TG as a sensitive fluorescence probe for the quantitative detection of (1)O2, which holds particular promise for detecting (1)O2 in DNA-related biological surroundings.

Coordination chemistry of 6-thioguanine derivatives with cobalt: toward formation of electrical conductive one-dimensional coordination polymers.

In this work we have synthetized and characterized by X-ray diffraction five cobalt complexes with 6-thioguanine (6-ThioGH), 6-thioguanosine (6-ThioGuoH), or 2'-deoxy-6-thioguanosine (2'-d-6-ThioGuoH) ligands. In all cases, these ligands coordinate to cobalt via N7 and S6 forming a chelate ring. However, independently of reagents ratio, 6-ThioGH provided monodimensional cobalt(II) coordination polymers, in which the 6-ThioG(-) acts as bridging ligand. However, for 2'-d-6-ThioGuoH and 6-ThioGuoH, the structure directing effect of the sugar residue gives rise to mononuclear cobalt complexes which form extensive H-bond interactions to generate 3D supramolecular networks. Furthermore, with 2'-d-6-ThioGuoH the cobalt ion remains in the divalent state, whereas with 6-ThioGuoH oxidation occurs and Co(III) is found. The electrical and magnetic properties of the coordination polymers isolated have been studied and the results discussed with the aid of DFT calculations, in the context of molecular wires.