Observation of Enhanced Dissociative Photochemistry in the Non-Native Nucleobase 2-Thiouracil.

We present the first study to measure the dissociative photochemistry of 2-thiouracil (2-TU), an important nucleobase analogue with applications in molecular biology and pharmacology. Laser photodissociation spectroscopy is applied to the deprotonated and protonated forms of 2-TU, which are produced in the gas-phase using electrospray ionization mass spectrometry. Our results show that the deprotonated form of 2-thiouracil ([2-TU-H]-) decays predominantly by electron ejection and hence concomitant production of the [2-TU-H]· free-radical species, following photoexcitation across the UVA-UVC region. Thiocyanate (SCN-) and a m/z 93 fragment ion are also observed as photodecay products of [2-TU-H]- but at very low intensities. Photoexcitation of protonated 2-thiouracil ([2-TU·H]+) across the same UVA-UVC spectral region produces the m/z 96 cationic fragment as the major photofragment. This ion corresponds to ejection of an HS· radical from the precursor ion and is determined to be a product of direct excited state decay. Fragment ions associated with decay of the hot ground state (i.e., the ions we would expect to observe if 2-thiouracil was behaving like UV-dissipating uracil) are observed as much more minor products. This behaviour is consistent with enhanced intersystem crossing to triplet excited states compared to internal conversion back to the ground state. These are the first experiments to probe the effect of protonation/deprotonation on thionucleobase photochemistry, and hence explore the effect of pH at a molecular level on their photophysical properties.

Cu(II) adsorption on 2-thiouracil-modified Luffa cylindrica biochar fibres from artificial and real samples, and competition reactions with U(VI).

The adsorption of Cu(II) ions by biochar fibres prior and after modification with 2-thiouracil on real and artificial samples has been studied by batch-type adsorption experiments, FTIR and XPS spectroscopy and competition reactions using U(VI) ions as competitor cations. The experimental data of the artificial samples clearly show that the modified material presents extraordinary higher affinity for Cu(II) ions even in the acidic pH range, the spectroscopic data indicate the formation of inner-sphere complexes and the competition reactions significantly higher selectivity of the 2-thiouracil modified biochar fibres for Cu(II). The 2-thiouracil-modified biochar fibres have been successfully applied to acid mine drainage (AMD) samples regarding the selective separation of Cu(II) ions from "real" samples. Regarding the desorption of copper from the biochar surface, although 100% copper recovery was achieved by eluting the metal ion using 1 M HNO3, the deterioration of the modified biochar fibers due to extensive 2-thiouracil release from the biochar surface limits the applicability of the present adsorbent in routine and large-scale applications.

Cocrystals of 6-methyl-2-thiouracil: presence of the acceptor-donor-acceptor/donor-acceptor-donor synthon.

The results of seven cocrystallization experiments of the antithyroid drug 6-methyl-2-thiouracil (MTU), C(5)H(6)N(2)OS, with 2,4-diaminopyrimidine, 2,4,6-triaminopyrimidine and 6-amino-3H-isocytosine (viz. 2,6-diamino-3H-pyrimidin-4-one) are reported. MTU features an ADA (A = acceptor and D = donor) hydrogen-bonding site, while the three coformers show complementary DAD hydrogen-bonding sites and therefore should be capable of forming an ADA/DAD N-H...O/N-H...N/N-H...S synthon with MTU. The experiments yielded one cocrystal and six cocrystal solvates, namely 6-methyl-2-thiouracil-2,4-diaminopyrimidine-1-methylpyrrolidin-2-one (1/1/2), C(5)H(6)N(2)OS·C(4)H(6)N(4)·2C(5)H(9)NO, (I), 6-methyl-2-thiouracil-2,4-diaminopyrimidine (1/1), C(5)H(6)N(2)OS·C(4)H(6)N(4), (II), 6-methyl-2-thiouracil-2,4-diaminopyrimidine-N,N-dimethylacetamide (2/1/2), 2C(5)H(6)N(2)OS·C(4)H(6)N(4)·2C(4)H(9)NO, (III), 6-methyl-2-thiouracil-2,4-diaminopyrimidine-N,N-dimethylformamide (2/1/2), C(5)H(6)N(2)OS·0.5C(4)H(6)N(4)·C(3)H(7)NO, (IV), 2,4,6-triaminopyrimidinium 6-methyl-2-thiouracilate-6-methyl-2-thiouracil-N,N-dimethylformamide (1/1/2), C(4)H(8)N(5)(+)·C(5)H(5)N(2)OS(-)·C(5)H(6)N(2)OS·2C(3)H(7)NO, (V), 6-methyl-2-thiouracil-6-amino-3H-isocytosine-N,N-dimethylformamide (1/1/1), C(5)H(6)N(2)OS·C(4)H(6)N(4)O·C(3)H(7)NO, (VI), and 6-methyl-2-thiouracil-6-amino-3H-isocytosine-dimethyl sulfoxide (1/1/1), C(5)H(6)N(2)OS·C(4)H(6)N(4)O·C(2)H(6)OS, (VII). Whereas in cocrystal (I) an R(2)(2)(8) interaction similar to the Watson-Crick adenine/uracil base pair is formed and a two-dimensional hydrogen-bonding network is observed, the cocrystals (II)-(VII) contain the triply hydrogen-bonded ADA/DAD N-H...O/N-H...N/N-H...S synthon and show a one-dimensional hydrogen-bonding network. Although 2,4-diaminopyrimidine possesses only one DAD hydrogen-bonding site, it is, due to orientational disorder, triply connected to two MTU molecules in (III) and (IV).

Effects of metal primers on the bonding of an adhesive resin cement to noble metal ceramic alloys after thermal cycling.

STATEMENT OF PROBLEM: Although the effectiveness of primers for resin bonding to noble alloys has been demonstrated, no effective clinical technique for bonding to noble metal ceramic alloys has been established. PURPOSE: The purpose of this study was to evaluate the effects of metal primers on the shear bond strength of an adhesive resin to noble metal ceramic alloys after thermal cycling. MATERIAL AND METHODS: Sixty-three disk-shaped specimens (10 × 2.5 mm) were cast from high-gold-content alloys (Super Metal W-85: W85 or IFK88 GR: IFK88), a high-palladium-content alloy (Super Metal N-40: N40), and an Ag-Pd-Cu-Au alloy (Castwell M.C.12: MC12). Smaller-sized disk-shaped specimens (8 × 2.5 mm) were fabricated with MC12. Bonding surfaces were finished with 600-grit SiC-paper and airborne-particle abraded with 50-µm alumina. Pairs of disks were primed (V-Primer: VP; ML Primer: ML; or Metaltite: MT) and bonded with an adhesive resin (Super-Bond C&B). The bond strengths were determined before and after 20,000 and 50,000 thermal cycles (n=7). Data were analyzed by using a 3-way ANOVA and the Bonferroni test (α=.05). Failure modes were determined by optical microscope and SEM observation. RESULTS: Bond strengths to high-gold-content alloys with VP and MT significantly decreased after the thermal cycling (P<.001). Bond strengths to W85 (35.27 ±4.25 MPa) and IFK88 (33.57 ±3.56 MPa) after 50,000 thermal cycles obtained by ML were the highest (P<.001), and these groups showed combination failures. Bond strengths to N40 significantly decreased after 50,000 thermal cycles (P<.001), regardless of primers. CONCLUSIONS: Shear bond strengths (SBS) to high-gold-content alloys were not degraded up to 50,000 thermal cycles when primed with ML. None of the primers evaluated was effective for high-palladium-content alloy.

Antithyroid drugs and their analogues protect against peroxynitrite-mediated protein tyrosine nitration--a mechanistic study.

In this paper, the effect of some commonly used antithyroid drugs and their analogues on peroxynitrite-mediated nitration of proteins is described. The nitration of tyrosine residues in bovine serum albumin (BSA) and cytochrome c was studied by Western blot analysis. These studies reveal that the antithyroid drugs methimazole (MMI), 6-n-propyl-2-thiouracil (PTU), and 6-methyl-2-thiouracil (MTU), which contain thione moieties, significantly reduce the tyrosine nitration of both BSA and cytochrome c. While MMI exhibits good peroxynitrite (PN) scavenging activity, the thiouracil compounds PTU and MTU are slightly less effective than MMI. The S- and Se- methylated compounds show a weak inhibitory effect in the nitration of tyrosine, indicating that the presence of a thione or selone moiety is important for an efficient inhibition. Similarly, the replacement of N-H moiety in MMI by N-methyl or N-m-methoxybenzyl substituents dramatically reduces the antioxidant activity of the parent compound. Theoretical studies indicate that the substitution of N-H moiety by N-Me significantly increases the energy required for the oxidation of sulfur center by PN. However, such substitution in the selenium analogue of MMI increases the activity of parent compound. This is due to the facile oxidation of the selone moiety to the corresponding selenenic and seleninic acids. Unlike N,N'-disubstituted thiones, the corresponding selones efficiently scavenge PN, as they predominantly exist in their zwitterionic forms in which the selenium atom carries a large negative charge.

Bond strength of resin cements to a zirconia ceramic with different surface treatments.

This study evaluated the influence of surface treatments and metal primers on the bond strength of resin cements to a yttrium-stabilized tetragonal zirconia (Y-TZP) ceramic. Two-hundred and forty plates of Y-TZP ceramic were randomly assigned to 24 groups (n = 10) according to the combination of surface treatment (none, air abrasion with Al2O3 particles, Er:YAG laser irradiation), metal primer (none, Alloy Primer, Metal Primer II or Metaltite) and resin cement (Calibra [Bis-GMA-based] or Panavia F2.0 [MDP-based]). Fragments of dentin with a cylindrical edge (0.8 mm in diameter) were fixed to ceramic surfaces with the resin cements. The micro-shear bond test was carried out at a 1 mm/minute speed until failure, and the ceramic surfaces were examined after debonding. Bond strengths were analyzed through three-way ANOVA/Tukey test with a 5% significance level. Changes in topography after surface treatments were evaluated with scanning electron microscopy. Surface treatments significantly modified the topography of the Y-TZP ceramic. Air abrasion resulted in increased bond strength for both resin cements. However, air abraded and laser irradiated specimens presented higher bond strength with the Bis-GMA-based resin cement than with the MDP-based cement. Both cements presented similar behavior on untreated surfaces. The three metal primers yielded a significant increase in bond strength, regardless of the surface treatment and resin cement. Adhesive failures were the most prevalent. Air abrasion with Al2O3 particles and the application of metal primers increased bond strength to Y-TZP surfaces for both resin cements.

Effects of primers containing sulfur and phosphate monomers on bonding type IV gold alloy.

OBJECTIVE: The purpose of the present study was to evaluate the effect of five primers (two sulfur, one phosphate, and two sulfur-phosphate dual-function primers) on the bond strength between a self-curing luting agent and gold-copper-silver (Au-Cu-Ag) alloy. METHODS: The primers used were two sulfur primers (V-Primer and Metaltite), one phosphate primer (Epricord), and two primers which contained a sulfur monomer and a phosphate monomer (Alloy Primer and Metaltite/Epricord). The surface of Au-Cu-Ag specimens were blasted with alumina, and then bonded with acrylic rods using a tri-n-butylborane-initiated self-curing luting agent. Shear bond strengths were determined after 5000 thermocycles. An additional alumina-blasted Au-Cu-Ag alloy specimen was subjected to X-ray photoelectron spectroscopy (XPS) analysis. RESULTS: The maximum shear bond strengths were obtained with Metaltite/Epricord (29.6+/-2.3 MPa) and Alloy Primer (23.0+/-1.6 MPa), followed by Metaltite (10.3+/-4.2 MPa), V-Primer (8.9+/-0.6 MPa), Epricord (6.4+/-1.5 MPa), and No primer control (2.0+/-0.5 MPa). The XPS analysis detected six chemical elements (Au, Cu, Ag, Al, O, and C) on the Au-Cu-Ag alloy. In addition to pure Au element, the metal oxide-states of Ag2O, AgO, Cu2O, and CuO were suggested. CONCLUSION: The combined use of a sulfur monomer and a phosphate monomer significantly improved the bond strength of resin to Au-Cu-Ag alloy which should be especially significant to clinicians.

Effects of primers containing thiouracil and phosphate monomers on bonding of resin to Ag-Pd-Au alloy.

The purpose of the present study was to evaluate the effects of four experimental primers on bond strength between a self-curing luting agent and silver-palladium-gold alloy. The experimental primers were in mixed solutions of a thiouracil primer (Metaltite) and a phosphate primer (Epricord, PM, PE, or PP), which were designated as Metaltite/Epricord, Metaltite/PM, Metaltite/PE, and Metaltite/PP respectively. Three primers (Metal Primer II, V-Primer, and Alloy Primer) were also prepared as controls. Alumina-blasted metal alloys were bonded with acrylic rods. After 5,000 thermocycles, the maximum shear bond strength was obtained with Metaltite/PE (27.8+/-2.4 MPa) and Metaltite/Epricord (27.6+/-5.9 MPa), followed by Metaltite/PP, Alloy Primer, Metaltite, Metaltite/PM, Metal Primer II, V-Primer, and Epricord. PE, PM, and PP showed the lowest bond strength. Results of this study revealed that the combined use of a thiouracil monomer and a phosphate monomer improved adhesive bonding. In this light, clinicians should pay attention to the types of functional monomers dissolved in a primer when fabricating resin-bonded prostheses.

Highly efficient strand invasion by peptide nucleic acid bearing optically pure lysine residues in its backbone.

Chiral PNA monomers (PNA = peptide nucleic acid), in which nucleobases are attached to N-(aminoethyl)-D-lysine, were introduced to PNAs bearing pseudo-complementary nucleobases (2,6-diaminopurine and 2-thiouracil). When these highly cationic PNAs targeted double-stranded DNA, they invaded there much more efficiently than conventional pseudo-complementary PNAs composed of achiral PNA monomers. Although introduction of N-(aminoethyl)-D-lysine backbone was effective for promotion of strand invasion, L-isomer never promote it. Simple incorporation of lysine groups to the termini of PNA was also ineffective, indicating that introduction of positive charges into PNA backbone is important. Even highly G-C rich sequence, which conventional pseudo-complementary PNAs never invade, was successfully targeted based on this strategy.

Evaluation of 2 thione primers and 3 resin adhesives for silver-palladium-copper-gold alloy bonding.

OBJECTIVE: The purpose of this study was to evaluate 2 thione primers and 3 resin adhesives for enhancement of bonding strength to a silver-palladium-copper-gold alloy. METHOD AND MATERIALS: Two different sized disk specimens (10- and 8-mm diameter by 2.5-mm thick) were prepared from a silver-palladium-copper-gold alloy (Castwell M.C. 12, GC). The specimens were airborne-particle abraded with 50-microm-grain alumina, conditioned either with a thiouracil primer (Metaltite, Tokuyama Dental) or with a triazine dithione primer (V-Primer, Sun Medical), and then bonded with 1 of 3 acrylic resins: a benzoyl peroxide-amine redox-initiated resin adhesive (Multi-Bond, Tokuyama Dental) or a tri-nbutylborane-initiated resin adhesive (Super-Bond C&B and Super-Bond Quick, Sun Medical). For each adhesive, unprimed specimens were prepared as experimental controls. Shear bond strength was determined after thermocycling (100,000 cycles). RESULTS: Use of primers significantly (P < .05) enhanced the bond strength of specimens in all adhesives. Irrespective of the type of primer, the strength of Multi-Bond adhesive was significantly (P < .05) lower than that of Super-Bond C&B and Super-Bond Quick adhesives. The strength of the 2 tri-n-butylborane-initiated adhesives did not differ significantly (P > .05). The mean strength of the Super-Bond C&B adhesive was 40.4 MPa with Metaltite and 37.8 MPa with V-Primer; that of Super-Bond Quick adhesive was 40.9 MPa with Metaltite and 36.5 MPa with V-Primer. CONCLUSION: Use of thione primers effectively enhanced the strength of the bond to the silver-palladium-copper-gold alloy. Furthermore, the combinations of primers and tri-n-butylborane-initiated adhesives were found to be more efficient for bonding.

Double duplex invasion by peptide nucleic acid: a general principle for sequence-specific targeting of double-stranded DNA.

Pseudocomplementary PNAs containing diaminopurine.thiouracil base pairs have been prepared and are shown to bind with high specificity and efficiency to complementary targets in double-stranded DNA by a mechanism termed "double duplex invasion" in which the duplex is unwound and both DNA strands are targeted simultaneously, each by one of the two pseudocomplementary peptide nucleic acids (PNAs). On the basis of our results we predict that (for decameric targets) more than 80% of all sequences can be targeted by straightforward Watson-Crick base pairing by using this approach in its present form. Targeting of pseudocomplementary PNAs to the promoter of the T7 phage RNA polymerase effectively inhibits transcription initiation. These results have important implications in the development of gene therapeutic agents as well as for genetic diagnostic and molecular biology applications.