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PubMed1985

Use of physicochemical parameters in distance geometry and related three-dimensional quantitative structure-activity relationships: a demonstration using Escherichia coli dihydrofolate reductase inhibitors.

In earlier distance geometry related three-dimensional quantitative structure-activity relationships (Ghose, A. K.; Crippen, G. M. J. Med. Chem. 1984, 27, 901) the interactions of the ligand atom or group with the receptor site were evaluated empirically by using mathematical optimization techniques, without considering their physicochemical properties. In the present work we show how to use various physicochemical parameters in our three-dimensional receptor mapping. We have developed a model for E. coli DHFR using the inhibition data of 25 pyrimidines and 14 triazines. It gave a correlation coefficient of 0.893 and standard deviation of 0.530. It successfully predicted the binding data of five pyrimidines and five triazines.

Binding Sites↗
PubMed2003

Photochemistry of phenyl-substituted 1,2,4-thiadiazoles. 15N-labeling studies.

Irradiation of 5-phenyl-1,2,4-thiadiazole (6) resulted in the formation of benzonitrile (5), 3-phenyl-1,2,4-thiadiazole (4), phenyl- and diphenyl-1,3,5-triazines (7 and 8), and a trace quantity of diphenyl-1,2,4-thiadiazole (9). The formation of 4,5, 7, and 8 can be explained in terms of photoinduced electrocyclic ring closure resulting in the formation of an intermediate 4-phenyl-1,3-diaza-5-thiabicyclo[2.1.0]pentene. 15N-labeling experiments revealed that sulfur can undergo sigmatropic shifts around all four sides of the diazetine ring. Thus, irradiation of 6-4-15N led to the formation of 6-2-15N and an equimolar mixture of 4-2-15N and 4-4-15N. The thiabicyclo[2.1.0]pentene intermediate is also suggested to undergo sulfur elimination resulting in the formation of phenyldiazacyclobutadiene, which can undergo complete fragmentation to benzonitrile or [4+2] cycloaddition leading to unstable tricyclic adducts, the suggested precursors of the 1,3,5-triazine products 7 and 8. The observed 15N distribution in 7 and 8 is consistent with this mechanism. Irradiation of 4 led only to the formation of 5. 15N-labeling experiments show that 4 does not undergo electrocyclic ring closure but reacts exclusively by photofragmentation of the thiadiazole ring.

Journal Article↗
PubMed1999

Theoretical Study on the Mechanism of Formation of Cyanate Resins.

Ab initio calculations of a formation reaction of a triazine ring were performed. From the model substrate, methyl cyanate, a concerted association path with C(3)(h)() symmetry was first examined. In terms of energy changes, this path was found to be unlikely. Second, a stepwise path assisted by a water cluster was tested. But, this path was found to be of a relatively high amount of activation energy in the first additional step. Third, a zinc formate was used as a catalyst, and the reaction was computed to have a reasonable stepwise route for formation of the six-membered triazine ring. Fourth, the reaction promoted by a hydronium ion was shown to generate a ring-closure mechanism similar to that caused by the zinc catalyst. Thus, the crucial role of catalysts coordinated to the sigma lone-pair orbital of the cyanate nitrogen atom was verified.

Journal Article↗
PubMed2005

Approximate additivity of anion-pi interactions: an ab initio study on anion-pi, anion-pi(2) and anion-pi(3) complexes.

We have studied the additivity of the anion-pi interaction using high level ab initio calculations. We have optimized chloride and bromide complexes with one, two and three aromatic units (such as trifluoro-s-triazine and s-triazine). We have analyzed the interaction using the atoms in molecules theory and studied the charge transfer using several methods for deriving atomic charges. The results revealed additivities of both the geometries and the binding energies. We have also proposed a neutral receptor for chloride based on multiple anion-pi interactions. Finally, we have simulated solvent effects within the self-consistent reaction field model.

Journal Article↗
PubMed2003

Highly diastereoselective synthesis of C6-functionalized dihydroimidazotriazines.

[reaction: see text] The first examples of C(6)-substituted 7-hydroxy-6,7-dihydro-5H-imidazo[1,2-b][1,2,4]triazines have been prepared by ring closure of different 5(2H)-1,2,4-triazin-3-ones 1a-c with 40% aqueous glyoxal and various nucleophiles (alcohols, thiols, or amines). The structure and exact stereochemistry of 2a was established by a single X-ray diffraction study and (1)H and (13)C NMR spectra analysis. The process was shown to be totally regio- and diastereoselective. A mechanism involving an imine intermediate was proposed.

Journal Article↗
PubMed2004

Lanthanide(III)/actinide(III) differentiation in coordination of azine molecules to tris(cyclopentadienyl) complexes of cerium and uranium.

Reaction of azine molecules L with the trivalent metallocenes [M(C5H4R)3](M = Ce, U; R = But, SiMe3) in toluene gave the Lewis base adducts [M(C5H4R)3(L)](L = pyridine, 3-picoline, 3,5-lutidine, 3-chloropyridine, pyridazine, pyrimidine, pyrazine, 3,5-dimethylpyrazine and s-triazine), except in the cases of M = U and L = 3-chloropyridine, pyridazine, pyrazine and s-triazine where oxidation of U(III) was found to occur. In the pairs of analogous compounds of Ce(III) and U(III), i.e.[M(C5H4But)3(L)](L = pyridine, picoline) and [M(C5H4SiMe3)3(L)](L = pyridine, lutidine, pyrimidine and dimethylpyrazine), the M-N and average M-C distances are longer for M = Ce than for M = U; however, within a series of azine adducts of the same metallocene, no significant variation is noted in the M-N and average M-C distances. The equilibria between [M(C5H4R)3], L and [M(C5H4R)3(L)] were studied by 1H NMR spectroscopy. The stability constants of the uranium complexes, KUL, are greater than those of the cerium counterparts, KCeL. The values of KML are much greater for R = SiMe3 than for R = But and a linear correlation is found between the logarithms of KML and the hydrogen-bond basicity pKHB scale of the azines. Thermodynamic parameters indicate that the enthalpy-entropy compensation effect holds for these complexation reactions. Competition reactions of [Ce(C5H4R)3] and [U(C5H4R)3] with L show that the selectivity of L in favour of U(III) increases with the [small pi] donor character of the metallocene and is proportional to the pi accepting ability of the azine molecule, measured by its reduction potential.

Actinoid Series Elements↗
PubMed2004

Kinetics and mechanism of the degradation of the pesticide metsulfuron methyl induced by excitation of iron(III) aqua complexes in aqueous solutions: steady state and transient absorption spectroscopy studies.

The degradation of the pesticide metsulfuron methyl induced by excitation of iron(III) aqua complexes was studied at 365 nm. The process involved the formation of hydroxyl radicals and was shown to depend on Fe(OH)(2+) concentration. The second order rate constant of the reaction of metsulfuron methyl and hydroxyl radical was evaluated by laser flash photolysis experiments to k = 3.5 x 10(9) mol(-1) L s(-1). Various products were formed. Among them 2-amino-4-methoxy-6-methyl-1,3,5-triazine, 2-(carbomethoxy) benzenesulfonamide and hydroxylated derivatives represent the major photoproducts. Compounds arising from the cleavage of the triazine moiety were also found. After prolonged irradiation, a total disappearance of metsulfuron methyl and a complete mineralization of the solution were obtained.

Journal Article↗
PubMed2004

Highly substituted pyridines via tethered imine-enamine (TIE) methodology.

A tethered imine-enamine methodology has been developed for the direct conversion of 1,2,4-triazines into highly substituted pyridines via the inverse electron demand Diels-Alder reaction which avoids the need for a discrete aromatisation step. This TIE methodology has also been applied in one pot reaction cascades involving 1,2,4-triazines and utilising MnO(2)-mediated tandem oxidation processes (TOPs).

Journal Article↗
PubMed2006

Linear and star-shaped polynuclear Ru(II) complexes of 2-(2'-pyridyl)benzimidazolyl derivatives: syntheses, photophysical properties and red light-emitting devices.

Two new star-shaped ligands with a 1,3,5-triphenylbenzene core, tmpb (1,3,5-tris[p-2-(2'-pyridyl)benzimidazolylphenyl]benzene), and a 2,4,6-tris(p-biphenyl)-1,3,5-triazine core, tmbt (2,4,6-tris[p-2-(2'-pyridyl)benzimidazolylbiphenyl]-1,3,5-triazine), have been synthesized. Their corresponding trinuclear Ru(II) complexes [Ru3(tmpb)(bpy)6](PF6)6 (3) and [Ru3(tmpt)(bpy)6](PF6)6 (4) have been obtained. Two dinuclear linear Ru(II) complexes with previously reported ligands bmb (1,4-bis[2-(2'-pyridyl)benzimidazolyl]benzene) and bmbp (4,4'-bis[2-(2'-pyridyl)benzimidazolyl]biphenyl) and formulae [Ru2(bmb)(bpy)4](PF6)4 (1) and [Ru2(bmbp)(bpy)4](PF6)4 (2) have also been synthesized. Photophysical and electrochemical properties of the new compounds have been investigated. All four compounds display a characteristic metal-to-ligand-charge transfer (MLCT) absorption band and emit a red light when excited at the maximum of the MLCT band with emission maximum at 624, 629, 623 and 625 nm, respectively in neat films at ambient temperature. The emission quantum efficiency of the four complexes in neat films was determined to be 0.15, 0.17, 0.04 and 0.05, respectively. Light emitting devices based on these four compounds were fabricated by spin-casting the compound as a neat film to an ITO substrate, followed by the deposition of an aluminium metal layer. All devices emit a deep red light and the device behavior resembles that of a light emitting electrochemical cell. The EL maximum of the devices 1, 2, 3, and 4 is at 637, 657, 678, and 655 nm, respectively. All four devices have a fast response time when a sufficiently high voltage is applied. The device based on 2 is the brightest with a maximum luminance of 133 cd m(-2) at 7 V. The performance of devices based on 1, 2, and 4 is in general much more efficient than the device based on [Ru(bpy)3](PF6)2, which was fabricated and evaluated under the same experimental conditions as for the devices based on 1-4.

Journal Article↗
PubMed1997

Determination of atrazine levels in whole saliva and plasma in rats: potential of salivary monitoring for occupational exposure.

Current biological monitoring techniques are often unable to provide accurate estimates of pesticide dose in exposed worker populations. This study was conducted to investigate the feasibility of pesticide biomonitoring using saliva. Atrazine [2-chloro-4-ethylamino-6-(isopropylamino)-s-triazine], a member of the triazine herbicides, was selected to investigate salivary excretion following direct gastric administration in rats. Concentrations of atrazine in whole saliva and arterial plasma samples were determined by enzyme-linked immunosorbent assay (ELISA). Atrazine reached its highest level in both arterial plasma (238 micrograms/L) and whole saliva (157 micrograms/L) 35 min after administration of 105 mg/kg of atrazine, and then decreased with time in a parallel fashion. Although saliva atrazine levels were lower than levels in arterial plasma, there was a very high correlation between whole saliva and arterial plasma atrazine concentrations (r2 = .95). In addition, pharmacokinetic analysis suggested that salivary levels of atrazine can be used to predict concentrations of atrazine in plasma. The mean whole saliva/arterial plasma atrazine concentration ratio (S/P) was 0.66 +/- 0.11 (n = 20). The S/P ratios did not vary significantly over time, and were not affected by salivary flow rate. This study demonstrates that atrazine is transported into saliva, and that a relatively constant concentration ratio between whole saliva and arterial plasma is maintained. Because the salivary concentrations of atrazine are independent of variation in salivary flow rate, salivary monitoring of atrazine in humans may prove useful and practical. Finally, this study suggests that other pesticides with chemical and physical properties similar to those of atrazine can be monitored in saliva.

Animals↗
PubMed2006

Sorption of pesticides on kaolinite and montmorillonite as a function of hydrophilicity.

Pesticides and other organic species are adsorbed by soil via different mechanisms, with bond strengths that depend on the properties of both the soil and the pesticide. Since the clay fraction in soil is a preferential sorbent for organic matter, reference kaolinite and montmorillonite are useful models for studying the mechanism and the strength of sorption. This paper presents the results of batch experiments to investigate the interactions of kaolinite KGa-1 and montmorillonite SWy-1 with the following pesticides and organic species resulting from the natural degradation of pesticides in the environment: atrazine (1-chloro-3-ethylamino-5-isopropylamino-2,4,6-triazine), simazine (1-chloro-3,5-bisethylamino-2,4,6-triazine), diuron [1,1-dimethyl-3-(3,4-dichlorophenyl)urea], aniline, 4-chlorophenol, 2,4-dichlorophenol, and 2,4,6-trichlorophenol. Each of these chemicals has different hydrophilicity. Systems containing 2.0 g of clay were put in contact with 100.0 mL of solutions of the pesticides at known concentration ranging from 1.0 to 5.0 mg/L, and the amount of solute adsorbed was evaluated through RP-HPLC analysis of the pesticide still present in the aqueous suspension. To test for electrostatic interactions between the clay surface and the pesticides, potentiometric titration was used to determine the permanent surface charge of clays. Experiments were performed at different pH values. The results indicate that, for the chemicals studied, neutral molecules are preferentially retained relative to ionized ones, and that montmorillonite is a more effective sorbent than kaolinite.

Adsorption↗
PubMed1999

Occurrence of pesticides and polychlorinated biphenyls in water of the Nile river at the estuaries of Rosetta and Damiatta branches, north of Delta, Egypt.

A study was conducted from summer 1995 to summer 1997 to assess the seasonal occurrence of pesticide residues and other organic contaminants, polychlorinated biphenyls (PCBs), in water at the estuaries of Rosetta and Damiatta branches of the Nile river. The results indicated that organochlorine compounds (OCs) including HCB, lindane, p,p'-DDE, p,p'DDD, p,p'-DDT, aroclor 1254 and aroclor 1260 were present in all the water samples at concentration levels ranging between 0.195-0.240, 0.286-0.352, 0.035-0.067, 0.019-0.033, 0.024-0.031, 0.390-0.70 and 0.166-0.330 microgram/l, respectively. The levels of these compounds were higher in water of Damiatta branch than those found in water of Rosetta branch. Aldrin, dieldrin and endrin were not detected in all water samples. Only 4 compounds from 36 organophosphorus insecticides, fungicides and s-triazine herbicides tested were detected in water samples collected during summer and autumn seasons from Rosetta branch. The concentration levels of these detected compounds, dimethoate, malathion, captan, and ametryne, ranged from 0.011 to 0.340 microgram/l, respectively. Similar compounds during the same seasons as found in water of Rosetta branch were also detected in water of Damiatta branch except ametryne. The levels of the detected compounds (dimethoate, malathion and captan) ranged between 0.030 and 0.330 microgram/l. The levels of detected organophosphorus insecticides, fungicides and s-triazine herbicides were in the order: dimethoate > malathion > captan > ametryne.

Egypt↗
PubMed1994

Different binding mode in AT and GC sequences for unfused-aromatic dications.

We have previously synthesized a 2,5-diphenylfuranamidine dication (4) and presented evidence that this compound binds to AT sequences in DNA by a minor-groove interaction mode but binds to GC sequences by intercalation (1,2). To probe these sequence-dependent binding modes in more detail, and particularly to obtain additional evidence for the binding mode in GC rich sequences, we have synthesized and studied the DNA complexes of 1-3 which have the furan ring of 4 replaced by 2,6-substituted pyridine (1), pyrimidine (2), or triazine (3) ring systems. The three compounds with a six-membered central ring system bind to AT DNA sequences more weakly than the furan compound, but retain the minor-groove binding mode. The pyridine and pyrimidine derivatives bind to GC sequences of DNA more strongly than the furan, but the triazine derivative binds more weakly. The aromatic proton signals of 1-3, as previously observed with 4 shift upfield by approximately 0.5 ppm or greater on complex formation with polyd(G-C)2. This and other spectroscopic as well as viscosity and kinetics results indicate that 1-4 bind to GC sites in DNA by intercalation. A nonclassical intercalation model, with the twisted-unfused, aromatic ring system intercalated into an intercalation site of matching structure can explain all of our and the literature results for the GC binding mode of these unfused, aromatic compounds.

Base Sequence↗
PubMed2001

Analysis of pesticide runoff from mid-Texas estuaries and risk assessment implications for marine phytoplankton.

During 1993, estuarine surface water samples were collected from the mid-Texas coast (Corpus Christi to Port Lavaca, TX). Agricultural watershed areas as well as tidal creeks immediately downstream were chosen as sampling sites along with adjoining bay sampling stations. Collections were made throughout the growing season (February to October 1993) before and after periods of significant (> 1.25 cm) rainfall. All samples were initially screened for the presence of pesticides using enzyme-linked immunosorbent assay (ELISA) test kits (EnviroGard) for triazine herbicides and carbamate insecticides. All samples were extracted and then analyzed using gas chromatography (GC) for quantification of atrazine. Only samples testing positive for carbamate insecticides via ELISA were further extracted for GC analysis to quantify aldicarb and carbofuran. Additionally, laboratory toxicity tests using phytoplankton were examined from published, peer-reviewed literature and compared with the atrazine field levels found in Texas. Results of ELISA screening indicated the presence of triazine herbicides in nearly all samples (>93%). GC analysis further confirmed the presence of atrazine concentrations ranging from <0.01-62.5 microg/L. Screening tests also found detectable levels of carbamate insecticides (aldicarb and carbofuran) that were also confirmed and quantified by GC. Comparison of measured concentrations of atrazine compared with published toxicity tests results indicated that there was a potential environmental risk for marine/estuarine phytoplankton in surface waters of Texas estuaries, particularly when the chronic nature of atrazine exposure is considered.

Atrazine↗
PubMed1984

Chemical synthesis of 5-azacytidine nucleotides and preparation of tRNAs containing 5-azacytidine in its 3'-terminus.

5-azacytidine-5'-triphosphate prepared from 5-azacytidine by chemical phosphorylation is a substrate for AMP (CMP) tRNA nucleotidyl transferase from yeast. tRNAsPhe from yeast containing 5-azacytidine in their 3'-termini were prepared enzymatically. tRNAPhe-Cpn5CpA and tRNAPhe-n5Cpn5CpA can be aminoacylated by phenylalanyl-tRNA synthetase from yeast and they are active in the poly(U)-dependent synthesis of poly(Phe) on E. coli ribosomes. The decomposition of 5-azacytidine via hydrolysis of the triazine ring is significantly accelerated by a phosphate group on the 5'-position of the nucleotide. After the incorporation of 5-azacytidine-5'-phosphate into a polynucleotide chain the rate of hydrolysis of the triazine ring decreases considerably.

Azacitidine↗
PubMed2002

Mapping of a hapten-binding site: molecular modeling and site-directed mutagenesis study of an anti-atrazine antibody.

A three-dimensional model of the variable domain of the atrazine-specific Fab fragment K411B was constructed by molecular modeling using known structures of highly homologous immunoglobulins as templates. Molecular dynamic simulations and cross-reactivity data were used to predict residues responsible for the binding of the hapten 4-chloro-6-(isopropylamino)-1,3,5-triazine-2-(6-aminohexanecarboxylic acid) (iPr/Cl/C6) instead of atrazine. Specific binding pockets could be defined for the chlorine, the isopropylamino group and the C6-spacer of the hapten. The influence of various amino acids on hapten binding was investigated by site-directed mutagenesis, and the effect of these mutations was analyzed by capture ELISA using the hapten iPr/Cl/C6 and 4-amino-6-chloro-1,3,5-triazine-2-(6-aminohexanecarboxylic acid) (H/Cl/C6). GlyH100a seems to be important in determining the conformation of the heavy-chain complementarity determining region H3; replacing it with any other residue prevented the binding of the hapten. Altering residues responsible for the binding of the chlorine atom (TrpH33, GluH50 and TyrL96) decreased the affinity significantly. Hapten-spacer recognition can be attributed to the interaction with PheL32; replacing PheL32 by leucine reduced the affinity towards iPr/Cl/C6. A triple mutant Fab fragment (GlnL89Glu, ValH37Ile and GluL3Val) showed an affinity 5-fold greater towards iPr/Cl/C6 compared to the wild-type K411B, as a result of better recognition of the isopropylamino group of iPr/Cl/C6.

Amino Acid Sequence↗
PubMed1983

Large-scale purification of enzymes.

Many standard procedures for the purification of proteins in the laboratory do not readily lend themselves to scaling up, whereas, on the other hand, some techniques relatively unsatisfactory in the laboratory are much more effective on a large scale. When producing gram or kilogram quantities of enzymes for use over an extended period, the storage properties and general tractability of the purified products become increasingly important. Hence enzymes from thermophilic sources frequently have advantages over those from mesophiles. The possible economic advantages of simultaneous large-scale multi-enzyme isolation over separate individual enzyme purifications are evaluated. Batchwise adsorption and elution from ion-exchange celluloses frequently replace traditional precipitation techniques in the early stages of a large-scale purification. Dialysis is replaced by concentration, dilution and reconcentration with the use of hollow-fibre ultrafiltration equipment. Antiphonally direct scaling-up of column chromatographic procedures is usually possible. Modifications to column geometry to maximize flow rates are often desirable but purification factors and recoveries comparable with those obtained on the laboratory scale can be achieved relatively easily. Classical affinity chromatographic techniques have not proved so amenable to large-scale work, mainly because of the enormous expense and rather short life of the matrices. However, the quasi-affinity chromatography afforded by the triazine dye conjugates has proved of great benefit. The materials are cheap to prepare. The coupling procedures are both simple and rapid and do not involve the use of noxious chemicals such as cyanogen bromide. Moreover the triazine linkage is more stable under a variety of conditions than the isourea formed in cyanogen bromide coupling. Considerable further exploitation of these versatile matrices is expected.

Bacterial Proteins↗
PubMed1983

Effects of Photosystem II Herbicides on the Photosynthetic Membranes of the Cyanobacterium Aphanocapsa 6308.

The effects of the photosystem II herbicides diuron (3-(3,4-dichlorophenyl)-1,1-dimethylurea) and atrazine (2-chloro-4-ethylamino-6-isopropylamino-s-triazine) on the photosynthetic membranes of a cyanobacterium, Aphanocapsa 6308, were compared to the effects on a higher plant, Spinacia oleracea. The inhibition of photosystem II electron transport by these herbicides was investigated by measuring the photoreduction of the dye 2,6-dichlorophenol-indophenol spectrophotometrically using isolated membranes. The concentration of herbicide that caused 50% inhibition of electron transport (I(50) value) in Aphanocapsa membranes for diuron was 6.8 x 10(-9) molar and the I(50) value for atrazine was 8.8 x 10(-8) molar. (14)C-labeled diuron and atrazine were used to investigate herbicide binding with calculated binding constants (K) being 8.2 x 10(-8) molar for atrazine and 1.7 x 10(-7) molar for diuron. Competitive binding studies carried out on Aphanocapsa membranes using radiolabeled [(14)C]atrazine and unlabeled diuron revealed that diuron competed with atrazine for the herbicide-binding site. Experiments involving the photoaffinity label [(14)C]azidoatrazine (2-azido-4-ethylamino-6-isopropylamino-2-triazine) and autoradiography of polyacrylamide gels indicated that the herbicide atrazine binds to a 32-kilodalton protein in Aphanocapsa 6308 cell extracts.

Journal Article↗