Ab initio study of the (5R)- and (5S)-TT pyrimidine h5(6-4) pyrimidone photoproducts. Implications on the design of new biologically relevant analogues

A computational study of a series of N(1)- and/or C(6)-alkyl-5,6-dihydrothymine diastereomers at theory levels up to MP4(SDTQ)/6-31G//HF/6-31G and MP2/6-311G//HF/6-31G has demonstrated the respective importance of the substituents at positions 1, 5, and 6 on the energetically favored conformation of each isomer. Results obtained both in the gas and condensed phase indicate that unsubstitution of the N(1)-position favors a half-chair conformation with the C(5) -and C(6)-substituents in the equatorial position. On the other hand, in the case of the (6S)-1,6-dimethyl-5,6-dihydrothymine, the C(6)-substituent adopts the axial position to minimize its van der Waals interactions with the N(1)-substituent. Furthermore, if the configuration at the C(5)-dihydrothymine position has no resultant influence on the total molecular free energy, when a pyrimidone substituent is introduced at the dihydrothymine C(6)-position, additional repulsive forces between the C(5)- and C(6)-substituents make the diaxially substituted half-chair conformation the most energetically favorable one. These results indicate that the observed C(6)-axially substituted conformation of the thymine-thymine pyrimidine h(5)(6-4) pyrimidone photoproducts is not necessarily induced by the macrocyclic structure. They also nicely explain the formation mechanism of these photoproduct derivatives, and allow the prediction of the conformation of new analogues.     

First synthesis of the beta-D-rhamnosylated trisaccharide repeating unit of the O-antigen from Xanthomonas campestris pv. campestris 8004

The trisaccharide repeating unit of the O-antigen of the lipopolysaccharide from Xanthomonas campestris pv. campestris 8004, a pathogen of cruciferous crops, presents some structural features that renders it a challenging synthetic target: the presence of a beta-D-rhamnosidic linkage, the steric crowd on a 1,2-cis-diglycosylated D-rhamnose, and finally the noncommercial availability of its monosaccharide constituents. The synthesis of this trisaccharide as methyl glycoside has been accomplished by exploiting a strategy whose key steps were the sequential beta-D-rhamnosylation with a 2-O-benzylsulfonyl-N-phenyltrifluoroacetimidate donor, debenzylsulfonylation, and coupling with a D-Fucp3NAc thioglycoside donor.     

Oxidative cyclization of some aldehyde semicarbazones induced by metallic salts

The oxidative cyclization of some aldehyde semicarbazones 10 with four different oxidizing agents has been effected. The structure of the semicarbazones and the nature of cyclizing agent affected the rate and yield of cyclization but they did not show any influence on the regiochemistry of reaction. In fact, 1,2,4-triazoline 20 was the only heterocyclic ring obtained by the cyclization reaction.     

Gamma-radiolysis DNA products: synthesis of 6-(alpha-thyminyl)-5,6-dihydro-4-thiothymine derivatives

Far-UV photolysis of 4-thiothymidylyl(3'-5')thymidine led to the formation of three stable derivatives: one resulting from a combination between a 3'-end methylene radical and a 5'-end C(4) radical [4-(alpha-thyminyl) derivative] and two formed after a combination between a 3'-end methylene radical and a 5'-end C(6) radical [6-(alpha-thyminyl) derivative]. In the latter series, two stereochemical pathways took place during the reaction between the methylene and C(6) radicals. The major pathway occurred when the 5'-base glycosidic bond had an anti conformation leading to an S configuration of the C(6) Tp-end. The minor pathway, which had never been reported before in this series, involved a 5'-base in a syn conformation leading consequently to the R configuration at the C(6) Tp-end. The 5,6-dihydrothymine moiety of these two adducts presented a 5,6-trans diaxial substitution that resulted from the epimerization, at the 5,6-dihydropyrimidine 5-position, of a less stable cis-disubstituted intermediate.     

An in-depth correlation of perturbation of the organic-inorganic interface topology,electronic structure,and transport properties within beta''-(BEDT-TTF)(4) x (guest)(n) x [Re(6)Q(6)Cl(8)], (Q=S,Se)

An in-depth analysis of a set of 21 layered structures of metallic pseudopolymorphs of general formulation, beta'-(BEDT-TTF)(4) x (guest)(n) x [Re(6)Q(6)Cl(8)], (BEDT-TTF=bis-ethylenedithiotetrathiafulvalene; Q = S, Se; guest = H(2)O, 1,4-dioxane, THF, CCl(4), C(2)H(5)OH, CHCl(3), CH(2)ClI, CH(2)ClBr, CH(2)Cl(2), CH(2)OH-CH(2)OH, C(5)H(5)N, CH(3)COCH(3), 2-hydroxy-tetrahydrofuran, CH(3)CN, CS(2), C(6)H(6)), with diverse low-temperature behaviors, which differ solely by the nature of the cosolvent molecule selectively included during the electrocrystallization process, reveals a precise set of weak HO-H...Cl-mu-Re, (C-H)(BEDT-TTF)...Cl-mu-Re, C-H...O(guest), (C-H)(guest)...Cl-mu-Re hydrogen bonds at the organic-inorganic interface, none of which dominates any of the others and whose balance is adjusted upon substitution of one guest molecule by another. The electronic structure of the host adjusts to the weak perturbation imposed by exchanging the guest molecules and by balancing the former interfacial interactions; this correlates to a net activation of up to 0.1 eV of the energy of the HOMO level of one of the two donors, while keeping the pattern of HOMO-HOMO intermolecular interactions in the donor layer essentially unaltered. It is suggested that this controls the stability of the metallic state at low temperature or the occurrence of a metal-to-insulator phase transition for particular guests along the series. It is concluded that by allowing for numerous tiny modifications at the organic-inorganic interface within a single, robust host structure, one sees a concerted, inherently weak structural response of the system that is proportional to the magnitude of the underlying, equally weak activation of the HOMO energy of a fraction of the pi-donor molecules within the slabs; this has a sizeable influence on the macroscopic transport properties of the system.     

Analysis of T-2 and HT-2 toxins in cereal grains by immunoaffinity clean-up and liquid chromatography with fluorescence detection

A sensitive, precise and accurate method has been developed for the simultaneous determination of T-2 and HT-2 toxins in cereal grains at ppb levels using high-performance liquid chromatography (HPLC) with fluorescence detection and 1-antroylnitrile (1-AN) as labeling reagent after immunoaffinity clean-up. Cereal samples were extracted with methanol/water (90:10, v/v), and the extracts were cleaned-up through commercially available immunoaffinity columns containing monoclonal anti-T-2 antibodies (T-2 test HPLC, Vicam). T-2 and HT-2 toxins were quantified by reversed-phase HPLC with fluorometric detection (excitation wavelength 381 nm, emission wavelength 470 nm) after derivatization with 1-AN. The monoclonal antibody showed 100% cross-reactivity with both T-2 and HT-2 toxin, and the immunoaffinity column clean-up was effective up to 1.4 microg of both toxins. The method was successfully applied to the analysis of T-2 and HT-2 toxins in wheat, maize and barley. Recoveries from spiked samples with toxin levels from 25 to 500 microg/kg ranged from 70% to 100%, with relative standard deviation generally lower than 8%. The limit of detection of the method was 5 microg/kg for T-2 toxin and 3 microg/kg for HT-2 toxin, based on a signal-to-noise ratio 3:1. HT-2 toxin was detected in ten naturally contaminated wheat samples out of 14 samples analyzed, with toxin levels ranging from 10 to 71 microg/kg; three of them contained also T-2 toxin up to 12 microg/kg.     

Classification of smooth solutions to 2×2 hyperbolic systems with boundary damping

We give a complete classification for breakdown in finite time or global existence of continuously differentiable solutions to hyperbolic systems of two first-order partial differential equations posed on a real bounded interval, with boundary damping and initial data of small magnitude. Structural bounds for the derivatives of the initial data which govern the behavior of the solution are brought to light. We isolate two classes of boundary damping: the strong damping forbids the solution to blow up after a time that we exhibit while the weak damping permits a late breakdown. The results are applied to nonlinear elasticity. © 1997 by B. G. Teubner Stuttgart–John Wiley & Sons Ltd.     

Determination of ochratoxin A in wine and beer by immunoaffinity column cleanup and liquid chromatographic analysis with fluorometric detection: collaborative study.

The accuracy, repeatability, and reproducibility characteristics of a liquid chromatographic method for the determination of ochratoxin A (OTA) in white wine, red wine, and beer were established in a collaborative study involving 18 laboratories in 10 countries. Blind duplicates of blank, spiked, and naturally contaminated materials at levels ranging from < or =0.01 to 3.00 ng/mL were analyzed. Wine and beer samples were diluted with a solution containing polyethylene glycol and sodium hydrogen carbonate, and the diluted samples were filtered and cleaned up on an immunoaffinity column. OTA was eluted with methanol and quantified by reversed-phase liquid chromatography with fluorometric detection. Average recoveries from white wine, red wine, and beer ranged from 88.2 to 105.4% (at spiking levels ranging from 0.1 to 2.0 ng/mL), from 84.3 to 93.1% (at spiking levels ranging from 0.2 to 3.0 ng/mL), and from 87.0 to 95.0% (at spiking levels ranging from 0.2 to 1.5 ng/mL), respectively. Relative standard deviations for within-laboratory repeatability (RSDr) ranged from 6.6 to 10.8% for white wine, from 6.5 to 10.8% for red wine, and from 4.7 to 16.5% for beer. Relative standard deviations for between-laboratories reproducibility (RSDR) ranged from 13.1 to 15.9% for white wine, from 11.9 to 13.6% for red wine, and from 15.2 to 26.1% for beer. HORRAT values were < or =0.4 for the 3 matrixes.     

Microbiological reductions of chromen-4-one derivatives

From the microbiological reductions of 2-acetyl or 2-benzoylchromen-4-one both enantiomers of the corresponding alcohols were obtained with high enantiomeric excess. The absolute configurations were determined directly by an X-ray structural determination. The results obtained showed that for most of the microorganisms tested, an inversion of the configuration of the alcohol occurred with the change of the substituent (methyl to phenyl group) in position 2, but also with the presence of a bromine atom in position 6 of the aromatic ring, positioned quite far from the prochiral centre.     

Dynamic assessment of reinforced concrete beams repaired with externally bonded FRP sheets

This research deals with RC beams strengthened with FRP. An experimental research is presented which is aimed at evaluating the capability of an experimental modal analysis to assess the stiffness decrease due to damage, as well as the stiffness recovery due to strengthening. Ten beams were tested. All of them were subjected to loading cycles with increasing load levels in order to induce cracking of different severity in them. The beams were then retrofitted by externally bonded FRP sheets. Three types of composites were used. The number of layers was varied, too. Modal tests were carried out after each loading-unloading cycle. The modal frequencies and damping ratios were determined for the first four vibration modes. The results obtained indicate that an experimental modal analysis can give useful information on the severity of damage and the effectiveness of strengthening. Russian translation published in Mekhanika Kompozitnykh Materialov, Vol. 42, No. 1, pp. 3–20, January–February, 2006.