Phenylsulfonyl ene-allenes as efficient precursors to bicyclic systems via intramolecular [2 + 2]-cycloaddition reactions

Various phenylsulfonyl allene derivatives were prepared with double bonds tethered either to the alpha-position or the gamma-position of the allene. These substrates underwent a highly regio- and stereospecific thermal [2 + 2]-cycloaddition across the nonactivated cumulene double bond, forming distal cycloadducts (i.e., 57) in the case of alpha-tethered allenes and proximal adducts (i.e., 25) in the case of gamma-tethered allenes. The mechanistic rationale for the observed stereospecificity involves initial diradical formation, followed by a rapid ring closure to the more stable cis-fused ring system. The tether may be equipped with heteroatoms, allowing for the formation of fused heterocycles (e.g., 61), and the cycloaddition can be facilitated by the introduction of sterically bulky groups and/or by conformational rigidity to the tether. Other modes of cyclization were observed in the presence of sodium benzenesulfinate or Lewis acids, in which cases polar mechanisms prevail. The chemoselectivity is reversed for [4 + 2]-cycloadditions, which prefer instead to engage the vinyl sulfone moiety, independent of whether the tether is attached to the alpha- or gamma-position of the allene.     

Isolation and structure of [HC[CH(SiMe(3))(SnMe(3))](2)](+): a remarkably stable sec-alkyl cation

The reaction of the tin-substituted propene Me3Sn(R)CHCH=CHR (R = SiMe3) with MCl4 in dichloromethane in the presence of Me3SnCl gives the first examples of isolable sec-alkyl carbocation salts, [HC{CH(R)SnMe3}2]+M2Cl9- (M = Zr, Hf). The compounds are thermally stable and, unlike previously isolated trialkyl carbocations, do not require superacidic media or weakly coordinating anions for stability. The crystal structure and DFT calculations suggest polarization of the Sn substituents and hyperconjugation as the reason for the unexpected stability. The stabilizing effect of tin is significantly stronger than that of Si. The carbocations are effective initiators for the polymerization of isobutene, isoprene, and alpha-methylstyrene.     

Neutron activation for precise nuclear data

Techniques for extracting neutron flux parameters pertaining to the irradiation positions and nuclear data pertaining to the isotopes concemed from measured reaction rates have been developed. An adjustment procedure based on the generalized least squares method with incorporates the, evaluated literature nuclear data, estimates of flux parameters and the experimentally measured reaction rates using high resolution -ray spectrometry is applied. More precise values of the nuclear data, i.e., thermal neutron cross sections, resonance integrals and -ray emission probabilities are thus generated. The irradiation of a multi-element standard (MES) containing 24 elements is carried out in six diffieret positions in two diverse types of reactor in the UK and Russia. It is shown that the improvement in nuclear data is revelaed in moire than 90% of the cases.     

Comparison of neutron activation analysis methods

This work shows that nuclear data and neutron flux parameters can be applied to calculate directly the elemental concentrations. Techniques for extracting neutron flux parameters pertaining to the irradiation positions and nuclear data pertaining to the isotopes concerned from measured reaction rates have been previously developed. This method is compared to the comparator and relative methods of activation analysis. The principles, advantages and disadvantages of each method and sources of errors are discussed. It also briefly discusses other factors such as accuracy and precision, sensitivity, detection limits and limit of quantitative determination. The three methods are applied to the analysis of five environmental reference materials. The concentrations of more than 20 elements are determined. The results show a good agreement with the certified and/or literature values.     

The selenium-mercury interaction: synthesis,spectroscopic and x-ray structural studies of methylmercury-selenourea complexes

The synthesis of selenium bound selenourea complexes of methylmercury, [CH₃HgSeC(NH₂)₂]X (X=Cl, Br, NO₃, ClO₄) are described. A single crystal X-Ray analysis of the nitrate salt has provided for the first time bond length data pertinent to the biologically important mercury-selenium interaction. Crystals of [CH₃HgSeC(NH₂)₂] NO₃ belong to the space group PnZic with a = 7.524(1), b = 11.204(2), c = 9.738(2)Å, and z = 4. The structure was solved and refined using 561 observed reflections measured on a Syntex P2₁ diffractometer to a final R value of 0.037. The mercury atom is linearly coordinated to the methyl group and the selenium atom of the selenourea with Hg-Se of 2.477(3)Å. Strong Hg-Se bonding is indicated by the X-Ray data and by the ¹H - ¹⁹⁹Hg nmr coupling constants. A comparison of spectroscopic data for analogous thio and selenourea complexes is presented.     

In vitro selection of aptamers with affinity for neuropeptide Y using capillary electrophoresis

Capillary electrophoresis-systematic evolution of ligands by exponential enrichment (CE-SELEX) was used to select aptamers for neuropeptide Y (NPY). This is the first example of a CE-SELEX selection for aptamers that bind a target molecule smaller than itself. One of the limitations of CE-SELEX is that the aptamer must exhibit a significant mobility shift when it binds the target to facilitate fraction collection. Before this study, it was not clear if smaller targets would be capable of inducing a large enough shift in mobility for CE-SELEX to be successful. NPY is a 36-amino acid peptide (MW = 4272 g/mol), much smaller than the 80-base ssDNA used in the selection ( approximately 25 kDa). NPY binding aptamers with 300-1000 nM dissociation constants were obtained after only four rounds of selection. The specificity of the aptamers was tested using human pancreatic polypeptide (hPP). hPP is a 36-amino acid peptide with approximately 50% homology with NPY. Aptamers with up to 42-fold selectivity for NPY over hPP were observed.     

Kinetic and regiospecific interrogation of covalent intermediates in the nonribosomal peptide synthesis of yersiniabactin

For interrogation of enzyme-bound intermediates in nonribosomal peptide synthetases (NRPSs), mass spectrometry is used to read out the kinetics and substrate specificity of this medicinally important class of enzymes. The protein HMWP2 (230 kDa) catalyzes 11 chemical reactions, four of which could be resolved by fast quench approaches combined with mass spectrometry. The rate of complex intermediate accumulation at the PCP1 active site was observed to occur with a rate of 19 s(-1), with the rate of cysteine acylation faster than that of intermediate translocation. Use of alternative substrates for salicylic acid (at the ArCP carrier domain) and l-cysteine (at the PCP1 carrier domain) revealed a high penalty for omission of the salicyl alcohol. For some substrates, large discrepancies were found between prior adenylation assays and the current MS-based readouts. Indirect evidence for condensation via a thiolate attack (vs an amino group) was also accumulated. This is the first report to correlate the percent occupancy of multiple active sites in parallel with kinetic and structural resolution of intermediates and provides new evidence of interdomain and intermodule communication within thiotemplate assembly lines.     

Exploring the Effect of Ligand Structural Isomerism in Langmuir–Blodgett Films of Chiral Luminescent EuIII Self‐Assemblies

Here we have investigated the influence of the antenna group position on both the formation of chiral amphiphilic Eu^III‐based self‐assemblies in CH₃CN solution and, on the ability to form monolayers on the surface of quartz substrates using the Langmuir–Blodgett technique, by changing from the 1‐naphthyl ( 2(R) , 2(S) ) to the 2‐naphthyl ( 1(R) , 1(S) ) position. The evaluation of binding constants of the self‐ assemblies in CH₃CN solution was achieved using conventional techniques such as UV/Visible and luminescence spectroscopies along with more specific circular dichroism (CD) spectroscopy. The binding constants obtained for EuL , EuL₂ and EuL₃ species in the case of 2‐naphthyl derivatives were comparable to those obtained for 1‐naphthyl derivatives. The analysis of the changes in the CD spectra of 1(R) and 1(S) upon addition of Eu^III not only allowed us to evaluate the values of the binding constants but the resulting recalculated spectra may also be used as fingerprints for assignment of the chiral self‐assembly species formed in solution. The obtained monolayers were predominantly formed from EuL₃ (≈85 %) with the minor species present in ≈15 % EuL₂ .     

A Multifunctional Surface That Simultaneously Balances Hydrophilic Enzyme Catalysis and Hydrophobic Water Repellency

The compatibility of multiple functions at a single interface is difficult to achieve, but is even more challenging when the functions directly counteract one another. This study provides insight into the creation of a simultaneously multifunctional surface formed by balancing two orthogonal functions; water repellency and enzyme catalysis. A partially fluorinated thiol is used to impart bulk hydrophobicity on the surface, and an N‐hydroxysuccinimide ester‐terminated thiol provides a specific anchoring sites for the covalent enzyme attachment. Different ratios of the two thiols are mixed together to form amphiphilic self‐assembled monolayers, which are characterized with polarization‐modulation infrared reflection–absorption spectroscopy and contact angle goniometry. The enzyme activity is measured by a fluorescence assay. With the results collected here, specific surface compositions are identified at which the orthogonal functions of water repellency and enzyme catalysis are balanced and exist simultaneously. An understanding of how to effectively balance orthogonal functions at surfaces can be extended to a number of higher‐scale applications.