Ab initio study of the structures and stabilities of the dimer of ethyl cation, (C(2)H(+)(5))(2) and related C(4)H(10)(2+) isomers.

Ab initio calculations at the MP4(SDTQ)/6-311G//MP2/6-31G level were performed to study the structures and stabilities of the dimer of ethyl cation, (C(2)H(+)(5))(2), and related C(4)H(10)(2+) isomers. Two doubly hydrogen bridged diborane type trans 1 and cis 2 isomers were located as minima. The trans isomer was found to be more favorable than cis isomer by only 0.6 kcal/mol. Several other minima for C(4)H(10)(2+) were also located. However, the global energy minimum corresponds to C-H (C(4) position) protonated 2-butyl cation 10. Structure 10 was computed to be substantially more stable than 1 by 31.7 kcal/mol. The structure 10 was found to be lower in energy than 2-butyl cation 13 by 34.4 kcal/mol.

Acid-catalyzed isomerization of pivalaldehyde to methyl isopropyl ketone via a reactive protosolvated carboxonium ion intermediate.

Quantitative rearrangement of pivalaldehyde to methyl isopropyl ketone is observed in acids such as trifluoromethanesulfonic acid, anhydrous HF, and trifluoroethyl alcohol-BF3 but not in trifluoroacetic acid. Studies in a mixture of trifluoroacetic acid and trifluoromethanesulfonic acid show that acids with H(o) < or = -11 are able to carry out complete isomerization. These results and density functional theory calculations at the B3LYP/6-31G level suggest that protonated pivalaldehyde undergoes further protosolvation at higher acidities to a reactive superelectrophilic species resulting in rearrangement. A mechanism for the pivalaldehyde rearrangement to methyl isopropyl ketone in strong protic acids involving a reactive protosolvated superelectrophilic intermediate is suggested. Aspects of the related mechanism of the reaction with isobutane with CO in HF/BF3 medium leading to methyl isopropyl ketone are also discussed.

Asymmetric synthesis of trifluoromethylated allylic amines using alpha,beta-unsaturated N-tert-butanesulfinimines.

[reaction: see text]. The trifluoromethide ion generated in situ from TMSCF(3) and TBAT (tetrabutylammonium triphenyldifluorosilicate), as well as TMAF (tetramethylammonium fluoride), adds to the alpha,beta-unsaturated N-tert-butanesulfinimines exclusively in a 1,2 fashion with high diastereoselectivities, affording the first examples of chiral trifluoromethylated allylic amines.

N6(2+) and N4(2+) dications and their n(12) and n(10) azido derivatives: DFT/GIAO-MP2 theoretical studies.

The structures and energies of N(6)(2+) and N(4)(2+) were calculated by using the density functional theory method at the B3LYP/cc-aug-pVTZ level. The C(2)(h)() symmetric form 1 and D(infinity)(h) form 5 were found to be the stable minima for N(6)(2+) and N(4)(2+), respectively. Dissociation of 1 into 5 and N(2) was computed to be endothermic by 25.1 kcal/mol. (15)N NMR chemical shifts and vibrational frequencies of 1 and 5 were also calculated. Interactions of 1 and 5 with azide ions were also probed representing N(12) and N(10).

Gitonic and distonic alkanonium dications (diprotonated alkane dications C(n)H(2n+4(2+)), n = 1-4)(1).

The structures and stabilities of gitonic and distonic alkanonium dications, i.e., diprotonated alkane dications C(n)H(2n+4)(2+) (n = 1-4), were investigated at the MP4(SDTQ)/6-311G**//MP2/6-31G** level. The global minimum energy structures (2, 4, 7, and 10) of the C(n)H(2n+4)(2+) dications are double C--H protonated alkanes to give structures with two two electron three-center (2e-3c) bonds. Two different dissociation pathways for the dications, viz deprotonation and demethylation, were also computed. Demethylation was found to be the favorable mode of dissociation.

Nafion-H catalysed sulfonylation of aromatics with arene/alkenesulfonic acids for the preparation of sulfones1a.

Synthesis of both symmetric and unsymmetric diaryl/aryl alkyl sulfones is easily achieved by Friedel-Crafts type sulfonylation of aromatics with suitable arene- or alkane-sulfonic acids in the presence of Nafion-H, a perfluorinated resinsulfonic acid catalyst.

Poly(p-hydroxystyrene) grafted polystyrene nanospheres: excellent hosts for silver and ruthenium nanoparticles.

A novel approach is described for the preparation of surface functionalized micro- and nanobeads using one pot synthesis by a core-shell method. Monodisperse poly(p-hydroxystyrene) is successfully prepared by grafting the p-acetoxystyrene monomer during the last 30 min of the fabrication of polystyrene bead core by emulsifier-free emulsion polymerization followed by hydrolysis of the acetoxy group by a base. The size of the resulting beads is dictated mostly by the size of the core. Hydroxyl derivatized polystyrene microspheres have been found useful as a high surface area and stable support for anchoring catalytically active silver and ruthenium nanoparticles. The bead formation, surface functionalization, and coating with metal nanoparticles have been studied using scanning electron microscopy, transmission electron microscopy, energy dispersive x-ray spectrometry, Fourier transform infrared spectrometry, and Auger analysis.

A facile stereocontrolled synthesis of anti-alpha-(trifluoromethyl)-beta-amino alcohols.

A short stereocontrolled preparation of anti-alpha-(trifluoromethyl)-beta-amino alcohols is described, involving an initial CF(3) transfer to cinnamaldehyde and a one-step, three-component condensation of 3,3,3-trifluorolactic aldehyde, an alkenyl (aryl) boronic acid, and an amine. Applying this methodology to chiral 3,3,3-trifluorolactic aldehyde allowed us to generate an amino alcohol enantioselectively in 92% ee.

Protonated and methylated dimethyl sulfoxide cations and dications. DFT/GIAO-MP2 NMR studies and comparison with experimental data.

Energies, electronic structures, and thermodynamics of protonated and methylated dimethyl sulfoxide (DMSO) cations and dications were calculated using the density functional theory (DFT) method. The O-protonated structure 2 was found to be 37.0 kcal/mol more stable than the S-protonated 3. For diprotonated DMSO dication, the O, O-diprotonated form 6 was found to be the global minimum, more stable by 20.8 kcal/mol than O,S-diprotonated 7. Interestingly, for dimethylated DMSO dication, O,O-dimethylated 11 and O,S-dimethylated 12 are isoenergetic. (13)C, (17)O, and (33)S NMR chemical shifts of the cations and dications were calculated using the GIAO-MP2 method and compared with the available experimental data.

Mutagenicity studies of methyl-tert-butylether using the Ames tester strain TA102.

Methyl-tert-butylether (MTBE) is an oxygenate widely used in the United States as a motor vehicle fuel additive to reduce emissions and as an octane booster [National Research Council, Toxicological and Performance Aspects of Oxygenated Motor Vehicle Fules, National Academy Press, Washington, DC, 1996]. But it is the potential for MTBE to enter drinking water supplies that has become an area of public concern. MTBE has been shown to induce liver and kidney tumors in rodents but the biochemical process leading to carcinogenesis is unknown. MTBE was previously shown to be non-mutagenic in the standard Ames plate incorporation test with tester strains that detect frame shift (TA98) and point mutations (TA100) and in a suspension assay using TA104, a strain that detects oxidative damage, suggesting a non-genotoxic mechanism accounts for its carcinogenic potential. These strains are deficient in excision repair due to deletion of the uvrB gene. We hypothesized that the carcinogenic activity of MTBE may be dependent upon a functional excision repair system that attempts to remove alkyl adducts and/or oxidative base damage caused by direct interaction of MTBE with DNA or by its metabolites, formaldehyde and tert-butyl alcohol (TBA), established carcinogens that are mutagenic in some Ames strains. To test our hypothesis, the genotoxicity of MTBE-induced DNA alterations was assayed using the standard Ames test with TA102, a strain similar to TA104 in the damage it detects but uvrB + and, therefore, excision repair proficient. The assay was performed (1) with and without Aroclor-induced rat S-9, (2) with and without the addition of formaldehyde dehydrogenase (FDH), and (3) with human S-9 homogenate. MTBE was weakly mutagenic when tested directly and moderately mutagenic with S-9 activation producing between 80 and 200 TA102 revertants/mg of compound. Mutagenicity was inhibited 25%-30% by FDH. TA102 revertants were also induced by TBA and by MTBE when human S-9 was substituted for rat S-9. We conclude that MTBE and its metabolites induce a mutagenic pathway involving oxidation of DNA bases and an intact repair system. These data are significant in view of the controversy surrounding public safety and the environmental release of MTBE and similar fuel additives.

Electrochemical preparation of tris(tert-butyldimethylsilyl)cyclopropene and its hydride abstraction to tris(tert-butyldimethylsilyl)cyclopropenium tetrafluoroborate.

Electrochemical reductive tert-butyldimethylsilylation of tetrachlorocyclopropene to 1,2,3-tris(tert-butyldimethylsilyl)cyclopropene, a potential strained precursor for Diels-Alder and related cycloaddition reactions, is described. By hydride abstraction with nitrosonium tetrafluoroborate, 1,2,3-tris(tert-butyldimethylsilyl)cyclopropene is ionized quantitatively to Hückeloid 2pi aromatic tris(tert-butyldimethylsilyl)cyclopropenium tetrafluoroborate.

Helionitronium trication (NO2He3+) and helionitrosonium trication (HeNO3+).

The structures and stabilities of helionitronium trication NO2He3+ and helionitrosonium trication HeNO3+ were calculated at the ab initio MP2/6-31G** level. The Cs symmetry structure was found to be a minimum for the NO2He3+ trication, which is isoelectronic and isostructural with the previously studied NO2H2+. Dissociation of the Cs symmetry structure into NO+ and OHe2+ is thermodynamically preferred by 183.1 kcal/mol (1 cal = 4.18 J), although a kinetic barrier of 12.4 kcal/mol has to be overcome. The Cinfinityv symmetry structure was also found to be a minimum for the HeNO3+ trication.

Structural changes measured by X-ray scattering from human flap endonuclease-1 complexed with Mg2+ and flap DNA substrate.

Human flap endonuclease-1 (FEN-1) is a member of the structure-specific endonuclease family and is essential in DNA replication and repair. FEN-1 has specific endonuclease activity for repairing nicked double-stranded DNA substrates that have the 5'-end of the nick expanded into a single-stranded tail, and it is involved in processing Okazaki fragments during DNA replication. Magnesium is a cofactor required for nuclease activity. We used small-angle x-ray scattering to obtain global structural information pertinent to nuclease activity from FEN-1, the D181A mutant, the wild-type FEN-1. 34-mer DNA flap complex, and the D181A.34-mer DNA flap complex. The D181A mutant, which has Asp-181 replaced by Ala, selectively binds to the flap structure, but has lost its cleaving activity. Asp-181 is thought to be involved in Mg2+ binding at the active site (Shen, B., Nolan, J. P., Sklar, L. A., and Park, M. S. (1996) J. Biol. Chem. 271, 9173-9176). Our data indicate that FEN-1 and the D181A mutant each have a radius of gyration of approximately 26 A, and the effect of Mg2+ on the scattering from the proteins alone is insignificant. The 34-mer DNA fragment was constructed such that it readily forms a 5'-flap structure. The formation of the flap conformation of the DNA substrate was evident by both the extrapolated Io scattering and radius of gyration and was supported by NMR spectrum and nuclease assays. In the absence of magnesium, the FEN-1.34-mer DNA flap complex has an Rg value of approximately 34 A, whereas the D181A.34-mer DNA flap complex self-associates, suggesting that a significant protein conformational change occurs by addition of the flap DNA substrate and that Asp-181 is crucial for proper binding of the protein to the DNA substrate. A time course change in the scattering profiles arising from magnesium activation of the FEN-1.34-mer DNA flap complex is consistent with the protein completely releasing the DNA substrate after cleavage.

Calculated 11B-13C NMR chemical shift relationship in hypercoordinate methonium and boronium ions.

The boronium-carbonium continuum was extended to include hypercoordinated protonated methanes and their boron analogs. The 11B NMR chemical shifts of the hypercoordinated hydriodo boron compounds and the 13C NMR chemical shifts of the corresponding isoelectronic and isostructural carbocations were calculated by using the GIAO-MP2 method. The data show good linear correlation between 11B and 13C NMR chemical shifts, which indicates that the same factors that determine the chemical shifts of the boron nuclei also govern the chemical shifts of carbon nuclei of these hypercoordinated hydriodo compounds.

Attempted hydrogen-deuterium exchange of the protio-trimethyloxonium dication (CH3)3OH2+, study of methylating ability of (CH3)3O+ in superacids and theoretical investigations.

Attempted hydrogen-deuterium exchange of trimethyloxonium ion, (CH3)3O+ with excess of 1:1 (2)HF/SbF5 superacid at -30 degreesC over a period of 30 days showed no exchange. Theoretical calculations at the MP2/6-31G** level are in accord with the lack of hydrogen-deuterium exchange in the methyl group of the (CH3)3O+ cation as protonation (protosolvation) prefers the oxygen lone pair of electrons, instead of a C---H bond. Methylation of aromatics with the (CH3)3O+CF3SO3- in CF3SO3H and 2CF3SO3H:B(O3SCF3)3 was also studied. Whereas in triflic acid no alkylation was observed, in triflatoboric acid, a powerful superacid, alkylation takes place, indicating protolytic activation of the trimethyloxonium ion.

Tertiary structural changes and iron release from human serum transferrin.

Iron release from human serum transferrin was investigated by comparison of the extent of bound iron, measured by charge transfer absorption band intensity (465 nm), with changes observed by small-angle solution X-ray scattering (SAXS) for a series of equilibrated samples between pH 5.69 and 7.77. The phosphate buffers used in this study promote iron release at relatively high pH values, with an empirical pK of 6.9 for the convolved release from the two sites. The spectral data reveal that the N-lobe release is nearly complete by pH 7.0, while the C-lobe remains primarily metal-laden. Conversely, the radius of gyration, Rg, determined from the SAXS data remains constant between pH 7.77 and 7.05, and the evolution of Rg between its value observed for the diferric protein at pH 7.77 (31.2+/-0.2 A) and that of the apo protein at pH 5.69 (33.9+/-0.4 A) exhibits an empirical pK of 6.6. While Rg is effectively constant in the pH range associated with iron release from the N-lobe, the radius of gyration of cross-section, Rc, increases from 16.9+/-0.2 A to 17.6+/-0.2 A. Model simulations suggest that two different rotations of the NII domain relative to the NI domain about a hinge deep in the iron-binding cleft of the N-lobe, one parallel with and one perpendicular to the plane of the iron-binding site, can be significantly advanced relative to their holo protein positions while yielding constant Rg and increased Rc values consistent with the scattering data. Rotation of the CII domain parallel with the C-lobe iron-binding site plane can partially account for the increased Rg values measured at low pH; however, no reasonable combined repositioning of the NII and CII domains yields the experimentally observed increase in Rg.