Factors affecting the selectivity of the oxidation of methyl p-toluate by cobalt(III)

The anaerobic oxidation of methyl p-toluate by cobalt(III) in acetic acid was investigated. Observed products were 4-carbomethoxybenzaldehyde (2), 4-carbomethoxybenzoic acid (3), 4-carbomethoxybenzyl acetate (1), 4,4'-dicarbomethoxybibenzyl (6), methyl 2,4-dimethylbenzoate (8), and methyl 3,4-dimethylbenzoate (9). Deuterium isotope labeling showed that 2 was not formed from 1, but appeared to be formed directly from methyl p-toluate via 4-carbomethoxybenzyl alcohol (5). The ratio of (2 + 3) to 1 was 0.5 with [py3Co3O(OAc)5OH[PF6] and 1.0 with cobaltic acetate. Cobaltic acetate was generated in situ by the reaction of cobaltous acetate and peracetic acid. When the oxidation was carried out in the presence of chromium (0.05 equiv based on cobalt), the ratio increased dramatically and no 6 was observed. Other transition metals such as vanadium, molybdenum, and manganese had a similar effect, but were not as effective as chromium. Chromium was observed to form a mixed-metal cluster complex with cobalt. Treatment of an acetic acid solution of cobaltous acetate and methyl isonicotinate with K2CrO4 produced a solid tentatively identified as [(MIN)3Co2CrO(OAc)6][CrO4H] (MIN = methyl isonicotinate). The selectivity for the oxidation of methyl p-toluate exhibited by the mixed-metal cluster complex was similar to that observed by the addition of chromium to oxidations using [py3Co3O(OAc)5OH[PF6].     

Ion/molecule reactions provide new evidence for the structure and origin of

Ion/molecule reactions leading to formation of the diagnostically useful [C(3)H(4)N](+) ion (m/z 54), from acetonitrile CI plasma, have been studied using a tandem quadrupole mass spectrometer. Evidence is presented to demonstrate that [C(3)H(4)N](+) is produced from an ion/molecule reaction between [C(2)H(2)N](+) (m/z 40) and neutral acetonitrile, via a [C(4)H(5)N(2)](+) (m/z 81) intermediate. Loss of HCN, where the H atom arises from neutral acetonitrile and the CN group from [C(2)H(2)N](+) (m/z 40), leads to the production of [C(3)H(4)N](+) (m/z 54). These results are consistent with a proposed concerted elimination of HCN, generating m/z 54 as a methylene vinylidene ammonium ion. Copyright 1999 John Wiley & Sons, Ltd.     

Matrix-assisted laser desorption ionization/mass spectrometry mapping of human immunodeficiency virus-gp120 epitopes recognized by a limited polyclonal antibody

In this study we have applied epitope excision and epitope extraction strategies, combined with matrix assisted laser desorption/ionization mass spectrometry, to determine the fine structure of epitopes recognized by a polyclonal antibody to human immunodeficiency virus envelope glycoprotein gp120. This is the first application of this approach to epitope mapping on a large, heavily glycosylated protein. In the epitope excision method, gp120 in the native form is first bound to the antibody immobilized on sepharose beads and cleaved with endoproteinase enzymes. In the epitope extraction method, the gp120 was first proteolytically cleaved and then allowed to react with the immobilized antibody. The fragments that remain bound to the antibody, after repeated washing to remove the unbound peptides, contain the antigenic region that is recognized by the antibody, and the bound peptides in both methods can be characterized by direct analysis of the immobilized antibody by matrix assisted laser desorption ionization/mass spectrometry. In this study we have carried out epitope excision and extraction experiments with three different enzymes and have identified residues 472–478 as a major epitope. In addition, antigenic regions containing minor epitopes have also been identified.     

Turbulent wake and vortex shedding for a stack partially immersed in a turbulent boundary layer

The effect of the jet-to-cross-flow velocity ratio, R, on the turbulent wake and Kármán vortex shedding for a cylindrical stack of aspect ratio AR=9 was investigated in a low-speed wind tunnel using thermal anemometry. The cross-flow Reynolds number was Re_D=2.3×10⁴, the jet Reynolds number ranged from Re_d=7.6×10³ to 4.7×10⁴, and R was varied from 0 to 3. The stack was partially immersed in a flat-plate turbulent boundary layer, with a boundary layer thickness-to-stack-height ratio of δ/H=0.5 at the location of the stack. From the behaviour of the turbulent wake and the vortex shedding, the flow around the stack could be classified into three regimes depending on the value of R, which were the downwash (R<0.7), cross-wind-dominated (0.7R<1.5), and jet-dominated (R1.5) flow regimes. Each flow regime had a distinct structure to the mean velocity (streamwise and wall-normal directions), turbulence intensity (streamwise and wall-normal directions), and Reynolds shear stress fields, as well as the variation of the Strouhal number and the power spectrum along the stack height.     

Hydrogen tunneling in an enzyme active site: a quantum wavepacket dynamical perspective

We study the hydrogen tunneling problem in a model system that represents the active site of the biological enzyme, soybean lipoxygenase-1. Toward this, we utilize quantum wavepacket dynamics performed on potential surfaces obtained by using hybrid density functional theory under the influence of a dynamical active site. The kinetic isotope effect is computed by using the transmission amplitude of the wavepacket, and the experimental value is reproduced. By computing the hydrogen nuclear orbitals (eigenstates) along the reaction coordinate, we note that tunneling for both hydrogen and deuterium occurs through the existence of distorted, spherical s-type proton wave functions and p-type polarized proton wave functions for transfer along the donor-acceptor axis. In addition, there is also a significant population transfer through distorted p-type proton wave functions directed perpendicular to the donor-acceptor axis (via intervening pi-type proton eigenstate interactions) which underlines the three-dimensional nature of the tunneling process. The quantum dynamical evolution indicates a significant contribution from tunneling processes both along the donor-acceptor axis and along directions perpendicular to the donor-acceptor axis. Furthermore, the tunneling process is facilitated by the occurrence of curve crossings and avoided crossings along the proton eigenstate adiabats.     

Quantum wavepacket ab initio molecular dynamics: an approach for computing dynamically averaged vibrational spectra including critical nuclear quantum effects

We have introduced a computational methodology to study vibrational spectroscopy in clusters inclusive of critical nuclear quantum effects. This approach is based on the recently developed quantum wavepacket ab initio molecular dynamics method that combines quantum wavepacket dynamics with ab initio molecular dynamics. The computational efficiency of the dynamical procedure is drastically improved (by several orders of magnitude) through the utilization of wavelet-based techniques combined with the previously introduced time-dependent deterministic sampling procedure measure to achieve stable, picosecond length, quantum-classical dynamics of electrons and nuclei in clusters. The dynamical information is employed to construct a novel cumulative flux/velocity correlation function, where the wavepacket flux from the quantized particle is combined with classical nuclear velocities to obtain the vibrational density of states. The approach is demonstrated by computing the vibrational density of states of [Cl-H-Cl]-, inclusive of critical quantum nuclear effects, and our results are in good agreement with experiment. A general hierarchical procedure is also provided, based on electronic structure harmonic frequencies, classical ab initio molecular dynamics, computation of nuclear quantum-mechanical eigenstates, and employing quantum wavepacket ab initio dynamics to understand vibrational spectroscopy in hydrogen-bonded clusters that display large degrees of anharmonicities.     

100-Gb/s undersea transmission with high spectral efficiency using pre-filtered QPSK modulation format

We provide a review of our recent 100-Gb/s, high spectral efficiency (SE) experiment targeting transoceanic and regional undersea transmission distances. We demonstrated that simple pre-filtering at the transmitter together with a maximum a posteriori probability (MAP) detection algorithm can significantly improve SE. We transmitted 96×100-Gb/s pre-filtered polarization division multiplexed return-to-zero quadrature phase shift keyed (PDM-RZ-QPSK) channels with 300% SE over 10608 km using 52-km spans of 150-μm 2 fiber and simple single-stage erbium-doped fiber amplifiers (EDFAs). We also achieved 400% SE over 4368 km using similar techniques.     

The connected Ramsey number

A graph G is totally connected if both G and ? (its complement) are connected. The connected Ramsey number r_c(F, H) is the smallest integer k ? 4 so that if G is a totally connected graph of order k then either F ? G or H ? ?. We show that if neither of F nor H contains a bridge, then r_c = r(F, H), the usual generalized Ramsey number of F and H. We compute r_c (P_mP_m), the connected Ramsey number for paths.     

Longest paths and cycles in K1,3-free graphs

In this article we show that the standard results concerning longest paths and cycles in graphs can be improved for K_1,3-free graphs. We obtain as a consequence of these results conditions for the existence of a hamiltonian path and cycle in K_1,3-free graphs.     

Current limits on the cold dark matter interaction cross section obtained by the UK collaboration

The UK Dark Matter Collaboration has run a 5–6 kg NaI detector in a well-shielded underground environment for about a year. Signatures of Cold Dark Matter (CDM) interactions are sought using pulse shape discrimination techniques by searching for the relatively short scintillation pulses arising from nuclear recoils among the residual, longer, background events from residual radioactivity in the detector and its environment. Here we report on an improvement to the limits on the CDM cross section for spin dependent interactions using our latest data from a crystal with improved detection efficiency and taking into account recent improved estimates of the spin factor correction between the nucleus and nucleon-supersymmetric particle cross sections.