Elimination of methane from protonated acetaldehyde: The Ab initio transition state

The transition state (TS) for loss of CH₄ from protonated acetaldehyde has been located at the second-order Moller-Plesset (MP2)/6-31G(d,p) level of theory. The activation energy is predicted to be 263.9 kJ/mol starting from the more stable form (methyl and hydrogen E) and 261.6 kJ/mol starting from the less stable form (methyl and hydrogen Z) that is required for reaction. The products (methane and the formyl ion) are predicted to lie 136.6 kJ/mol below the TS for their formation. MP2 methods underestimate the heats of formation of both the TS and the reaction products by about 40 kJ/mol when compared with experiment. Restricted Hartree-Fock (RHF) calculations give much more accurate relative energies. The MP2 TS leads directly to fragmentation and is described as a protonation of the methyl group by the acidic proton on oxygen. Under RHF theory the reaction is stepwise. An RHF TS similar to the MP2 TS leads to a nonclassical intermediate (which is stable at this level of theory) that has one of the C---H bonds protonated. This mechanism (protonation of an alkyl group) appears to be a general one for high energy 1,2 eliminations from organic cations. (J Am Soc Mass Spectrom 1994, 5, 1102-1106)     

Optimization of solid-phase clean-up prior to liquid chromatographic analysis of ochratoxin A in roasted coffee

An extraction and clean-up method for ochratoxin A (OA) in roasted coffee has been developed and the HPLC method optimized. An interfering compound with a similar retention time as OA was adsorbed by the aminopropyl (NH2) material at < or = 5% NaHCO3. Residual OA on the column was recovered by washing with the extraction solution followed with methanol. Fractions were mixed together for further clean-up with Ochratest immunoaffinity columns (IACs). Analysis by HPLC resulted in a well resolved OA peak and reduction in matrix interferences. Recoveries ranged from 72 to 84% and the detection limit was 1 ng/g.     

Fast Dimension Reduction Using Rademacher Series on Dual BCH Codes

The Fast Johnson–Lindenstrauss Transform (FJLT) was recently discovered by Ailon and Chazelle as a novel technique for performing fast dimension reduction with small distortion from ℓ ₂ ^d to ℓ ₂ ^k in time O(max {dlog d,k ³}). For k in [Ω(log d),O(d ^1/2)], this beats time O(dk) achieved by naive multiplication by random dense matrices, an approach followed by several authors as a variant of the seminal result by Johnson and Lindenstrauss (JL) from the mid 1980s. In this work we show how to significantly improve the running time to O(dlog k) for k=O(d ^1/2−δ ), for any arbitrary small fixed δ. This beats the better of FJLT and JL. Our analysis uses a powerful measure concentration bound due to Talagrand applied to Rademacher series in Banach spaces (sums of vectors in Banach spaces with random signs). The set of vectors used is a real embedding of dual BCH code vectors over GF(2). We also discuss the number of random bits used and reduction to ℓ ₁ space. The connection between geometry and discrete coding theory discussed here is interesting in its own right and may be useful in other algorithmic applications as well.     

Reactions within fluorobenzene-ammonia heterocluster ions: experiment and theory

Reactions occurring within gas phase fluorobenenze-ammonia heterocluster cations (FC(6)H(5)-(NH(3))(n=1-4)) have been studied through the use of a triple quadrupole mass spectrometer as well as employing density functional theory (DFT). Collision induced dissociation (CID) experiments were conducted in which mass selected cluster ions are accelerated into a cell containing argon gas and the resulting products then subsequently mass analyzed. Two dominate reaction channels are observed. The first is simple evaporative loss of neutral ammonia from the cluster ion. The second involves a substitution reaction occurring within the cluster ion to form the aniline cation, C(6)H(5)NH(2)(+), where the reactivity was found to vary as a function of cluster size. DFT calculations have been performed to both help analyze the structure and the reactivity of these cluster ions. Pronounced differences in activation energies were found that provide an explanation for the observed variation of reactivity as a function of cluster size. An ad hoc model based upon the Arrhenius equation was developed to fit both the experimental collision energy dependence of the reaction and the observed lowering of the reaction barrier to aniline formation as a function of cluster size.     

Helix formation in a pentapeptide: experiment and force-field dependent dynamics

We used a combined approach of experiment and simulation to determine the helical population and folding pathway of a small helix forming blocked pentapeptide, Ac-(Ala)(5)-NH(2). Experimental structural characterization of this blocked peptide was carried out with far UV circular dichroism spectroscopy, FTIR, and NMR measurements. These measurements confirm the presence of the α-helical state in a buffer solution. Direct molecular dynamics and replica-exchange simulations of the pentapeptide were performed using several popular force fields with explicit solvent. The simulations yielded statistically reliable estimates of helix populations, melting curves, folding, and nucleation times. The distributions of conformer populations are used to measure folding cooperativity. Finally, a statistical analysis of the sample of helix-coil transition paths was performed. The details of the calculated helix populations, folding kinetics and pathways vary with the employed force field. Interestingly, the helix populations, folding, and unfolding times obtained from most of the studied force fields are in qualitative agreement with each other and with available experimental data, with the deviations corresponding to several kcal/mol in energy at 300 K. Most of the force fields also predict qualitatively similar transition paths, with unfolding initiated at the C-terminus. Accuracy of potential energy parameters, rather than conformational sampling may be the limiting factor in current molecular simulations.     

Zeeman effect measurements in SO2: An internal magnetic field calibrant

It is proposed that the Zeeman effect of the 2₀₂ → 1₁₁ transition in sulfur dioxide be used as a magnetic field calibration standard. The recent accurate beam maser measurements on this transition by Ellenbroek and Dymanus have been confirmed by electric resonance measurements in the 1₁₁ level. The Zeeman splittings most useful for calibration purposes depend linearly on the magnetic field. For perpendicular selection rules the Zeeman coefficient is 0.5282(3) MHz/kG while parallel transitions have a 0.3722(3) MHz/kG Zeeman coefficient. The sign of the rotational magnetic moment of SO₂ has been determined to be negative.     

A note on the Hecke hypothesis and the determination of imaginary quadratic fields with class-number 1

The authors propose a hypothesis whose proof would provide, in particular, a solution of the “class-number 1” problem, recently solved by Stark and Baker.     

Development of a colloidal gold-based lateral-flow immunoassay for the rapid simultaneous detection of zearalenone and deoxynivalenol

A multianalyte lateral-flow technique using colloidal gold-labeled monoclonal antibodies was developed for the rapid simultaneous detection of deoxynivalenol (DON) and zearalenone (ZEA). The results of this qualitative one-step test were interpreted visually. A very simple and fast sample preparation was used, and the assay procedure could be accomplished within 10 min. When applied to spiked wheat samples, the technique gave accurate and reproducible results. Cut-off levels of 1500 and 100 μg kg⁻¹ for DON and ZEA, respectively, were observed. The described multianalyte format can be used as a reliable, rapid and cost-effective on-site screening technique for the simultaneous determination of mycotoxins in grain samples.     

Dense Fast Random Projections and Lean Walsh Transforms

Random projection methods give distributions over k×d matrices such that if a matrix Ψ (chosen according to the distribution) is applied to a finite set of vectors x _i ∈ℝ ^d the resulting vectors Ψx _i ∈ℝ ^k approximately preserve the original metric with constant probability. First, we show that any matrix (composed with a random ±1 diagonal matrix) is a good random projector for a subset of vectors in ℝ ^d . Second, we describe a family of tensor product matrices which we term Lean Walsh. We show that using Lean Walsh matrices as random projections outperforms, in terms of running time, the best known current result (due to Matousek) under comparable assumptions.