Improved ground state rotational parameters of deuterium fluoride, DF

The rotational spectrum of DF in the 1.3-3.3 THz frequency region has been observed by means of a tunable far-infrared spectrometer. The J + 1 ← J, with J = 1-4, rotational transitions of DF have been recorded with an accuracy of the order of 50-200 kHz. These measurements, in conjunction with the hyperfine components of the J = 1 ← 0 transition recently observed [Cazzoli and Puzzarini, J. Mol. Spectrosc. 231 (2005) 124-130] and the rotational transitions up to J = 47 [R.S. Ram, Z. Morbi, B. Guo, K.-Q. Zhang, P.F. Bernath, J. Vander Auwera, J.W.C. Johns, S.P. Davies, Astrophys. J. Suppl. Series 103 (1996) 247-254] consented to improve the ground state rotational parameters of DF.     

Disposable Electrospun Electrodes Based on Conducting Nanofibers

A new approach for the fabrication of disposable electrochemical surfaces is presented. Conducting fibers have been produced by a two‐step process. Firstly, a fine web mat composed of nylon‐6 and ferric chloride fibers (with diameters of about 200 nm) were electrospun on a flat collector. Afterwards, the electrospun mat was exposed to pyrrole vapor under controlled dry atmosphere until each nylon thread was fully coated with a conducting polypyrrole layer. The conducting fiber mat was finally integrated within an electrochemical cell and used as a disposable sensor device. The suitability of the sensor was tested using phosphate and carbonate organic anions. The large scale fabrication of conducting tissue, in the form of a fine web of spun nanofibers, is relatively fast and inexpensive, and it paves the way to the build up of new disposable electrodes.     

Asymmetric synthesis of cyclopropanes from lithiated aryloxiranes and alpha,beta-unsaturated Fischer carbene complexes

A diastereo- and enantiospecific formation of tetrasubstituted cyclopropane carbene complexes and cyclopropanecarboxylates from lithiated aryloxiranes and alpha,beta-unsaturated Fischer carbene complexes is described.     

Dynamical origin of the quantum Zeno effect

The quantum Zeno effect is often studied and understood in term of nonunitary evolutions, involving projections à la von Neumann (measurements). We propose a dynamical explanation of this effect, which involves only unitary operators. The limit of infinitely frequent measurements is critically discussed: it is unphysical, yet interesting and peculiar.     

Energy-based prediction of amino acid-nucleotide base recognition

Despite decades of investigations, it is not yet clear whether there are rules dictating the specificity of the interaction between amino acids and nucleotide bases. This issue was addressed by determining, in a dataset consisting of 100 high-resolution protein-DNA structures, the frequency and energy of interaction between each amino acid and base, and the energetics of water-mediated interactions. The analysis was carried out using HINT, a non-Newtonian force field encoding both enthalpic and entropic contributions, and Rank, a geometry-based tool for evaluating hydrogen bond interactions. A frequency- and energy-based preferential interaction of Arg and Lys with G, Asp and Glu with C, and Asn and Gln with A was found. Not only favorable, but also unfavorable contacts were found to be conserved. Water-mediated interactions strongly increase the probability of Thr-A, Lys-A, and Lys-C contacts. The frequency, interaction energy, and water enhancement factors associated with each amino acid-base pair were used to predict the base triplet recognized by the helix motif in 45 zinc fingers, which represents an ideal case study for the analysis of one-to-one amino acid-base pair contacts. The model correctly predicted 70.4% of 135 amino acid-base pairs, and, by weighting the energetic relevance of each amino acid-base pair to the overall recognition energy, it yielded a prediction rate of 89.7%.     

A low-cost and low-tech electrochemical flow system for the evaluation of total phenolic content and antioxidant power of tea infusions

A disposable, small scale and low-cost electrochemical "wall-jet" flow-cell was build up together with a graphite lead mounted onto a pencil holder able to modulate the current response by the control of the surface area of the electrode. The total phenolic content of different types of tea infusions was determined and compared with the conventional Folin-Ciocolteau method. An index of antioxidant power was also proposed and data were compared with those obtained by the DPPH* assay.     

Regio- and stereoselective lithiation of terminal oxazolinylaziridines: the aziridine N-substituent and the oxazolinyl group effect

The regioselective lithiation of terminal oxazolinylaziridines has been investigated. The steric hindrance of the nitrogen substituent in 1-trityl-2-oxazolinylaziridine 3a, combined with the coordinating ability of the oxazolinyl group, causes beta-lithiation, whereas a completely regioselective alpha-lithiation is observed with the much less sterically demanding 1-benzyl-2-oxazolinylaziridine 3c and a competition between alpha- and beta-lithiation occurs with 1-cumyl-2-oxazolinylaziridine 3b in which the N-substituent has a steric hindrance in between the trityl and the benzyl groups. The application of the lithiation-trapping sequence for the preparation of enantioenriched 2,3-cis-disubstituted oxazolinylaziridines and aziridino-gamma-lactones is also reported.     

Nitrogen dynamics and reactivity of chiral aziridines: generation of configurationally stable aziridinyllithium compounds

Diastereomeric oxazolinylaziridines (R,R)-9 and (R,S)-9 have been regioselectively lithiated at the α-position with respect to the oxazolinyl ring. The resulting aziridinyllithium compounds proved to be chemically and configurationally stable under the experimental conditions used, thus furnishing, upon trapping with electrophiles, chiral 2,2-disubstituted aziridines, in contrast to the corresponding α-lithiated oxazolinyloxiranes that have been reported to be chemically stable but configurationally unstable. This peculiar behavior of the nitrogen-bearing heterocycle has been rationalized on the basis of DFT calculations and the observed dynamics of the aziridine nitrogen atom. The DFT analysis allowed the disclosure of a solvent-dependent differing stability of diastereomeric lithiated aziridines (R,R)-9-Li and (R,S)-9-Li, suggesting η(3)-coordinated oxazolinylaziridinyllithium compounds as likely intermediates. Such intermediates could be the result of a dynamically controlled lithiation that relies on the preliminary formation of a complex between the lithiating agent and the oxazolinyl ring. According to this model, the competing complexation of the lithiating agent by the lone pair of electrons on the aziridine nitrogen would cause addition to the oxazoline C=N bond, thus ending up with the formation of oxazolidines, which are precursors of useful chiral ketoaziridines. The proposed model has been also supported by estimating the nitrogen inversion barrier by dynamic NMR spectroscopic experiments.