Density functional theory study of the cycloaddition reaction of furan derivatives with masked o-benzoquinones. Does the furan act as a dienophile in the cycloaddition reaction?

The molecular mechanism for the cycloaddition reaction between 2-methylfuran and a masked o-benzoquinone has been characterized using quantum mechanical calculations at the B3LYP/6-31G theory level. An analysis of the results on the reaction pathway shows that the reaction takes place along a polar stepwise mechanism. The first and rate-determining step corresponds to the nucleophilic attack of the furan ring on the doubly conjugated position of the 2,4-dienone system present at the masked o-benzoquinone to give a zwitterionic intermediate. Closure of this intermediate affords the formally [2 + 4] cycloadduct. For the second step two reactive channels have been characterized corresponding to the formation of the formally [2 + 4] and [4 + 2] cycloadducts. Analysis of the energetic results indicates that while the first is the meta regiocontrolling and endo stereocontrolling step, the second one is responsible for the formation of the unexpected formally [2 + 4] cycloadduct. The global and local electrophilicity/nucleophilicity power of the reactants and intermediate have been evaluated to rationalize these results. Density functional theory analysis for these cycloadditions is in complete agreement with the experimental outcome, explaining the reactivity and selectivity of the formation of the formally [2 + 4] cycloadducts.     

Glycoform characterization of intact erythropoietin by capillary electrophoresis-electrospray-time of flight-mass spectrometry

Glycosylated proteins often show a large variation in their glycosylation pattern, complicating their structural characterization. In this paper, we present a method for the accurate mass determination of intact isomeric glycoproteins based on capillary electrophoresis-electrospray-time of flight-mass spectrometry. Human recombinant erythropoietin has been chosen as a showcase. The approach enables the on-line removal of nonglycosylated proteins, salts, and neutral and negatively charged species. More important, different glycosylation forms are separated both on the base of differences in the number of negatively charged sialic acid residues and the size of the glycans. Thus, 44 glycoforms and in total about 135 isoforms of recombinant human erythropoietin, taking also acetylation into account, could be distinguished for the reference material from the European Pharmacopeia. Distinct glycosylation differences for samples from different suppliers are clearly observed. Based on the accurate mass an overall composition of each single isoform is proposed, perfectly in agreement with data on glycan and glycopeptide analysis. This method is an ideal complement to the established techniques for glycopeptide and glycan analysis, not differentiating branching or linkage isoforms, but leading to an overall composition of the glycoprotein. The presented strategy is expected to improve significantly the ability to characterize and quantify isomeric glycoforms for a large variety of glycoproteins.     

Ion-pair extraction and fluorimetric determination of ultratraces of strontium with cryptand 2.2.2 and eosin

A highly sensitive and selective fluorimetric determination of strontium is proposed, based on solvent extraction of the ion-pair formed between the cationic complex of Sr²⁺ with cryptand 2.2.2 and eosinate as counter ion. A linear working range from 0.7 ng/ml (limit of detection) to 500 ng/ml of strontium and a relative standard deviation of 3.5% at the 100 ng/ml level are obtained. The metal: ligand: counter ion molecular ratio in the extracted mononuclear ion-pair is 1 1 1. The equilibrium constants involved in the extraction process were calculated.     

Numerical analysis of a mixed elliptic problem with flux and convective boundary conditions to obtain a discrete solution of nonconstant sign

We consider a material that occupies a convex polygonal bounded domain Ω ⊂ ℝⁿ, with regular boundary Γ = Γ₁ ∪ Γ₂ (with Γ ∩ Γ = ∅︁) with meas (Γ₁) = |Γ₁| > 0 and |Γ₂| > 0. We assume, without loss of generality, that the melting temperature is 0°C. We consider the following steady‐state heat conduction problem in Ω: with α, q, B = Const > 0, and q and α represent the heat flux on Γ₂ and the heat transfer coefficient on Γ₁, respectively. In a previous article (Tabacman‐ Tarzia, J Diff Eq 77 (1989), 16– 37) sufficient and/or necessary conditions on data α, q, B, Ω, Γ₁, Γ₂ to obtain a temperature u of nonconstant sign in Ω (that is, a multidimensional steady‐state, two‐phase, Stefan problem) were studied. In this article, we consider a regular triangulation by finite element method of the domain Ω with Lagrange triangles of the type 1, with h > 0 the parameter of the discretization. We study sufficient (and/or necessary) conditions on data α, q, B, Ω, Γ₁, and Γ₂ to obtain a change of phase (steady‐state, two‐phase, discretized Stefan problem) in corresponding discretized domain, that is, a discrete temperature of nonconstant sign in Ω. Moreover, error bounds as a function of the parameter h, are also obtained. © 1999 John Wiley & Sons, Inc. Numer Methods Partial Differential Eq. 15: 355–369, 1999     

Experimental and theoretical study of radon levels and entry mechanisms in a mediterranean climate house

An experimental study has been carried out in an inhabited single-family house. Radon concentration in the different rooms of the house and in its garden soil has been measured with Nuclear Track Detectors. No high differences of radon concentration have been observed between the different rooms of the house, so that the proximity of the room level to the soil seems not to affect the radon concentration. The annual radon concentration obtained indoors and in the soil has been respectively 35 Bq m⁻³ and 24 kBq m⁻³. Since radon generation in the source, entry into indoor air and accumulation indoors depend on several parameters, the effect of a specific parameter on indoor radon concentration is difficult to explain from the radon measurements only. The RAGENA (RAdon Generation, ENtry and Accumulation indoors) model has been adapted to the room in the basement of the house. The mean radon concentration values obtained with the model are compared to experimental results derived from measurements using Nuclear Track Detectors. The use of the model, together with the experimental study, has allowed characterising radon sources, levels and entry mechanisms in the house. The concrete walls have been found to be the most relevant radon source, while the contribution of the soil is negligible in this case. The indoor radon level is given by the balance of the permanent exhalation from concrete and the removal due to ventilation. The indoor radon levels are close to the average value for the Barcelona area which, in turn, is close to the world averaged value.     

A multilinear Phelps' Lemma

We prove a multilinear version of Phelps' Lemma: if the zero sets of multilinear forms of norm one are `close', then so are the multilinear forms.

     

Relativistic action-at-a-distance interactions: Lagrangian and Hamiltonian to terms of second order

Relativistic systems of particles interacting pairwise at a distance (interactions not mediated by fields) in flat spacetime are studied. It is assumed that the interactions propagate at the speed of light in vacuum and that all masses are scalars under Poincaré transformations. The action functional of the theory depends on multiple times (the proper times of the particles). In the static limit, the theory has three components: a linearly rising potential, a Coulomb-like interaction and a dynamical component to the Poincaré invariant mass. In this Letter we obtain explicitly, to terms of second order, the Lagrangian and the Hamiltonian with all the dynamical variables depending on a single time. Approximate solutions of the relativistic two-body problem are presented.     

Experimental and theoretical push‐pull Chemo‐ and regioselectivity in 1,3‐Dipolar cycloaddition reactions: the case of benzotriazepin‐5‐one with mesitylnitrile oxide

A novel heterocyclic compound 3‐mesityl‐5‐methyl‐4,5,11,11a‐tetrahydro‐6H‐[1,2,4]oxadiazolo [5,4‐b][1,3,4]benzotriazépin‐6‐one 4 has been synthesised by a 1,3 dipolar cycloaddition (13DC) reaction of 1,3,4‐benzotriazepin‐5‐one 1 with mesitylnitrile oxide 3 . The reaction, beside its synthetic interest, has shown to be completely chemo‐ and regioselective. The structure of the compound was determined by X‐ray crystallography and analysed by spectral methods (NMR and mass spectrometry). The molecular mechanism for the reaction has been studied using quantum mechanical calculations at the B3LYP/6‐31G* theory level. Two mechanisms are possible for the formation of the cycloadduct 4 . The first one involves a 13DC reaction between 1 , as dipolarophile and 3 , as dipole. Analysis of the results indicates that it takes place along asynchronous concerted bond‐formation process with a very low polar character. The regioselectivity obtained from the calculations are in complete agreement with the unique formation of the cycloadduct 4 . The second mechanism is initiated by the nucleophilic attack of the N3 nitrogen of the tautomer form of 2 , to the C5 carbon of the nitrile oxide 3 to yield an amidoxime. However, the large energy involved in this addition prevents this mechanism. The large energy difference between the tautomers 1 and 2 , makes that only the C?N site of benzotriazepin‐5‐one 1 could act as a dipolarophile site. This fact makes the 13DC reaction to be chemoselective. The analysis of global electrophilicity of the reagents allows explaining the low polar character of these 13DC reactions. Copyright © 2007 John Wiley & Sons, Ltd.