Diels-alder reactions with inverse electron demand. II. The reaction of benzamidine with π-deficient heteroaromatic compounds.

Benzamidine (I) reacts with several s-tetrazines to give as-triazines in moderate to good yield, together with some s-triazines; separate experiments have shown that these are due to the fact that (1) also undergoes Diels-Alder- reactions with as- and even with s-triazines.     

A kinetic investigation of the thermal rearrangement of N-pyridino-2-vinyl-aziridines.

The first-order rate constants of the thermal rearrangement of several N-pyridino-2-vinyl-aziridines into the corresponding pyrido-azepines have been determined. Substituent effects on this isomerization were found to be completely analogous to the related benzenic O-Claisen rearrangement.     

Synthesis and rearrangements of alkyl-quadricyclanones

Both 1-isopropyl-6- and 7-methoxycarbonyl-quadricyclanones, the first alkyl-substituted molecules of this type, have been synthesized. Cycloaddition reactions with methylpropiolate failed. Their different behaviour in rearrangement and degradation reactions is discussed.     

An Improved Method for Calculating Zeta-Potentials from Measurements of the Electrokinetic Sonic Amplitude

The surface electric properties of the commercially available silica, Monospher 1000 (Fa. Merck), have been studied by conductivity and ESA (electrokinetic sonic amplitude) experiments. It could be shown that accounting for the contribution of the stagnant layer to surface conductivity is indispensable in the interpretation of electrokinetic data at low ionic strength. A general method has been put forward which allows to take into account the total, experimentally accessible surface conductivity in the evaluation of ESA data of moderately concentrated suspensions. This includes additional conductivity measurements which serve for the independent estimation of the total relative surface conductivity. The resulting zeta-potentials are clearly higher than those obtained after neglecting the contribution of the stagnant layer to surface conductivity. In addition, the ionic mobilities of potassium and magnesium in the hydrodynamically stagnant layer have been investigated in some detail. It has been found that the ionic mobility of potassium is of the same order of magnitude as in the bulk solution while the mobility of magnesium is significantly reduced. Copyright 2000 Academic Press.     

Rotational invariance of INDO theories including d-orbitals into the basis set

It is proved that in general the INDO approximation to the full Roothaan theory does not lead to expressions which are invariant under a rotation of local atomic axes. However, when only s- and p-functions are used in the atomic basis set, the equations obtained are invariant due to the special properties of the p-functions. When d-orbitals are included into the basis set, rotational invariance is lost but can be restored if a supplementary approximation is introduced.     

A CDOE/INDO LMO study of the nuclear spin-spin coupling constants between directly bonded C-H and C-C atoms

Müller-Pritchard (MP) type relations are used to study¹ J _CH and¹ J _CC coupling constants in a series of unsubstituted as well as in heterosubstituted hydrocarbons. In the case of the coupling constant between two atoms connected by a multiple bond, a generalised MP-type relation is derived.Aangesteld Navorser of the Belgian National Foundation for Scientific Research (N.F.W.O.)     

An enhanced microfluidic chip coupled to an electrospray Qstar mass spectrometer for protein identification

The combination of microfabricated fluidic systems (muFAB) and electrospray mass spectrometers (ESI-MS) will provide multiplexed and integrated analytical systems for proteins and other biomolecules. Implementation of this novel approach requires the development of robust and user-friendly muFAB devices. Here, we present new approaches that improve the robustness, user friendliness and performance of muFAB devices coupled to MS. First, we present the development of a convenient mount to connect a muFAB device to the ESI-MS and the incorporation of filters in the reservoirs and exit of the muFAB. This mount facilitates interfacing and significantly reduces the chemical noise observed by the MS. Furthermore, we demonstrate improvements in sample handling and delivery by using either a nonaqueous electrolyte or a cationic coating on the surfaces in the muFAB device and transfer capillary. These improvements are applied to protein analysis by continuous infusion of proteolytic digests.     

Systematic determination of ion score cutoffs based on calculated false positive rates: application for identifying ubiquitinated proteins by tandem mass spectrometry

We report a simple approach for determining ion score cutoffs that permit the confident identification of ubiquitinated proteins by tandem mass spectrometry (MS/MS). Initial experiments involving the analysis of gel bands containing multi-Ubiquitin chains with quadrupole time-of-flight and quadrupole ion trap mass spectrometers revealed that standard ion score cutoffs used for database searching were not sufficiently stringent. We also found that false positive and false negative rates (FPR and FNR) varied significantly depending on the cutoff scores used and that appropriate cutoffs could only be determined following a systematic evaluation of false positive rates. When standard cutoff scores were used for the analysis of complex mixtures of ubiquitinated proteins, unacceptably high FPR were observed. Finally, we found that FPR for ubiquitinated proteins are affected by the size of the protein database that is searched. These observations may be applicable for the study of other post-translational modifications.     

Identification of lysophosphatidylcholine (LPC) and platelet activating factor (PAF) from PC12 cells and mouse cortex using liquid chromatography/multi-stage mass spectrometry (LC/MS3)

Lipids play essential roles in cellular structural support, energy storage and signal transduction. Recently, mass spectrometry (MS) has been used to produce three-dimensional maps that elucidate the lipid composition of complex cellular lysates. The identification of individual lipids within these maps is slow and requires the synthesis and spiking of each candidate lipid. We present a novel MS-based technique that rapidly elucidates the atomic connectivity of the fatty acid/alcohol substituent on the sn-1 position of several different families of glycerophosphocholine-containing lipids within the confines of a chromatographic separation. Sodiated lipid species were fragmented to produce radical cations which lost successive methylene groups upon further collisional activation to reveal the identity of the parent molecule. This approach was demonstrated to be effective on isobaric members of the lysophosphatidylcholine (LPC) and platelet activating factor (PAF) families of glycerophospholipids. We demonstrate the application of this technique to unambiguously identify these species within complex cellular lysates and tissue extracts.     

Diels-Alder Reactions with Inverse Electron Demand -III- New Synthesis of Amino-Pyridazines; Evidence for an Acetamidine - Ketene - NN - Acetal Tautomerism

As part of our investigations in the field of the so-called “Diels-Alder reactions with inverse electron demand” involving various π-deficient heteroaromatic compounds as the diene¹, we describe herein the reaction of N,N′,N′-trisubstituted acetamidines with several s-tetrazines. The ability of open-chain CN multiple bonds to act as dienophiles in this type of reaction has indeed untill recently largely escaped investigation^1,2.