Properties of matrix-assisted laser desorption. Measurements with a time-to-digital converter.

Some properties of matrix-assisted laser desorption have been studied using single-ion-counting methods and a time-to-digital converter. The methods allow examination of the process for irradiances near the reported threshold for observation with a transient recorder. All measurements were made using bovine insulin as a test compound. We present direct evidence that an irradiance threshold near 10(6) W cm-2 exists for ion production, and that the process is a collective effect, either involving a large number of molecular ions (approximately 10(4) in a successful event or none at all. Above the threshold, the yield is found to scale with a high power (4th to 6th) of the irradiance. Measurements of initial velocity distributions indicate an axial velocity spread corresponding to approximately 50 eV and a radial velocity spread corresponding to approximately 2.4 eV. Thus the ejection or extraction mechanism appears to be strongly asymmetric.     

On-membrane digestion of beta-casein for determination of phosphorylation sites by matrix-assisted laser desorption/ionization quadrupole/time-of-flight mass spectrometry

This article discusses the features of a newly developed matrix-assisted laser desorption/ionization quadrupole/time-of-flight (MALDI-QqTOF) mass spectrometer that is useful in the analysis of phosphorylated peptides. Aliquots of beta-casein, a commonly used phosphorylated protein standard, were digested with trypsin directly on a non-porous polyurethane membrane used as sample support in MALDI-QqTOF mass spectrometry (MS) experiments. Although a complete peptide map was obtained, it was difficult to obtain sequence information for some of the tryptic fragments, in particular T1-2, which bears four phosphate groups and is thus difficult to ionize in positive mode. This article focuses on the sequencing of this particular fragment by comparing MS/MS spectra obtained using different precursor ions. These precursors associated with T1-2 were [M + H](+), [M + H](2+), and [M + H - nH(3)PO(4)](+) ions. Typically, phosphorylated ions showed facile unimolecular losses of phosphoric acid moieties, and produced limited backbone fragmentation. The abundance of [M + H](2+) ions of T1-2 in the full mass spectrum was low relative to that of [M + H](+). [M + H - 4H(3)PO(4)](+) ions as MS/MS precursors underwent backbone fragmentations, with phosphoserine residues transformed into dehydroalanines or serines. Unusual b + 18 u fragments were observed, although only for segments with previously phosphorylated serines. These partly interfered with c-ions, and were noticeable due to overlapping isotopic envelopes. It was possible to establish the sequence of phosphorylated tryptic fragment T1-2 and the location of phosphate groups using the mass of dehydroalanine residues (69 Da) and b + 18 u fragments as markers. All MS and MS/MS spectra obtained with fully phosphorylated beta-casein were compared with spectra acquired with dephosphorylated beta-casein obtained commercially. These comparisons helped assess the spectral differences caused by the presence of phosphate groups. Also, they highlighted the potential usefulness of conducting dephosphorylation directly on the probe prior to MALDI analysis in future studies.     

Chiral calixarene derived from resorcinol. Part 4: Diastereoselective closure of the oxazine ring

A method for the preparation of a single diastereomer of the oxazine derivative of calixresorc[4]arene 3 has been developed. Lewis acids were employed for opening of the oxazine ring.     

Stereoselective E/Z photoisomerization of oxazolidinone functionalized enecarbamates: direct and triplet sensitized irradiation

Oxazolidinone-functionalized enecarbamates undergo diastereoselective E/Z photoisomerization upon direct and triplet sensitized irradiations with chiral/achiral sensitizers, showing that the enhanced product diastereoselectivity depends on the solvent and temperature.     

Die potentiometrische Bestimmung von Jod und jod-freisetzenden Oxydationsmitteln

Zusammenfassung Zwischen dem Redoxpotential und dem log der Jodkonzentration jodidhaltiger wäßriger Lösungen besteht unabhängig vom pH-Wert über 3–4 Zehnerpotenzen ein weitgehend linearer Zusammenhang, der eine einfache und genaue quantitative Bestimmung von Jod ermöglicht. Da unter diesen Bedingungen sehr viele Oxydationsmittel äußerst rasch und quantitativ unter Bildung von Jod (bzw. Trijodid) reagieren, kann die Methode sehr vielseitig angewandt werden (z. B. zur Bestimmung von Chlor, Brom, O- und N-Halogenverbindungen, Ozon u. a.).Die Leistungsfähigkeit der Methode ist vor allem gekennzeichnet durch die rasche Redoxpotentialeinstellung in jodidhalügen Jodlösungen, die absolute Konstanz der als Meßelektrode dienenden Platinelektrode, die unter sorgfältiger Einhaltung der gewählten Meßbedingungen (Temperatur, Jodidkonzentration, Gesamtionenstärke) eine Nacheichung praktisch nicht notwendig macht, sowie die Möglichkeit, mit den gleichen Reagenzien den Konzentrationsbereich 10^–1–2,5×10^–8 M/l zu vermessen. Mit den relativen Standardabweichungen 0,7–1,2% im Bereich 10^–3–10^–5 M/l, 1,5–1,7% bei 10^–6 M/l bzw. 4,4–9,1% bei 10^–7 M/l stellt die potentiometrische Jodbestimmung eines der genauesten analytischen Verfahren im Bereich niederer Konzentrationen dar, das zudem durch einen äußerst geringen meßtechnischen Aufwand gekennzeichnet ist.

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The potentiometric determination of iodine and iodine releasing oxidants

Summary Between the redox potential and the log of the iodine concentration of aqueous iodine solutions (pH<9) containing an excess amount of iodide exists independent of the pH-value over a concentration range of 3–4 powers a nearly linear relation, which enables a simple and precise determination of iodine. Since under these conditions a great many oxidants react very fast and quantitatively forming iodine (resp. tri-iodide) the method can be used very versatile (e. g. for the determination of chlorine, bromine, O- and N-halogene compounds, ozone). The efficiency of the method is characterized above all by the fast adjustment of the redox potential in iodine solutions containing iodide, the constancy of the Pt-electrode, by which carefully observing the choosen measuring conditions (temperature, iodide concentration, total ionic strength) a recalibration is virtually unnecessary, and the possibility to measure with the very same reagents the concentration range 10^–1–2.5×10^–8 M/l. With relative standard deviations of 0.7–1.2% in the range 10^–3–10^–5 M/l, 1.5–1.7% at 10^–6 M/l resp. 4.4–9.1% at 10^–7 M/l the potentiometric determination of iodine, though requiring only a very simple instrumental fitting out, represents one of the most precise analytical methods at low concentration levels.

     

Conformationally controlled (entropy effects), stereoselective vibrational quenching of singlet oxygen in the oxidative cleavage of oxazolidinone-functionalized enecarbamates through solvent and temperature variations

On photooxygenation (methylene blue as sensitizer) of E/Z enecarbamates, equipped with the oxazolidinone chiral auxiliary, the oxidative cleavage of the alkenyl functionality releases the enantiomerically enriched methyldesoxybenzoin (MDB) product. The extent (% ee) as well as the sense (R vs S) of the stereoselectivity in the MDB formation depends on the choice of the alkene configuration; the efficacy of stereocontrol may be tuned by appropriate solvent and temperature conditions. Highlighted is the finding that the formation of the preferred MDB enantiomer (R or S) depends for the E isomer on the chosen solvent and temperature, but not for the corresponding Z isomer. The activation parameters for the various solvents disclose that differential entropy effects (ΔΔS^‡) dominate the conformationally more flexible E diastereomers. As mechanistic rationale for this unprecedented conformationally imposed stereochemical behavior, we propose the competitive action of stereoselective vibrational quenching of the attacking singlet oxygen by the enecarbamate versus sterically controlled stereoselective oxidative cleavage of its double bond.     

On the origin of geminal regioselectivity in the ene reaction of singlet oxygen with substituted alkenes

The geminal regioselectivity observed in the ene reaction between singlet oxygen and alkenes with anion-stabilizing groups is rationalized on the basis of a perepoxide intermediate, in which in analogy to the nucleophilic attack on protonated epoxides, the perepoxide is opened preferentially at the C---O bond weakened by the substituent.