Indirect exciton transition in silicon under [111] uniaxial compression

Transverse electroabsorption and absolute absorption spectra in silicon at 1.8° K in the presence of [111] uniaxial compression are reported. The relative strengths of the phonon-assisted transitions and the change of exciton binding energy are found to be in agreement with predictions. The effect of strain upon spectral broadening is also discussed.     

Microwave-assisted synthesis of isonitriles: a general simple methodology

[reaction: see text] A facile conversion of formamides to isonitriles under very mild conditions and microwave irradiation is described. This simple and efficient method has been applied for the synthesis of both aliphatic and aromatic isonitriles in high yields.     

Terpene biosynthesis in the nudibranch Doriopsilla areolata

Biogenesis of the enantiomeric sesquiterpenes 1 and 5 of the marine nudibranch Doriopsilla areolata was investigated by feeding of [1-(13)C]glucose, [1,2-(13)C(2)]glucose, and [1,2-(13)C(2)]acetate. Evidence is presented that supports de novo origin of both compounds via mevalonic acid.     

Tryptophan phosphorescence of ribonuclease T1 as a probe of protein flexibility

The phosphorescence properties of Trp-59 of ribonuclease T₁ fromAspergillus oryzae were monitored as a function of temperature, pH, salt concentration, and complex formation with substrate analogues and, also, in the presence of glycerol as viscogenic cosolvent. The results establish a rough correlation between the internal flexibility of the macromolecule, as derived from the triplet lifetime, and its stability (G orT _m ) toward unfolding. Below 10°C or in 70% glycerol the triplet probe distinguishes at least two gross conformations for the protein, which are characterized by a large difference in phosphorescence lifetime. It is pointed out that such structural heterogeneity does not correspond with the heterogeneity inferred from fluorescence decays and acrylamide quenching rates. Further, implications of the phosphorescence data with regard to the interpretation of acrylamide quenching of fluorescence are discussed.     

Selektive Bestimmung verschiedener Stickstoff-Formen mit der Mikrokjeldahl-Methode

Zusammenfassung Durch Anwendung der Methode vonDumas-Zimmermann und einer modifiziertenKjeldahl-Methode nebeneinander ist die selektive Bestimmung der verschiedenen Stickstoff-Formen in heterocyklischen Verbindungen möglich.Auf Grund der Ergebnisse, die einerseits bei Pyridin-, Chinolin- und Isochinolinverbindungen sowie anderseits bei hydrierten Ringsystemen erhalten wurden, werden einige Regeln festgesetzt, die den Anwendungsbereich des Verfahrens begrenzen.

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Summary By the joint application of theDumas-Zimmermann procedure and a modifiedKjeldahl method, it is possible to make a selective determination of the various forms of nitrogen in heterocyclic compounds. On the basis of the findings, which were obtained on one hand with pyridine-, quinoline-, and isoquinoline compounds, and with hydrogenated ring systems on the other, several rules were set up limiting the field of application of the procedure.

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Résumé Par emploi simultané de la méthodeDumas-Zimmermann et d'une méthode deKjeldahl modifiée, le dosage sélectif des différentes formes d'azote dans les composés hétérocycliques a été rendu possible. D'après les résultats, on a établi quelques règles qui limitent le domaine d'emploi des procédés. Ces résultats sont obtenus, d'une part avec la pyridine, la quinoléine, l'isoquinoléine, d'autre part, avec les cycles hydrogénés.

     

Shuttling dynamics in an acid-base-switchable [2]rotaxane.

Molecular shuttles are an intriguing class of rotaxanes which constitute prototypes of mechanical molecular machines and motors. By using stopped-flow spectroscopic techniques in acetonitrile solution, we investigated the kinetics of the shuttling process of a dibenzo[24]crown-8 ether (DB24C8) macrocycle between two recognition sites or "stations"--a secondary ammonium (-NH2+-)/amine (-NH-) center and a 4,4'-bipyridinium (bipy2+) unit--located on the dumbbell component in a [2]rotaxane. The affinity for DB24C8 decreases in the order -NH2+- > bipy2+ > -NH-. Hence, shuttling of the DB24C8 macrocycle can be obtained by deprotonation and reprotonation of the ammonium station, reactions which are easily accomplished by addition of base and acid to the solution. The rate constants were measured as a function of temperature in the range 277-303 K, and activation parameters for the shuttling motion in both directions were determined. The effect of different counterions on the shuttling rates was examined. The shuttling from the -NH2+- to the bipy2+ station, induced by the deprotonation of the ammonium site, is considerably slower than the shuttling in the reverse direction, which is, in turn, activated by reprotonation of the amine site. The results show that the dynamics of the shuttling processes are related to the change in the intercomponent interactions and structural features of the two mutually interlocked molecular components. Our observations also indicate that the counterions of the cationic rotaxane constitute an important contribution to the activation barrier for shuttling.     

Interaction of water with the model ionic liquid [bmim][BF4]: molecular dynamics simulations and comparison with NMR data

We report on molecular dynamics simulations of the ionic liquid [bmim][BF 4] and its mixtures with water, from zero up to 0.5 mol fraction of water. All of the simulations are carried out with two published force fields. The results are compared with each other and with published as well as new NMR data on the same mixtures, whenever possible. We perform extensive analyses of structural quantities, such as pair correlation functions, nearest-neighbor analysis and size distribution of the water clusters formed at higher concentrations. We show that the water clusters are formed almost exclusively by linear chains of hydrogen-bonded molecules. There is a nanoscale structuring of the mixtures but no macroscopic phase separation among the components, in agreement with experiment. Roughly, we identify two solvation regimes. At low water content, the ions are selectively coordinated by individual water molecules, but their ionic network is largely unperturbed. At high water content, the ionic network is somewhat disrupted or swollen in a nonspecific way by the water clusters.     

New diastereomeric bis-sesquiterpenes from Hainan marine sponges Axinyssa variabilis and Lipastrotethya ana

Three unprecedented diastereoisomeric dimers, cis-dimer A (1), cis-dimer B (2) and trans-dimer C (3), exhibiting a bis-bisabolene skeleton, and a new sesquiterpene, dehydrotheonelline (4), their potential precursor, have been isolated from the South China Sea sponges Axinyssa variabilis and Lipastrotethya ana, along with known related sesquiterpenes. The structure of the novel molecules has been determined by extensive NMR spectral analysis.