Thermisch stimulierte Lumineszenzerscheinungen II: Einfluß der Vorbehandlung auf das Verhalten verschiedener Metallpulver

Thermoluminescence of metals is not expected on ground of the theory. The effects which have been found for Cu, Fe, Cr, Zn, W and Mo might be regarded as due to the formation of oxygen containing superstructures with semiconducting properties in the surface regions. Both the emitted thermoluminescence energy and the temperature at which its peak in the glow curve is found for Cu and Mo, are influenced by the mode of the pretreatment of the sample. The values are found lower after previous cooling and higher after previous grinding or heating in a vacuum. For all metals under investigation the phenomenon of the so-called contact-memory has been found. The effects of thermal or mechanical pretreatment of the metal powders are also reflected in corresponding changes in adsorption behaviour. The results allow certain conclusions about the energy distribution over the various hierarchic levels.

     

Volumetrische Mikrobestimmung von Selen in organischen Substanzen

Zusammenfassung Es wird ein maßanalytisches Mikroverfahren zur Bestimmung von Selen in organischen Verbindungen angegeben, bei dem der Aufschluß mit Natriumperoxyd erfolgt. Durch diese Art des Aufschlusses werden organische Selensubstanzen rasch und quantitativ zerstört, doch konnte die eigentliche Bestimmung des Selens bisher nur gravimetrisch vorgenommen werden. Dies ist zeitraubend und wegen der ungünstigen Wägeform (elementares Selen) für Mikrobestimmungen nicht verwendbar.Zur Mikro-Selenbestimmung schien aus diesen Gründen ein volumetrisches Verfahren zweckmäßiger. Für dessen Ausarbeitung war die Feststellung wesentlich, daß Selen in organischen wie auch anorganischen Verbindungen durch Peroxydaufschluß ausschließlich zu Selenat oxydiert wird. Um das gebildete Selenat volumetrisch bestimmen zu können, wurde ein Mikroverfahren entwickelt, das auf der Reaktion zwischen Selenat und Jodid in salzsaurer Lösung beruht. Hierbei werden für 1 Selen 6 Äquivalente Jod frei, die man mit Thiosulfat titriert. Oxydierende Stoffe, die die jodometrische Bestimmung durch zusätzliche Jodausscheidung stören würden (beim Peroxydaufschluß: Wasserstoffperoxyd und gegebenenfalls Jodat), werden vor der Titration in geeigneter Weise reduziert, ohne daß Selenat angegriffen wird.Dieses Verfahren ermöglicht eine rasche und genaue Bestimmung von Selen in organischen Substanzen mit Einwaagen von 1 bis 4 mg. Ebenso konnte damit Selen in anorganischem Material bestimmt werden.

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Summary A volumetric micro method is given for the determination of selenium in organic compounds, in which the compound is decomposed with sodium peroxide. This type of disintegration destroys organic selenium compounds rapidly and quantitatively, but up to now the actual determination of the selenium could only be accomplished gravimetrically. This is time-consuming and because of the unfavorable weighing form (elementary selenium) it is not applicable for micro determinations.For these reasons, a volumetric procedure seemed to be more appropriate for micro determinations of selenium. For its development, it was essential to establish that selenium in organic as well as inorganic compounds is oxidized exclusively to selenate by peroxide decomposition. In order to determine the resulting selenate volumetrically, a micro method was developed which is based on the reaction between selenate and iodide in hydrochloric acid solution. In this, 6 equivalents of iodine are liberated for 1 of selenium; the iodine is then titrated. Oxidants, which interfere with the iodometric determination by releasing additional iodine (in the peroxide decomposition: hydrogen peroxide and perhaps iodate) are reduced before the titration by suitable means without attacking the selenate.This procedure makes possible a rapid and accurate determination of selenium in organic compounds with samples of 1 to 4 mg. It also can be applied to the determination of selenium in inorganic compounds.

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Résumé On donne un micro-procédé de dosage du sélénium dans les composés organiques pour lesquels la mise en solution réussit par le peroxyde de sodium. Par ce moyen d'attaque, les substances organiques sélénifères sont détruites rapidement et quantitativement et à partir de là le dosage proprement dit du sélénium n'a pu être effectué que par voie gravimétrique mais celui-ci prend beaucoup de temps et, à cause de la forme défavorable de pesée (sélénium élémentaire) ne parait pas utilisable pour un microdosage.A cause de ces remarques une méthode volumétrique paraissait plus convenable. Pour son établissement, il était indispensable que l'on pouvait constater, que le sélénium contenu aussi bien dans les matières organiques que minérales fut complètement transformé en séléniate sous l'action du peroxyde. Pour pouvoir doser volumétriquement le séléniate, on a mis au point un micro-procédé qui repose sur la réaction entre séléniate et iodure en solution chlorhydrique. Il en résulte que pour un atome de sélénium 6 équivalents d'iode sont libérés que l'on titre au thiosulfate. Les matières oxydantes qui pourraient gêner par libération supplémentaire d'iode (au cours de l'attaque au peroxyde: eau oxygénée et iodate résultant) sont réduites de manière convenable auparavant sans que le séléniate ne soit attaqué.Ce procédé permet un dosage rapide et précis du sélénium dans les matières organiques avec des prises d'essai de 1 à 4 mg. En outre, on a pu généraliser au dosage du sélénium dans les matières minérales.

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Mit 1 Abbildung.     

HgCl2(Caf): Co‐crystallization of Mercuric Chloride and Caffeine

Colourless single crystals of the co‐crystallizate of mercuric chloride and caffeine, HgCl₂(Caf), were obtained from an ethanolic solution of mercuric chloride, HgCl₂, and caffeine (Caf) and recrystallized from hot water. The crystal structure (monoclinic, P2₁, Z = 2, a = 398.36(8), b = 1964.5(4), c = 809.6(2) pm, β = 99.24(3)°, Z = 2, R₁ = 0.0584 for 1430 F_o > 4σ(F_o)) contains helical chains (parallel to the 2₁ screw axis) of almost unaffected HgCl₂ molecules and caffeine molecules which are very weakly bound to one keto‐oxygen atom (O4) of one and N9 of a second caffeine molecule at distances of 282 and 281 pm, respectively. To the contrary, theoretical calculations show that the molecule HgCl₂(Caf)₂ is stable (in the gas phase at T = 0 K) with surprisingly strong bonding as indicated by the “tetrahedrization” of the molecule.     

Enantioselective synthesis of non-natural aromatic alpha-amino acids

We present two complementary methods for the stereoselective synthesis of non-natural alpha-amino acids with aromatic or heteroaromatic side chains. One approach is based on the chemical transformation of methionine, whereas the other applies the stereoselective Myers alkylation of glycine. The resulting product types differ in the linker length between glycine and the aromatic substituent. Since methionine and pseudoephedrine are available in both absolute configurations, R- or S-configured enantiopure amino acids with either C(2) or C(3) linkers can be obtained on gram scales. In each case the key step of the synthesis is hydroboration of the unsaturated building blocks 9 and 17, followed by palladium-catalyzed Suzuki cross-coupling with aryl halides. Attention must in certain cases be paid to the stereochemical integrity when basic Suzuki conditions are applied. Our initial difficulties are reported as well as the final "racemization-proof" procedures. The protecting groups chosen for the alpha-amino acids should be compatible with solid-phase peptide synthesis. This was confirmed by the successful synthesis of a series of tripeptides.     

Facile migratory insertion of a N-heterocyclic carbene into a ruthenium-carbon double bond: a new type of reaction of a NHC ligand

The reaction of [RuCp(IPri)(CH3CN)2]PF6 (IPri = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) with HCCR (R = COOMe, COOEt, COMe) yields the allyl carbene complexes [RuCp(=C(R)-eta3-CHC(R)CH-IPri)]PF6. This conversion involves selective head-to-tail coupling of two alkynes and an unusual migratory insertion of the N-heterocyclic carbene into the ruthenium-carbon double bond of a ruthenacyclopentatriene intermediate.     

Randomizing the Unfolded State of Peptides (and Proteins) by Nearest Neighbor Interactions between Unlike Residues

To explore the influence of nearest neighbors on conformational biases in unfolded peptides, we combined vibrational and 2D NMR spectroscopy to obtain the conformational distributions of selected “GxyG” host–guest peptides in aqueous solution: GDyG, GSyG, GxLG, GxVG, where x/y=A, K, L, V. Large changes of conformational propensities were observed due to nearest‐neighbor interactions, at variance with the isolated pair hypothesis. We found that protonated aspartic acid and serine lose their above‐the‐average preference for turn‐like structures in favor of polyproline II (pPII) populations in the presence of neighbors with bulky side chains. Such residues also decrease the above‐the‐average pPII preference of alanine. These observations suggest that the underlying mechanism involves a disruption of the hydration shell. Thermodynamic analysis of ³J(H^N,H^α) (T) data for each x,y residue reveals that modest changes in the conformational ensemble masks larger changes of enthalpy and entropy governing the pPII?β equilibrium indicating a significant residue dependent temperature dependence of the peptides’ conformational ensembles. These results suggest that nearest‐neighbor interactions between unlike residues act as conformational randomizers close to the enthalpy–entropy compensation temperature, eliminating intrinsic biases in favor of largely balanced pPII/β dominated ensembles at physiological temperatures.     

Stereocontrol in Dinuclear Triple Lithium‐Bridged Titanium(IV) Complexes: Solving Some Stereochemical Mysteries

Compounds 1 a – f ‐H₂ form “monomeric” triscatecholate titanium(IV) complexes [Ti( 1 a – f )₃]^2?, which in the presence of Li cations are in equilibrium with the triple lithium‐bridged “dimers” [Li₃(Ti( 1 a – f )₃)₂]^?. The equilibrium strongly depends on the donor ability of the solvent. Usually, in solvents with high donor ability, the stereochemically labile monomer is preferred, whereas in nondonor solvents, the dimer is the major species. In the latter, the stereochemistry at the complex units is “locked”. The configuration at the titanium(IV) triscatecholates is influenced by addition of chiral ammonium countercations. In this case, the induced stereochemical information at the monomer is transferred to the dimer. Alternatively, the configuration at the metal complexes can be controlled by enantiomerically pure ester side chains. Due to the different orientation of the ester groups in the monomer or dimer, opposite configurations of the triscatecholates were observed by circular dichroism (CD) spectroscopy for [Ti( 1 c – e )₃]^2? or [Li₃(Ti( 1 c – e )₃)₂]^?. A surprising exception was found for the dimer [Li₃(Ti( 1 f )₃)₂]^?. Herein, the dimer is the dominating species in weak donor (methanol), as well as strong donor (DMSO), solvents. This is due to the bulkiness of the ester substituent destabilizing the monomer. Due to the size of the substituent in [Li₃(Ti( 1 f )₃)₂]^? the esters have to adopt an unusual conformation in the dimer resulting in a stereocontrol of the small methyl group. Following this, opposite stereocontrol mechanisms were observed for the central metal‐complex units of [Li₃(Ti( 1 c – e )₃)₂]^? or [Li₃(Ti( 1 f )₃)₂]^?.     

Highly Luminescent and Color‐Tunable Salicylate Ionic Liquids

High quantum yields of up to 40.5 % can be achieved in salicylate‐bearing ionic liquids. A range of these ionic liquids have been synthesized and their photoluminescent properties studied in detail. The differences noted can be related back to the structure of the ionic liquid cation and possible interionic interactions. It is found that shifts of emission, particularly in the pyridinium‐based ionic liquids, can be related to cation–anion pairing interactions. Facile and controlled emission color mixing is demonstrated through combining different ILs, with emission colors ranging from blue to yellow.     

Protease Probes that Enable Excimer Signaling upon Scission

Peptide‐based probes that fluoresce upon proteolytic cleavage are invaluable tools for monitoring protease activity. The read‐out of protease activity through pyrene excimer signaling would be a valuable asset because the large Stokes shift and the long lifetime of the excimer emission facilitate measurements in autofluorescent media such as blood serum. However, proteolytic cleavage abolishes rather than installs the proximity relationships required for excimer signaling. Herein, we introduce a new probe architecture to enable the switching on of pyrene excimer emission upon proteolytic scission. The method relies on hairpin‐structured peptide nucleic acid (PNA)/peptide hybrids with pyrene units and anthraquinone‐based quencher residues positioned in a zipper‐like arrangement within the PNA stem. The excimer hairpin peptide beacons afforded up to a 50‐fold enhancement of the pyrene excimer emission. Time‐resolved measurements allowed the detection of matrix metalloprotease 7 in human blood serum.