Evolution of Organized Lariat Ether Alcohol and Diol Hydrate Macrostructures in the Solid State

Lariat ether alcohols and diols derived from dibenzo-14-crown-4crystallize in the anhydrous form or as hydrogen-bondedmonohydrates or networks depending upon thepolyether ring substitutents and the composition of the aqueous organic crystallization solvent. Supramolecular structures are created when the lariat ether alcohol sym-(hydroxy)dibenzo-14-crown-4 (1) with ``sticky ends' of ether oxygens and hydroxyl protons, hydrogen bond to each other via bridging water molecules to form three-dimensional networks. For crystallization of 1 from 30% aqueous acetonitrile, this self-association is sufficiently strong to spontaneously generate a supramolecular structure with water channels.     

FLP-type nitrile activation and cyclic ether ring-opening by halo-borane nonagermanide-cluster Lewis acid–base pairs

Even though homoatomic nine-atom germanium clusters are known for two decades, their chemical properties are still rarely investigated. We now discovered that Zintl ion main group-element clusters possess a reactive lone pair of electrons, and we show a new pathway to bind ligands with functional groups to the [Ge₉] cluster core through Ge–C bond formation. We report on the reactivity of [Ge₉{Si(TMS)₃}₂]²⁻ (TMS = trimethylsilyl) towards a series of Lewis acidic bromo-boranes. The reaction of [Ge₉{Si(TMS)₃}₂]²⁻ and DAB^o-tol–Br (DAB = 1,3,2-diazaborolidine; o-tol = 2-methylphenyl) resulted, depending on the reaction protocol, either in the formation of [Ge₉{Si(TMS)₃}₂DAB^o-tol]⁻ (1a) with direct Ge–B interactions, or in [Ge₉{Si(TMS)₃}₂(CH₂)₄O–DAB^o-tol]⁻ (2a) featuring a ring-opened thf moiety. Ring opening reactions occur for all bulkier DAB^R–Br [R: o-xyl (2,6-dimethylphenyl), Mes (2,4,6-trimethylphenyl), Dipp (2,6-diisopropylphenyl)], DAB(ii)^Dipp–Br and acyclic (ⁱPr₂N)₂BBr without Ge–B bond formation as shown for the structural characterization of the ring-opened products of thf (3, 4) and trimethylene oxide (5). In contrast to thf, the activation of CH₃CN requires the simultaneous presence of Lewis-acid and Lewis-basic reactants allowing the formation of [Ge₉{Si(TMS)₃}₂CH₃C N–DAB^Mes]⁻ (6a). Within the presented compounds, 3 and 4 show an unusual substitution pattern of the three ligands at the [Ge₉] core in the solid state. The [Ge₉] cluster/borane systems correspond to intermolecular frustrated Lewis pairs (FLPs), in which the [Ge₉] cluster with several lone pairs represents the Lewis base, and the borane is the Lewis acid.

The reactivity of the lone pairs in polyhedral Zintl anions is shown by the reaction of the bis-silylated cluster [Ge₉{Si(TMS)₃}₂]²⁻ accomplishing cyclic-ether ring-opening or nitrile activation according to a FLP-like mechanism with bromo-boranes.     

Das unterschiedliche massenspektrometrische Fragmentierungsverhalten von Lysinmethylester und dessen N,N′-Diacetylderivat. 27. Mitteilung über das massenspektrometrische Verhalten von Stickstoffverbindungen

The Different Behaviour of Lysine Methyl Ester and its N,N′-Diacetyl Derivative under Electron Impact The base peak in the spectrum of lysine methyl ester is due to the fragment ion C₅H₁₀N (m/e 84), for which the cyclic structure g (Scheme 1) is deduced. During its formation from the [M-COOCH₃]-ion an equilibration of both nitrogen atoms takes place (ion c , Scheme 1). The cyclic nature of ion m/e 84 is in agreement with the intensity of the corresponding ions in the spectra of homologues of lysine methyl ester (Fig. 1). Although in comparison with lysine methyl ester ( 1 ) N,N′-diacetyl-lysine methyl ester ( 7 ) shows the same general fragmentation pathway with formation of the ions [M-COOCH₃] and [M-COOCH₃-H₂NCOCH₃] (m/e 126), the exact fragmentation mechanism proves to the different. Two mechanisms are discussed for the formation of the ion m/e 126 from 7 (Schemes 2 and 3). The results are based on the spectra of labelled derivatives.     

Syntheses of thyrotropin-releasing hormones (TRH) (Schaf) and related peptides

The syntheses of seven tripeptide isomers containing L -histidine, L -proline and L -glutamic acid residues, the same as found in the natural thyrotropin-releasing hormone (TRH), are reported. In addition L -pyroglutamyl-L -histidyl-L -proline and its amide as well as N^α-acetyl-L -glutamyl-L -histidyl-L -proline are described. Whereas eight peptides are inactive and L -pyroglutamyl-L -histidyl-L -proline shows a slight TRH activity, L -pyroglutamyl-L -histidyl-L -proline-amide has the full biological activity of the isolated thyrotropin-releasing hormone and, at the present state of knowledge, seems to be identical with it.     

FREE RADICALS FROM PHOTOREDUCTION OF HEMATOPORPHYRIN IN AQUEOUS SOLUTION

—The anaerobic photoreduction of hematoporphyrin by organic reducing agents in fluid aqueous solutions has been studied by electron spin resonance spectroscopy. Efficient photoreduction occurs with many hydrogen-atom or electron donors, including catechols, pyrogallol. hydroquinone. ascorbate, p -phenylenediamine, cysteine and glutathione. A combination of direct and spin-trapping measurements (using 2-methyl-2-nitrosopropane) has been used to confirm production of radicals from both the porphyrin and the reducing agent in each system.     

IspH protein of Escherichia coli: studies on iron-sulfur cluster implementation and catalysis

The ispH gene of Escherichia coli specifies an enzyme catalyzing the conversion of 1-hydroxy-2-methyl-2-(E)-butenyl diphosphate into a mixture of isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP) in the nonmevalonate isoprenoid biosynthesis pathway. The implementation of a gene cassette directing the overexpression of the isc operon involved in the assembly of iron-sulfur clusters into an Escherichia coli strain engineered for ispH gene expression increased the catalytic activity of IspH protein anaerobically purified from this strain by a factor of at least 200. For maximum catalytic activity, flavodoxin and flavodoxin reductase were required in molar concentrations of 40 and 12 microM, respectively. EPR experiments as well as optical absorbance indicate the presence of a [3Fe-4S](+) cluster in IspH protein. Among 4 cysteines in total, the 36 kDa protein carries 3 absolutely conserved cysteine residues at the amino acid positions 12, 96, and 197. Replacement of any of the conserved cysteine residues reduced the catalytic activity by a factor of more than 70 000.     

On the importance of carbohydrate-aromatic interactions for the molecular recognition of oligosaccharides by proteins: NMR studies of the structure and binding affinity of AcAMP2-like peptides with non-natural naphthyl and fluoroaromatic residues

The specific interaction of a variety of modified hevein domains to chitooligosaccharides has been studied by NMR spectroscopy in order to assess the importance of aromatic-carbohydrate interactions for the molecular recognition of neutral sugars. These mutant AcAMP2-like peptides, which have 4-fluoro-phenylalanine, tryptophan, or 2-naphthylalanine at the key interacting positions, have been prepared by solid-phase synthesis. Their three-dimensional structures, when bound to the chitin-derived trisaccharide, have been deduced by NMR spectroscopy. By using DYANA and restrained molecular dynamics simulations with the AMBER 5.0 force field, the three-dimensional structures of the protein-sugar complexes have been obtained. The thermodynamic analysis of the interactions that occur upon complex formation have also been carried out. Regarding binding affinity, the obtained data have permitted the deduction that the larger the aromatic group, the higher the association constant and the binding enthalpy. In all cases, entropy opposes binding. In contrast, deactivation of the aromatic rings by attaching fluorine atoms decreases the binding affinity, with a concomitant decrease in enthalpy. The role of the chemical nature of the aromatic ring for establishing sugar contacts has been thus evaluated.     

Synthesis and selectivity of sym-hydroxydibenzo-14crown-4 ionophores for protons,alkali metal cations,and alkaline earth cations in polymeric membranes

Six derivatives of sym-hydroxydibenzo-14-crown-4 have been prepared and incorporated into solvent-polymeric membranes. Responses of the membranes to protons, alkali metal cations, and alkaline earth cations have been determined. The preferred uptake of protons is attributed to proton complexation by stable crown ether alcohol and diol monohydrate species.     

Eine neue Version der Epoxyketon → Alkinon-Fragmentierung: Thermischer Zerfall der Hydrazone aus α,β-Epoxycarbonyl-verbindungen und N-Amino-aziridinen. Vorläufige Mitteilung

A new version of the epoxyketone → alkynone fragmentation is described. Hydrazones derived from cyclic α,β-epoxy-ketones and N-amino-aziridines decompose thermally into molecular nitrogen, an olefinic fragment and an acetylenic carbonyl compound (compare scheme 1). The reaction of phenylglyoxal with N-amino-phenyl-aziridine at room temperature produces styrene and diazo-acetophenone in high yield (compare scheme 3).