Doubly Protonated Benzene in the Gas Phase^†

Structural aspects and the unimolecular fragmentations of doubly protonated benzene are studied by means of tandem-mass spectrometry. The corresponding dications are generated by electron ionization (EI) of 1,3- and 1,4-cyclohexadienes, respectively. It is suggested that EI of 1,3-cyclohexadiene leads to the singlet state of doubly protonated benzene, whereas EI of 1,4-cyclohexadiene yields a mixture of singlet and triplet states. Unimolecular fragmentation of doubly protonated benzene exclusively proceeds via dehydrogenation leading to the benzene dication. The proton affinities (PAs) of protonated benzene amount to PA(C6H7+)meta = 1.9 ± 0.3 eV for protonation taking place at the meta-position, PA(C6H7+)ortho = 1.5 ± 0.2 eV, and PA(C6H7+)para = 0.9 ± 0.2 eV, respectively. Various facets of the experiments are compared with density functional theory calculations and generally good agreement is found

Chirality in Copper Nanoalloy Clusters

It is shown that chirality is common in bimetallic clusters. Specifically, a detailed computational study of two copper clusters, Cu_n⁺ (<i>n</i> = 9,11), demonstrates that exchange of one copper atom with another metal atom (Ni, Zn, Ag, or Au) at various locations, leads, in most cases, to chirality in the a priori achiral cluster (<i>n</i> = 9) and always preserves it in the a priori chiral one (<i>n</i> = 11). Chirality was evaluated on a quantitative level, employing the Continuous Chirality Measure methodology, in two versions: a purely geometric structure analysis, and an analysis which takes into account the different nature of the atoms. Physical aspects of chirality were demonstrated by emergence of vibrational circular dichroism signals and by the emergence of parity violation (PV) energy difference, which is calculated by employing a quasi-relativistic approach. In the case of AgCu₁₀⁺(p9), the PV energy splitting value is about ∼10^–15 Hartree, bringing this nanoalloy close to the range of systems that have been discussed as promising candidates for a measurement of this phenomenon on the molecular level

Biosynthesis of Guatrypmethine C Implies Two Different Oxidases for <i>exo</i> Double Bond Installation at the Diketopiperazine Ring

Heterologous expression of a silent gene cluster from Streptomyces cinnamoneus led to identification of guatrypmethine C, a guaninylated cyclo-l-Trp-l-Met derivative with two exo double bonds at the diketopiperazine ring. Structural elucidation was achieved using MS, 1H, 13C, and 15N NMR analyses including 1H–13C and 1H–15N HMBC. Gene combination and biochemical investigation proved that the formation of the two CC bonds are catalyzed by two distinct enzyme families (i.e., cyclodipeptide oxidase and FeII/2OG-dependent oxygenase)

Pseudotetrahedral Polyhalocubanes: Synthesis, Structures, and Parity Violating Energy Differences

All possible pseudotetrahedral, stable polyhalocubanes were prepared, and their structures were proven by NMR spectroscopy and X-ray crystallography. Parity violating energy differences (ΔEpv) and vibrational frequency shifts were computed. The ΔEpv values are predicted to be one to two orders of magnitude smaller than those for the corresponding polyhalomethanes. However, the ΔEpv energy ordering is the same as that for the methane analogues. For both substance classes, the (S) isomers are, with the exception of the bromochlorofluoroiodo derivatives, more stable than the (R) forms

Additional file 2: Figure S2. of Larval superiority of Culex pipiens to Aedes albopictus in a replacement series experiment: prospects for coexistence in Germany

Pupal size. Abdominal length [AL, mm, mean ± SD] of female Ae. albopictus and Cx. pipiens in pure and mixed cohorts in dependence of temperature and larval food regime. Line - linear regression. Dotted lines - 95% confidence interval. (PPTX 558 kb

Combined Experimental and Theoretical Study of the Vibronic Spectra of Perylenecarboximides

Absorption and emission spectra of perylene-3,4-dicarboximide (PMI) and perylene-3,4,9,10-tetracarboxdiimide (PDI) derivatives embedded in a thin polymer film were measured by room-temperature bulk and low-temperature single-molecule spectroscopy. In contrast to bulk line narrowing spectra, the low-temperature single-molecule data allowed to unambiguously resolve the vibrational fine structure of the emission spectra. Additionally, the emission spectra were calculated by quantum chemical methods within the Franck−Condon approximation for various N-substituted derivatives of PMI and PDI. The experimental as well as calculated emission spectra are dominated by two spectral regions of high vibronic activity, a band system ranging from the 0−0 transition (at ΔE0−0) down to 600 cm−1 below ΔE0−0 and a band system between approximately 1250 and 1700 cm−1 below ΔE0−0. Apart from the wavenumber region close to ΔE0−0 (down to 100 cm−1 below ΔE0−0), good agreement is found between the calculated and experimental spectra, allowing a clear-cut assignment of the dominant vibrational modes. There are, however, discrepancies in the intensities in particular for low-frequency vibrational modes. These differences between theory and experiment are tentatively attributed to linear electron−phonon coupling which is completely neglected in the calculations and hindered internal rotation that is not properly accounted for in the harmonic approximation. Furthermore, in the experimental spectra, at the bulk as well as the single-molecule level, significant differences between PMI and PDI are observed which are attributed to stronger interactions with the matrix environment in the case of PMI due to the permanent electric dipole moment of that molecule

A MassQL-Integrated Molecular Networking Approach for the Discovery and Substructure Annotation of Bioactive Cyclic Peptides

The marine sponge-derived fungus Stachylidium bicolor 293 K04 is a prolific producer of specialized metabolites, including certain cyclic tetrapeptides called endolides, which are characterized by the presence of the unusual amino acid N-methyl-3-(3-furyl)-alanine. This rare feature can be used as bait to detect new endolide-like analogs through customized fragment pattern searches of tandem mass spectrometry data using the Mass Spec Query Language (MassQL). Here, we integrate endolide-specific MassQL queries with molecular networking to obtain substructural information guiding the targeted isolation and structure elucidation of the new proline-containing endolides E (1) and F (2). We showed that endolide F (but not E) is a moderate antagonist of the arginine vasopressin V1A receptor, a member of the G protein-coupled receptor superfamily

Metastable Ni₇B₃: A New Paramagnetic Boride from Solution Chemistry, Its Crystal Structure and Magnetic Properties

We trapped an unknown metastable boride by applying low-temperature solution synthesis. Single-phase nickel boride, Ni₇B₃, was obtained as bulk samples of microcrystalline powders when annealing the amorphous, nanoscale precipitate that is formed in aqueous solution of nickel chloride upon reaction with sodium tetrahydridoborate. Its crystal structure was solved based on a disordered Th₇Fe₃-type model (hexagonal crystal system, space group <i>P</i>6₃<i>mc</i>, no. 186, <i>a</i> = 696.836(4) pm, <i>c</i> = 439.402(4) pm), using synchrotron X-ray powder data. Magnetic measurements reveal paramagnetism, which is in accordance with quantum chemical calculations. According to high-temperature X-ray diffraction and differential scanning calorimetry this nickel boride phase has a narrow stability window between 300 and 424 °C. It crystallizes at ca. 350 °C, then starts decomposing to form Ni₃B and Ni₂B above 375 °C, and shows an exothermic effect at 424 °C

BOOST -- A Satellite Mission to Test Lorentz Invariance Using High-Performance Optical Frequency References

BOOST (BOOst Symmetry Test) is a proposed satellite mission to search for violations of Lorentz invariance by comparing two optical frequency references. One is based on a long-term stable optical resonator and the other on a hyperfine transition in molecular iodine. This mission will allow to determine several parameters of the standard model extension in the electron sector up to two orders of magnitude better than with the current best experiments. Here, we will give an overview of the mission, the science case and the payload

Bis(arylvinyl)pyrazines, -pyrimidines, and -pyridazines As Imaging Agents for Tau Fibrils and β‑Amyloid Plaques in Alzheimer’s Disease Models

The in vivo diagnosis of Alzheimer’s disease (AD) is of high socioeconomic interest and remains a demanding field of research. The biopathological hallmarks of the disease are extracellular plaques consisting of aggregated β-amyloid peptides (Aβ) and tau protein derived intracellular tangles. Here we report the synthesis and evaluation of fluorescent pyrazine, pyrimidine,and pyridazine derivatives in vitro and in vivo aiming at a tau-based diagnosis of AD. The probes were pre-evaluated on human brain tissue by fluorescence microscopy and were found to label all known disease-related alterations at high contrast and specificity. To quantify the binding affinity, a new thiazine red displacement assay was developed and selected candidates were toxicologically profiled. The application in transgenic mouse models demonstrated bioavailability and brain permeability for one compound. In the course of histological testing, we discovered an AD-related deposition of tau aggregates in the Bowman’s glands of the olfactory epithelium, which holds potential for an endoscopic diagnosis of AD in the olfactory system