Crystal structure of tetragonal boron related to α-AlB12

Single crystals of the so-called β-tetragonal (or tetragonal II or III) boron modification have been obtained from boron deposits prepared by hydrogen reduction of BBr₃ on tantalum filaments at 1200°C. Chemical analysis of the samples shows that this phase can be regarded as a true modification of pure elemental boron in contrast to α-tetragonal phases which require small amounts of foreign atoms to stabilize their boron framework.The lattice parameters (a = 10.14(1)Å; c = 14.17(1)Å) were obtained and refined from single crystal data. The unit cell contains four chemical units, B₂₁ · 2B₁₂ · B_2.5 resulting in d_c = 2.34 g cm^?3 (d_m = 2.36(2) g cm^?3). The systematic extinctions are compatible with space group P4₁ or P4₃.The structure was determined from 1009 independent reflexions using a model derived from the recently solved structure of α - AlB₁₂ (a = 10.161Å; c = 14.283Å; space group P4₁2₁2 or P4₃2₁2). The final R value (unweighted data) is 9.6%.Basically, the structure of this tetragonal form of boron consists of the same three-dimensional boron skeleton, built upon simple and twinned icosahedra, as that of α-AlB₁₂. However, the defective twinned icosahedral B₁₉ units in α-AlB₁₂ are now completed (B₂₁ units) in the related tetragonal boron. A number of interstitial sites, located at positions different from those occupied by aluminum in α-AlB₁₂, are totally or partially filled by boron atoms and very probably increase the stability of the boron framework.     

Carbocyanine dyes as efficient reversible single-molecule optical switch

We demonstrate that commercially available unmodified carbocyanine dyes such as Cy5 (usually excited at 633 nm) can be used as efficient reversible single-molecule optical switch, whose fluorescent state after apparent photobleaching can be restored at room temperature upon irradiation at shorter wavelengths. Ensemble photobleaching and recovery experiments of Cy5 in aqueous solution irradiating first at 633 nm, then at 337, 488, or 532 nm, demonstrate that restoration of absorption and fluorescence strongly depends on efficient oxygen removal and the addition of the triplet quencher beta-mercaptoethylamine. Single-molecule fluorescence experiments show that individual immobilized Cy5 molecules can be switched optically in milliseconds by applying alternating excitation at 633 and 488 nm between a fluorescent and nonfluorescent state up to 100 times with a reliability of >90% at room temperature. Because of their intriguing performance, carbocyanine dyes volunteer as a simple alternative for ultrahigh-density optical data storage. Measurements on single donor/acceptor (tetramethylrhodamine/Cy5) labeled oligonucleotides point out that the described light-driven switching behavior imposes fundamental limitations on the use of carbocyanine dyes as energy transfer acceptors for the study of biological processes.     

“Badge-type” passive sampler for monitoring ambient ammonia concentrations

A passive “badge-type” sampling device for the determination of gaseous ammonia was developed. The collection substrate is phosphoric acid. The sampler can be used for outdoor and indoor sampling of ammonia in the concentration range from 0.05 μg/m³ to 10 mg/m³. The performance was tested in the laboratory and in the field against an annualar denuder, a filter pack and an impinger technique. The intercalibration showed that the passive sampler compares very well with active samplers (r²=0.99; k=1.05). The average reproducibility of the sampler was 8%. Hence the badge sampler is well suited for the determination of ammonia in a wide range of concentrations and particularly for application under rural background conditions. The sampling rate of the device was calculated according to a simple multi-layer model.     

Integrable multicomponent perfect fluid multidimensional cosmology. I

Multidimensional cosmological models with a space-time consisting ofn (n 2) Einstein spaces are investigated for a special class of multicomponent perfect fluid as a matter source. The dynamical behaviour of the universe is described. In the case of static internal spaces the external space evolves like a Friedmann universe with changing effective equation of state. Some of the models considered are integrable and classical as well as quantum solutions are found. Some of them represent wormholes. Quantum wormholes have a discrete spectrum.     

Direct coupling of a flow–flow electromembrane extraction probe to LC-MS

A fully integrated and automated electromembrane extraction LC-MS (EME-LC-MS) system has been developed and characterized. Hyphenation of a flow–flow EME probe to LC-MS was accomplished by using an in-built 10-port switching valve of the LC-MS system. The 10-port switching valve decoupled the high pressure of the UHPLC-system from the low pressure required for operation of the EME-probe by automated switching between a sample extraction/analysis and a sample load position. In the sample load position the extracted analytes were loaded into a HPLC sample loop. By switching the valve to the sample extraction/analysis position the setup allowed simultaneous analysis of previously loaded analytes while extracting a new sample. Performance of the system was characterized with respect to precision and linearity (RSD < 2.5%, R²: 0.998) and the setup was applied for studying the in-vitro metabolism of methadone by rat liver microsomes. As the metabolic reaction proceeded, methadone and its metabolites were extracted and analyzed in parallel by LC-MS using either isocratic or gradient elution. Compared to a conventional in-vitro metabolism analysis based on protein precipitation followed by LC-MS analysis the fully automated EME-LC-MS system offers a significant time saving and in addition demonstrates increased sensitivity as the analytes were automatically enriched during the extraction process. The experiment revealed 6 to 16 times higher S/N ratios of the EME-LC-MS method compared to protein precipitation followed by LC-MS and thus concomitantly lower LOD and LOQ. The setup integrates a complete analytical workflow of rapid extraction, enrichment, separation and detection of analytes in a fully automated manner. These attributes make the developed system a powerful alternative approach for a wide range of analytical applications.     

A simple sheathless CE-MS interface with a sub-micrometer electrical contact fracture for sensitive analysis of peptide and protein samples

Online coupling of capillary electrophoresis (CE) to electrospray ionization mass spectrometry (MS) has shown considerable potential, however, technical challenges have limited its use. In this study, we have developed a simple and sensitive sheathless CE-MS interface based on the novel concept of forming a sub-micrometer fracture directly in the capillary. The simple interface design allowed the generation of a stable ESI spray capable of ionization at low nanoliter flow-rates (45–90 nL/min) for high sensitivity MS analysis of challenging samples like those containing proteins and peptides. By analysis of a model peptide (leucine enkephalin), a limit of detection (LOD) of 0.045 pmol/μL (corresponding to 67 attomol in a sample volume of ∼15 nL) was obtained. The merit of the CE-MS approach was demonstrated by analysis of bovine serum albumin (BSA) tryptic peptides. A well-resolved separation profile was achieved and comparable sequence coverage was obtained by the CE-MS method (73%) compared to a representative UPLC-MS method (77%). The CE-MS interface was subsequently used to analyse a more complex sample of pharmaceutically relevant human proteins including insulin, tissue factor and α-synuclein. Efficient separation and protein ESI mass spectra of adequate quality could be achieved using only a small amount of sample (30 fmol). In addition, analysis of ubiquitin samples under both native and denatured conditions, indicate that the CE-MS setup can facilitate native MS applications to probe the conformational properties of proteins. Thus, the described CE-MS setup should be useful for a wide range of high-sensitivity applications in protein research.     

Binding of serotonin to the human serotonin transporter. Molecular modeling and experimental validation

Molecular modeling and structure-activity relationship studies were performed to propose a model for binding of the neurotransmitter serotonin (5-HT) to the human serotonin transporter (hSERT). Homology models were constructed using the crystal structure of a bacterial homologue, the leucine transporter from Aquifex aeolicus, as the template and three slightly different sequence alignments. Induced fit docking of 5-HT into hSERT homology models resulted in two different binding modes. Both show a salt bridge between Asp98 and the charged primary amine of 5-HT, and both have the 5-HT C6 position of the indole ring pointing toward Ala173. The difference between the two orientations of 5-HT is an enantiofacial discrimination of the indole ring, resulting in the 5-hydroxyl group of 5-HT being vicinal to either Ser438/Thr439 or Ala169/Ile172/Ala173. To assess the binding experimentally, binding affinities for 5-HT and 17 analogues toward wild type and 13 single point mutants of hSERT were measured using an approach termed paired mutant-ligand analogue complementation (PaMLAC). The proposed ligand-protein interaction was systematically examined by disrupting it through site-directed mutagenesis and re-establishing another interaction via a ligand analogue matching the mutated residue, thereby minimizing the risk of identifying indirect effects. The interactions between Asp98 and the primary amine of 5-HT and the interaction between the C6-position of 5-HT and hSERT position 173 was confirmed using PaMLAC. The measured binding affinities of various mutants and 5-HT analogues allowed for a distinction between the two proposed binding modes of 5-HT and biochemically support the model for 5-HT binding in hSERT where the 5-hydroxyl group is in close proximity to Thr439.     

Hydrogen atom scrambling in selectively labeled anionic peptides upon collisional activation by MALDI tandem time-of-flight mass spectrometry

We have previously shown that peptide amide hydrogens undergo extensive intramolecular migration (i.e., complete hydrogen scrambling) upon collisional activation of protonated peptides (Jørgensen et al. J. Am. Chem. Soc. 2005, 127, 2785–2793). The occurrence of hydrogen scrambling enforces severe limitations on the application of gas-phase fragmentation as a convenient method to obtain information about the site-specific deuterium uptake for proteins and peptides in solution. To investigate whether deprotonated peptides exhibit a lower level of scrambling relative to their protonated counterparts, we have now measured the level of hydrogen scrambling in a deprotonated, selectively labeled peptide using MALDI tandem time-of-flight mass spectrometry. Our results conclusively show that hydrogen scrambling is prevalent in the deprotonated peptide upon collisional activation. The amide hydrogens (¹H/²H) have migrated extensively in the anionic peptide, thereby erasing the original regioselective deuteration pattern obtained in solution.     

Electron-capture—Induced dissociation of protoporphyrin IX ions

Electron-capture induced dissociation of protoporphyrin cations and anions has been studied. The cations captured two electrons in two successive collisions and were converted to the corresponding even-electron anions. About one fifth of the ions lost a hydrogen atom to become radical anions but otherwise very little fragmentation was observed. The anions captured an electron to become dianions. No hydrogen loss occurred, and the only fragmentation channel observed was loss of CO2H, to give a doubly charged carbanion. Our results indicate that protoporphyrin ions are very efficient in accommodating one or even two electrons in the lowest unoccupied molecular orbital of the porphyrin macrocycle, and that electron capture induces only limited dissociation.