Multistep energy transfer in single molecular photonic wires

We demonstrate the synthesis and spectroscopic characterization of an unidirectional photonic wire based on four highly efficient fluorescence energy-transfer steps (FRET) between five spectrally different chromophores covalently attached to double-stranded DNA. The DNA-based modular conception enables the introduction of various chromophores at well-defined positions and arbitrary interchromophore distances. While ensemble fluorescence measurements show overall FRET efficiencies between 15 and 30%, single-molecule spectroscopy performed on four spectrally separated detectors easily uncovers subpopulations that exhibit overall FRET efficiencies of up to approximately 90% across a distance of 13.6 nm and a spectral range of approximately 200 nm. Fluorescence trajectories of individual photonic wires show five different fluorescence intensity patterns which can be ascribed to successive photobleaching events.     

Analysis of spin states, spin barriers, and trans-effects involved in the coordination and stabilization of dinitrogen by biomimetic iron complexes

Coordination of dinitrogen to Sellmann-type iron (II) complexes in a sulfur-dominated coordination sphere, which emulates the environment of iron centers in the FeMo-cofactor of nitrogenase, is analyzed with respect to spin states, spin barriers, and the effect of trans-ligands. Such detailed investigations became only recently feasible when the reliability of density functional methods, which are the only quantum chemical methods capable of describing large transition metal complexes, could significantly be improved for the calculation of energies for states of different spin. It is found that the actual binding energy of dinitrogen is of sufficient magnitude for a reasonably strong fixation of N₂ by Sellmann-type coordination compounds. However, potential fixation is determined by additional factors which reduce the binding energy. One factor is the change in spin state of the N₂-free metal fragment, which lowers the total energy and quenches the thermodynamic stabilization effect of the binding energy. In addition, the metal fragment rearranges and gains even more stabilization energy for the un-coordinated state. Apart from these thermodynamical effects, the existence of spin barriers, which must be overcome upon binding of dinitrogen, leads to kinetical effects, which cannot be neglected.     

Design,synthesis, and evaluation of 6-carboxyalkyl and 6-phosphonoxyalkyl derivatives of 7-oxo-8-ribitylaminolumazines as inhibitors of riboflavin synthase and lumazine synthase

A series of 6-carboxyalkyl and 6-phosphonoxyalkyl derivatives of 7-oxo-8-D-ribityllumazine were synthesized as inhibitors of both Escherichia coli riboflavin synthase and Bacillus subtilis lumazine synthase. The compounds were designed to bind to both the ribitylpurine binding site and the phosphate binding site of lumazine synthase. In the carboxyalkyl series, maximum activity against both enzymes was observed with the 3'-carboxypropyl compound 22. Lengthening or shortening the chain linking the carboxyl group to the lumazine by one carbon resulted in decreased activity. In the phosphonoxyalkyl series, the 3'-phosphonoxypropyl compound 33 was more potent than the 4'-phosphonoxybutyl derivative 39 against lumazine synthase, but it was less potent against riboflavin synthase. Molecular modeling suggested that the terminal carboxyl group of 6-(3'-carboxypropyl)-7-oxo-8-D-ribityllumazine (22) may bind to the side chains of Arg127 and Lys135 of the enzyme. A hypothetical molecular model was also constructed for the binding of 6-(2'-carboxyethyl)-7-oxolumazine (15) in the active site of E. coli riboflavin synthase, which demonstrated that the active site could readily accommodate two molecules of the inhibitor.     

Stereoselective synthesis of unnatural α-amino acid derivatives through photoredox catalysis

A protocol for stereoselective C-radical addition to a chiral glyoxylate-derived N-sulfinyl imine was developed through visible light-promoted photoredox catalysis, providing a convenient method for the synthesis of unnatural α-amino acids. The developed protocol allows the use of ubiquitous carboxylic acids as radical precursors without prior derivatization. The protocol utilizes near-stoichiometric amounts of the imine and the acid radical precursor in combination with a catalytic amount of an organic acridinium-based photocatalyst. Alternative mechanisms for the developed transformation are discussed and corroborated by experimental and computational studies.

A protocol for stereoselective C-radical addition to a chiral glyoxylate-derived N-sulfinyl imine was developed through visible light-promoted photoredox catalysis, providing a convenient method for the synthesis of unnatural α-amino acids.     

Neptunium determination by inductively coupled plasma mass spectrometry (ICP-MS)

The determination of neptunium-237 (²³⁷Np) traditionally has been performed by alpha spectrometry or neutron activation analysis. These methods are labor intensive and require several days for completion. Inductively Coupled Plasma Mass Spectrometry (ICP-MS) is a possible alternative for²³⁷Np determinations. This paper describes the analytical method developed for samples that have significant levels of uranium present. The lower reporting limits achievable by ICP-MS are competitive with the counting methods, but the real advantage for this laboratory lies in the lower cost and faster turnaround time provided by ICP-MS.     

Complex formation of isocytosine tautomers with PdII and PtII

Isocytosine (ICH) exists in solution as two major tautomers, the keto form with N1 carrying a proton (1a) and the keto form with N3 being protonated (1b). In water, 1a and 1b exist in equilibrium with almost equal amounts of both forms present. Reactions with a series of Pd(II) and Pt(II) am(m)ine species such as (dien)Pd(II), (dien)Pt(II), and trans-(NH(3))(2)Pt(II) reveal, however, a distinct preference of these metals for the N3 site, as determined by (1)H NMR spectroscopy. Individual species have been identified by the pD dependence of the ICH resonances. pK(a) values (calculated for H(2)O) for deprotonation of the individual tautomers complexes are 6.5 and 6.4 for the N3 linkage isomers of dienPd(II) and dienPt(II), respectively, as well as 6.2 and 6.0 for the N1 linkage isomers. The dimetalated species [(dienM)(2)(IC-N1,N3)](3+) (M = Pd(II) or Pt(II)) are insensitive over a wide range of pD. The crystal structure analysis of [(dien)Pd(ICH-N3)](NO(3))(2) is reported. Ab initio calculations have been performed for tautomer compounds of composition [(NH(3))(3)Pt(ICH)](2+), cis- and trans-[(NH(3))(2)PtCl(ICH)](+), as well as trans-[(NH(3))(2)Pt(ICH)(2)](2+). Without exception, N3 linkage isomers are more stable, in agreement with experimental findings. As to the reasons for this binding preference, an NBO (natural bond orbital) analysis for [(NH(3))(3)Pt(ICH-N3)](2+)strongly suggests that intramolecular hydrogen bonding between trans-positioned NH(3) ligands and the two exocyclic groups of the ICH is of prime importance. The calculations furthermore show a marked pyramidalization of the NH(2) group of ICH in the complex once the heterocyclic ligand forms a dihedral angle <90 degrees with the Pt coordination plane.     

Untersuchungen an Alkylmetallalkoxiden der Elemente Aluminium,Gallium und Indium. III. Eigenschaften und Schwingungsspektren von Dialkylgallium- und Indiumalkoxiden

Studies on Alkyl Metal Alkoxides of Aluminium, Gallium, and Indium. III. Properties and Vibrational Spectra of Dialkylgallium- and Indium Alkoxides The preparation and the properties of dialkyl metal alkoxides R₂MOR′ (with R = CH₃, CD₃, C₂H₅; R′ = CH₃, CD₃ and M = Ga, In) are described. The vibrational data (IR and Raman) of the trimeric methyl derivatives point to puckered six-membered M₃O₃-ring systems.     

Analysis of saccharides in beer samples by flow injection with electrospray mass spectrometry

Saccharides in foods play important roles, as they are essential substrates for fermentation processes. In brewing, the concentration of maltooligosaccharides influences the characteristics of beers and therefore their determination is of great practical interest. Electrospray ionization mass spectrometry (ESI-MS) was applied to identify and characterise maltooligosaccharides in beer samples. The effects due to different cation concentrations and dilution of samples were studied. Furthermore, quantitative analyses of maltooligosaccharides by means of flow-injection ESI-MS (FI/ESI-MS) of 1-microL beer samples (diluted 1000-fold) are described.