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.     

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.     

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.     

Characterization of CNS disorders and evaluation of therapy using structural and functional MRI

Modern drug development requires technologies that allow rapid translation from the preclinical to the clinical stage. It is obvious that non-invasive imaging modalities such as magnetic resonance imaging (MRI) will play a central role in this regard. This article reviews the use of structural and functional MRI readouts for characterization of central nervous system (CNS) disorders and evaluation of the efficacy of potential CNS drugs. Examples comprise dementia of Alzheimer's type, cerebral ischemia, and neuroinflammation covering both clinical and preclinical aspects. In these examples MRI has been used to obtain relevant structural information on brain atrophy, on the location and extent of ischemic brain areas, and on the number and distribution of demyelinated plaques. These structural data are complemented by readouts assessing the functional consequences associated with the pathomorphological changes. In the last decade, MRI has evolved into a standard tool for the development of CNS drugs. With regard to target-specific/molecular imaging applications MRI is limited by its inherently low sensitivity; complementary imaging modalities utilizing optical and radionuclear reporter systems will thus be required.     

Über Chalkogenidhalogenide des Chroms Synthese,Kristallstruktur und Magnetismus von Chromsulfidbromid,CrSBr

Chalkogenidehalides of Chromium. Synthesis, Crystal Structure, and Magnetism of Chromiumsulfidebromide, CrSBr CrSBr is obtained from chromium metal and S₂Br₂ in a sealed quartz ampoule at 880°C. It forms air stable, black crystals. The crystal structure determination (space group Pmmn, lattice constants a = 476.7(2), b = 350.6(2), c = 796.5(4) pm, Z = 2, R = 0.026) shows, that CrSBr crystallizes in the FeOCl structure type. The structure consists of layers which are stacked perpendicular to the c axis. The layers are formed by distorted, edge sharing CrS₄Br₂ octahedra. The interatomic distances are Cr? S 239.7 and 241.5 pm, Cr? Br 249.4 pm. To explain the unusual temperature factor of the Br atom the structure determination was additionaly carried out at 205 K and 118 K. A linear decrease of the coefficients of the anisotropic temperature factors of all atoms was found. The coefficients can be extrapolated to zero for 0 K. This shows the large displacement parameter U₁₁ of the Br atom to be caused by thermal vibrations. Even under forced conditions CrSBr does not form intercalation compounds with pyridine or tetracyanoethylene. CrSBr shows a marked antiferromagnetic behavior with a Néel temperature of 132 K and a critical field of 0.35 Tesla at 4.2 K.     

Expedient Diels-Alder assembly of 4-aryl-4-phenylsulfonyl cyclohexanones

The efficient preparation of 4-aryl-4-phenylsulfonyl cyclohexanones, containing a quaternary sulfone-bearing carbon centre, is described. Their synthesis proceeds in 38-78% overall yield by way of three steps: (i) sulfinate alkylation; (ii) methylenation; and (iii) regioselective Diels-Alder condensation with 2-trimethylsiloxybutadiene. The scope and limitations of the one-pot Mannich-type methylenation described were examined.     

Kinetic studies on the deaquation-anation of aquapentammine-cobalt(III) chloride

Summary The deaquation-anation of solid aquapentamminecobalt(III) chloride was studied isothermally and non-isothermally. Kinetic data were obtained from t.g.a. and were analysed using 17 rate laws known for solid state reactions. This reaction, long interpreted as S_N2, was found to obey an A1.5 rate law from both types of experiments. From the isothermal experiments, an E _a of 97.0 kJ mol^-1 was found.     

The Reactivity of Bis(para‐methoxyphenyl)telluroxide towards Triflic Acid and Diphenylphosphinic Acid. Theoretical Considerations of the Protonation and Hydration Process of Diorganotelluroxanes

The reaction of (p‐MeOC₆H₄)₂TeO with two equivalents of HO₃SCF₃ and HO₂PPh₂ provided the tetraorganoditelluroxanes (F₃CSO₃)(p‐MeOC₆H₄)₂TeOTe(p‐MeOC₆H₄)₂(O₃SCF₃) ( 1 ) and (Ph₂PO₂)(p‐MeOC₆H₄)₂TeOTe(p‐MeOC₆H₄)₂(O₂PPh₂)·2 Ph₂PO₂H ( 2 ) in good yields. Compounds 1 and 2 were characterized by solution and solid‐state ³¹P and ¹²⁵Te NMR spectroscopy, IR spectroscopy, electrospray mass spectrometry, conductivity measurements and single crystal X‐ray diffraction. In solution, compound 1 undergoes an electrolytic dissociation and reversibly reacts with traces of water to give the mononuclear cation [(p‐MeOC₆H₄)₂TeOH]⁺ and triflate anions. Theoretical aspects of the protonation and hydration of model telluroxanes R₂TeO (R = H, Me, Ph) were investigated by preliminary DFT calculations and compared to the corresponding selenoxanes R₂SeO. The tellurium dihydroxides R₂Te(OH)₂ seem to be more stable than the hydrogen‐bonded complexes R₂TeO·H₂O.