Stabilization of a K+-(bis-Cage-Annulated 20-Crown-6) Complex by Bidentate Picrate

Bis-cage-annulated 18-crown-6 and 20-crown-6 macrocyclic ethers (i.e., 1 and 2, respectively) have been synthesized, and their alkali metal picrate extraction profiles have been determined. Host system 1 proved to be a significantly more avid alkali metal cation complexant than 2 and somewhat more avid than 18-crown-6. Both 1 and 18-crown-6 display modest selectivity toward K⁺ and Rb⁺. A stable host–guest complex was prepared by slow evaporation of a CH₂Cl₂–hexane solution of an equimolar mixture of 2 and potassium picrate. The X-ray crystal structure of this complex reveals that picrate anion functions as a bidentate ligand therein. The gas-phase interaction energy between the 2 K⁺ complex and picrate anion was calculated to be ca. –64.9 kcal mol^–1, thereby indicating that participation of picrate anion as an additional bidentate ligand results in significant stabilization of complex 10.     

Energetics of radical transfer in DNA photolyase

Charge separation and radical transfer in DNA photolyase from Escherichia coli is investigated by computing electrostatic free energies from a solution of the Poisson-Boltzmann equation. For the initial charge separation 450 meV are available. According to recent experiments [Aubert et al. Nature 2000, 405, 586-590] the flavin receives an electron from the proximal tryptophan W382, which consequently forms a cationic radical WH(*)(+)382. The radical state is subsequently transferred along the triad W382-W359-W306 of conserved tryptophans. The radical transfer to the intermediate tryptophan W359 is nearly isoenergetic (58 meV uphill); the radical transfer from the intermediate W359 to the distal W306 is 200 meV downhill in energy, funneling and stabilizing the radical state at W306. The resulting cationic radical WH(*)(+)306 is further stabilized by deprotonation, yielding the neutral radical W(*)306, which is 214 meV below WH(*)(+)306. The time scale of the charge recombination process yielding back the resting enzyme with FADH(*) is governed by reprotonation of W306, with a calculated lifetime of 1.2 ms that correlates well with the measured lifetime of 17 ms. In photolyase from Anacystis nidulans the radical state is partially transferred to a tyrosine [Aubert et al. Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 5423-5427]. In photolyase from Escherichia coli, there is a tyrosine (Y464) close to the distal tryptophan W306 that could play this role. We show that this tyrosine cannot be involved in radical transfer, because the electron transfer from tyrosine to W306 is much too endergonic (750 meV) and a direct hydrogen transfer is likely too slow. Coupling of specific charge states of the tryptophan triad with protonation patterns of titratable residues of photolyase is small.     

Cobalt(II) Chloride Complex Formation in Acetamide–Calcium Nitrate Tetrahydrate Melts

The absorption spectra of Co(II) chloride complexes, containing variable concentrations of chloride ligand, in a molten mixture of 80 mol% acetamide–20 mol% calcium nitrate tetrahydrate, were studied at 313, 333, 353, and 363 K, in the wavelength range 400-800 nm. The melt contains three possible ligands (CH₃CONH₂, H₂O, and NO₃ ^-) for competition with added chloride ligand. Addition of chloride caused a shift of the absorption maximum of octahedral cobalt(II) nitrate towards lower energies and pronounced changes in the shape of the initial spectrum of cobalt(II) nitrate. The effect of temperature changes on the molar absorption coefficient of the Co(II) species was dependent on the chloride concentration and was attributed to the structural changes occurring in the cobalt(II) species. The STAR and STAR FA programs were applied to identify the complex ionic species and to calculate the stability constants of Co(II) complexes formed in this solvent. The results indicate the highest probability of formation of the following complex species: Co(NO₃)₄ ^2-, Co(NO₃)₂Cl₂ ^2-, and CoCl₄ ^2-. Stability constants of each complex were presented for the equilibria occurring at 313, 333, 353, and 363 K. Distribution of the Co(II) species was also calculated over the ranges of chloride concentration and temperature investigated.     

Nitrite catalyzes reductive nitrosylation of the water-soluble Ferri-Heme model FeIII(TPPS) to FeII(TPPS)(NO)

Quantitative investigation of the reaction of the ferri-heme model compound Fe(III)(TPPS)(H(2)O)(2) (1) to give Fe(II)(TPPS)(NO) (2) (TPPS = tetra(4-sulfonato-phenyl)porphinato) in buffered aqueous solution demonstrates a slow pH-independent reductive nitrosylation pathway in the pH range 4-6. The rate of this reaction is subject to modest general base catalysis. In the course of this study, a surprising catalytic pathway whereby nitrite ion (NO(2)(-)) strongly catalyzes the reduction of 1 to 2 under reductive nitrosylation conditions was demonstrated.     

Electron microscopy of V2O5 microcrystals in the V2O5-NH3-H2O colloid system

Summary The formation of V₂O₅ fibrous microcrystals in the V₂O₅-NH₃-H₂O system has been investigated by electron microscopy. The morphology of these microcrystals is dependent on the concentration of the particular components in the colloidal system and on the time of aging. In some colloid systems the orientation of V₂O₅ fibrous crystals with their long axes parallel to one another is also possible. The microcrystal growth in these systems can be attributed to very effective conditions for the recrystallization of vanadium pentoxide crystals in equilibrium with ammonium vanadate in solution.With 3 figures     

A simple procedure for the quantitative microdetermination of rhamnose

Summary A simple spectrophotometric procedure for microdetermination of rhamnose is described. It is based on breakdown of the sugar with sulphuric acid followed by treatment withp-hydroxydiphenyl. The coloured product is measured spectrophotometrically at 560 nm. A linear relationship exists between the amount of rhamnose (10-0g–) and the absorbance. The method is recommended for determination of rhamnose in samples containing rhamnose, different neutral sugars, amino-sugars, sugar alcohols, uronic acids, and muramic acid.

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Zusammenfassung Ein einfaches spektralphotometrisches Verfahren zur Mikrobestimmung von Rhamnose wurde beschrieben. Es beruht auf der Spaltung des Zuckers mit Schwefelsäure und nachfolgender Behandlung mit p-Hydroxydiphenyl. Das gefärbte Reaktionsprodukt wird spektralphotometrisch bei 560 nm gemessen. Die Extinktion steht mit der Rhamnosemenge in linearer Beziehung. Das Verfahren wird für die Rhamnosebestimmung in Proben empfohlen, die verschiedene Neutralzucker, Aminozucker, Zuckeralkohole, Uronsäuren und Muramsäure enthalten.

     

Potentiometric determination of the protonation constants of 1,3-propanediamine-N,N'-diacetate-N,N'-di-3-propionate

The protonation constants of 1,3-propanediamine-N,N'-diacetate-N,N'-di-3-propionate (1,3-pddadp) were determined in sodium nitrate solution of various concentrations and in 0.1M potassium nitrate medium. The values of the thermodynamic functions (DeltaG, DeltaH and DeltaS) for the successive protonation equilibria, in 0.1M (Na)NO(3) medium, at 25 degrees , are also reported.     

Infrared spectroscopic evidence for Π back-bonding in molecular adducts of pyridine

The observation of the dependence of the in-phase, out-of-plane CH bending on π electron density in aromatics made by Kross was applied to pyridine molecular complexes. Comparison of systems with and without possible back bonding and the existence of a series resembling the Irving-Williams' series, assert the π back bonding to be responsible for the behaviour of the pyridine “umbrella” band.     

Elektronenspektren einiger Thioamide und ihrer S- und N-Derivate

Zusammenfassung Wir diskutieren und veröffentlichen die Elektronenspektren einiger Thioamide und die einiger S- und N-Derivate (N-Aryl-, N-Methyl-N-aryl-, S-Methyl-N-aryl-, N-Acyl-) davon.

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The electronic spectra of some thioamides and their S- and N-derivatives (N-aryl, N-methyl-N-aryl, S-methyl-N-aryl, N-acyl) have been reported and discussed.

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Aus der vonN. Stojanac eingereichten Dissertation, die einen Teil der zur Erlangung des Titels eines Doktor der Chemie (Ph. D.) der Universität Zagreb gestellten Anforderungen erfüllt.     

Composite Ferric Oxyhydroxide-Containing Phases Formed in Neutral Aqueous Solutions of Tryptophan and Indole-3-Acetic Acid

Mössbauer, FTIR and XRD analyses showed that in aqueous medium in air in the presence of L-tryptophan (Trp) or indole-3-acetic acid (IAA) the ambient-temperature ageing of the precipitates formed from ferrous sulphate at pH7 gave composite phases with varying proportions of -FeOOH (a dominating crystalline phase), -FeOOH (both fine-grained, showing superparamagnetic behaviour at 298 K, and relatively better crystallized) and amorphous ferric hydroxide. The experimental data suggested a competition for adsorption sites at the oxyhydroxide surface in the suspension during phase transformations, as well as the transformation of -FeOOH (and/or amorphous ferric hydroxide) to -FeOOH via the dissolution-reprecipitation mechanism. The formation of certain ferric oxyhydroxide phases in the presence of Trp and IAA — released e.g., in the course of bacterial and plant metabolism — can contribute to the regulation of soil mineral composition.