Sorptionseffekte an Eisen(III)-hydroxid-F?llungen

Zusammenfassung Um Material zur Deutung des Mechanismus der Sorption von Sr²⁺-Ionen an Eisen(III)-hydroxid zu gewinnen, werden Meßreihen mitgeteilt, in denen die pH-Abhängigkeit der Sorption sowie deren Beeinflussung bei Fällung mit verschiedenen Basen (NaOH, Ammoniak, Trimethylamin, Benzylamin) untersucht wird. Aus den charakteristischen Unterschieden der Sorptionskurven muß gefolgert werden, daß die Art der Base für die Sorption eine wichtige Rolle spielt.

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Summary The sorption of Sr²⁺ ions on iron(III) hydroxide was studied as a function of the pH, of the precipitating reagent (NaOH, ammonium hydroxide, benzylamine, trimethylamine), of the temperature and of the total concentration.The curves of the relative sorption vs. pH are found to show characteristic differences which have to be explained in a detailed theory of the mechanism.

     

Electron microscopic phase analysis of BN-thin films

A series of BN films was deposited by means of r.f. magnetron sputtering of a h-BN target onto Si(1OO) surfaces. Hereby, the substrate bias voltage was varied. Special interest is focussed to the influence of the deposition parameters on the orientation of the growing hexagonal BN film with respect to the substrate. For structural investigation, cross section samples were prepared. In addition to HRTEM and diffraction investigations, especially electron energy loss spectroscopy (EELS) was applied successfully for phase identification. For negative bias voltages of U _B =–300 V and U _B =–350V, we found a phase system consisting of a first-grown 25 nm thick layer of hexagonal structure with the c axis parallel to the substrate surface followed by the cubic phase.Dedicated to Professor Dr. rer. nat. Dr. h.c. Hubertus Nickel on the occasion of his 65th birthday     

CsMoO2(HO3P–CH2–PO3): a new metallodiphosphonate with a hybrid framework

Crystals of caesium molybdeno­methyl­enedi­phospho­nate, [CsMoO₂(CH₃O₆P₂)], were hydro­thermally synthesized at 473 K. The monoclinic structure, as determined from single‐crystal X‐ray diffraction, is two‐dimensional and consists of stacked mixed layers of corner‐sharing tetrahedral di­phosphon­ate groups and MoO₆ octahedra, between which Cs⁺ cations are intercalated.     

Recognition of G-1:C73 atomic groups by Escherichia coli histidyl-tRNA synthetase

This work focuses on the RNA-protein interactions necessary for efficient aminoacylation of tRNAHis by Escherichia coli histidyl-tRNA synthetase (HisRS). The E. coli tRNAHis acceptor stem is characterized by a unique "extra" G-1:C73 base pair. Previous in vivo and in vitro studies showed that G-1:C73 is a major recognition element for E. coli HisRS. To further probe the role of the G-1:C73 base pair in specific aminoacylation, we carried out atomic group "mutagenesis" studies. Systematic base analogue substitutions at the -1:73 position of chemically synthesized microhelixHis substrates suggest that the G-1 base serves to position the 5'-monophosphate, which is critical for aminoacylation. Additionally, the C73 and G-1 bases contain major groove exocyclic atomic groups that contribute to HisRS recognition.     

β-Lactam Derivatives as Enzyme Inhibitors: Peptidic Derivatives of ( RS )-2-Oxo-4-phenylazetidine-1-alkanoic Acids

4-Phenylazetidine-2-one was transformed into 4-phenylazetidine-1-alkanoic acids, which were reacted in the presence of diphenylphosphoroazidate with amino acid esters and dipeptide esters yielding β-lactam peptides with different spacers between the lactam ring and the peptide moiety. All structures were established by elementary analyses, HPLC, optical rotation, and spectroscopic data and all new compounds were tested as inhibitors of PPE using standard procedures. Four compounds exhibited a weak activity compared with the standard inhibitor trifluoroacetyl-l-val-l-tyr-l-val.     

Dynamics and energetics of the self-assembly of a hydrophobically modified polyelectrolyte: Naphthalene-labeled poly(acrylic acid)

Steady-state and time-resolved fluorescence studies were performed on aqueous solutions of poly(acrylic acid) hydrophobically modified with two very different levels of naphthalene (Np). It is demonstrated that unique information on association phenomena involving hydrophobe-modifed polymers can be obtained from an extended fluorescence study by using data for a less-modified polymer as a reference. For the more highly modified polymer, the presence of excited-state (as well as ground-state) dimers in addition to monomer emission due to locally excited naphthalene gives evidence for hydrophobic association between naphthalene groups. This association becomes, as expected, much less important at higher pH due to the electrostatic repulsion between different chain segments. However, it is noted that even at high pH there is a significant self-association. The coexistence of static and dynamic quenching phenomena of the Np monomer label was also revealed in the time-resolved fluorescence data. The data are compatible with the existence of two types of monomers and one excimer and suggest that the essential contribution to the monomer emission comes from isolated chromophores, whereas excimer formation arises from both a dynamic route (excited Np chromophores able to produce a dynamic excimer) and a static route (excitation of ground-state Np dimers). At room temperature, the dissociative reaction, excimer-to-monomer, can be neglected, and thus the rate constant for excimer formation and decay could be obtained with and without considering the influence of preformed dimers. Temperature has shown to induce different behavior in the polymer photophysics. In the case of the less-labeled polymer, the decays were found to be single-exponential with the fluorescence lifetime decreasing with increasing temperature. From the temperature dependence of the steady-state fluorescence data, the activation energy for excimer formation and the binding energy of the excimer were evaluated at different pH values, through the Stevens-Ban-type plots of the excimer-to-monomer intensity ratio. With the time-resolved data, measured in the temperature range of 5-60 degrees C, it was possible to extract the intrinsic activation energies for excimer formation. The thermodynamic driving force for the intrapolymeric association was found to be dependent on a balance between hydrophobic and electrostatic interactions, which are dependent on the pH, temperature, and hydrophobic content of the polymer.     

Synthesis of (±)-Desmarestene and (±)-Viridiene,the Two Sperm Releasing and Attracting Pheromones from the Brown Algae Desmarestia aculeata and Desmarestia viridis

Desmarestene 1 6-(1Z,3-butadienyl)-1,4-cycloheptadiene and viridiene 3 cis-3-(1Z, 3-butadienyl)-4-vinylcyclopentene are chemical messengers for male gametes of the brown algae Desmarestia aculeate and Desmarestia viridis. Total syntheses of 1 , 3 and their stereoisomers 1a , 3a - c are reported. Gas-chromatographic comparison of synthetic 1 and 3 with the corresponding natural products has established their structural identity.     

Carbohydrate chain of ganglioside GM1 as a ligand: identification of the binding strategies of three 15 mer peptides and their divergence from the binding modes of growth-regulatory galectin-1 and cholera toxin

The branched pentasaccharide chain of ganglioside GM1 is a prominent cell surface ligand, for example, for cholera toxin or tumor growth-regulatory homodimeric galectins. This activity profile via protein recognition prompted us to examine the binding properties of peptides with this specificity. Our study provides insights into the mechanism of molecular interaction of this thus far unexplored size limit of the protein part. We used three pentadecapeptides in a combined approach of mass spectrometry, NMR spectroscopy and molecular modelling to analyze the ligand binding in solution. Availability of charged and hydrophobic functionalities affected the intramolecular flexibility of the peptides differently. Backfolding led to restrictions in two cases; the flexibility was not reduced significantly by association of the ligand in its energetically privileged conformations. Major contributions to the interaction energy arise from the sialic acid moiety contacting Arg/Lys residues and the N-terminal charge. Considerable involvement of stacking between the monovalent ligand and aromatic rings could not be detected. This carbohydrate binding strategy is similar to how an adenoviral fiber knob targets sialylated glycans. Rational manipulation for an affinity enhancement can now be directed to reduce the flexibility, exploit the potential for stacking and acquire the cross-linking capacity of the natural lectins by peptide attachment to a suitable scaffold.     

Multidrug resistance and cancer: the role of the human ABC transporter ABCG2

A variety of human cancers become resistant or are intrinsically resistant to treatment with conventional chemotherapy, a phenomenon called multidrug resistance. This broad-based resistance results in large part, but not solely, from overexpression of members of the ATP-binding cassette (ABC) superfamily of membrane transporters, including P-glycoprotein, various members of the multidrug resistance associated proteins (MRPs), and ABCG2, also known as MXR1, BCRP, and ABCP. When overexpressed in cell lines, ABCG2 has the ability to confer high levels of resistance to anthracyclines, mitoxantrone, bisantrene, and the camptothecins topotecan and SN-38. This review focuses on the discovery, the biochemistry and the normal physiology of human ABCG2, a novel ABC half transporter expressed abundantly in placenta, as well as in liver, intestine and stem cells. ABCG2 may serve a protective function by preventing toxins from entering cells as well as potentially playing a role in regulating stem cell differentiation. We also discuss the involvement of ABCG2 in multidrug resistance in cancer, especially with regard to acute myeloid leukemia. The mechanism by which substrates are recognized by ABCG2 and how the energy of ATP hydrolysis is transduced into transport remain elusive. A complete understanding of the mechanism and biological function of ABCG2 will be important for understanding its normal physiology as well as potentially for the development of novel chemotherapeutic treatment strategies.     

Glycosylidene Carbenes. Part 6. Synthesis of alkyl and fluoroalkyl glycosides

The syntheses of glycosides from the diazirine 1 and a range of alcohols under thermal and/or photolytic conditions are described. Yields and diastereoselectivities depend upon the pK_HA values of the alcohols, the solvent, and the reaction temperature. The glycosidation of weakly acidic alcohols (MeOH, EtOH, i-PrOH, and t-BuOH, 1 equiv. each) in CH₂Cl₂ at room temperature leads to the glycosides 2–5 in yields between 60 and 34% (Scheme 1 and Table 1). At ?70 to ?60°, yields are markedly higher. In CH₂Cl₂, diastereoselectivities are very low. In THF, at ?70 to ?60°, however, glycosidation of i-PrOH leads to α-D -/β-D - 4 in a ratio of 8:92. More strongly acidic alcohols, such as CF₃CH₂OH, (CF₃)₂ CHOH, and (CF₃)₂C(Me)OH, and the highly fluorinated long-chain alcohols CF₃(CF₂)₅(CH₂)₂OH ( 11 ) and CHF₂(CF₂)₉CH₂OH ( 13 ) react (CH₂Cl₂, r.t.) in yields between 73 and 85% and lead mainly to the β-D -glucosides β-D - 6 to β-D - 8 , β-D - 12 , and β-D - 14 (d.e. 14–68%). Yields and diastereoselectivities are markedly improved, when toluene, dioxane, 1,2-dimetoxyethane, or THF are used, as examined for the glycosidation of (CF₃)₂C(Me)OH, yielding (1,2-dimethoxyethane, 25°) 80% of α-D -/ β-D - 8 in a ratio of 2:98 (d.e. 96%; Table 4). In EtCN, (CF₃)₂C(Me)OH yields up to 55% of the imidate 10 . Glycosidation of di-O-isopropylideneglucose 15 leads to 16 (CH₂Cl₂, r.t.; 65%, α-D / β-D = 33:67). That glycosidation occurs by initial protonation of the intermediate glycosylidene carbene is evidenced, for strongly acidic alcohols, by the formation of 10 , derived from the attack of (CF₃)₂MeCO^? on an intermediate nitrilium ion (Scheme 4), and for weakly acidic alcohols, by the formation of α-D - 9 and β-D - 9 , derived by attack of i-PrO^? on intermediate tetrahydrofuranylium ions. A working hypothesis is presented (Scheme 3). The diastereoselectivities are rationalized on the basis of a protonation in the σ plane of the intermediate carbene, the stabilization of the thereby generated ion pair by interaction with the BnO? C(2) group, with the solvent, and/or with the alcohol, and the final nucleophilic attack by RO^? in the π plane of the (solvated) oxonium ion.