Heterogeneous structure of silk fibers from Bombyx mori resolved by 13C solid-state NMR spectroscopy

The molecular conformation of silk fibrion is characterized by solid-state 13C NMR before spinning (silk I structure) and after spinning (silk II structure). We compare native silk fibers with the quasi-crystalline Cp-fraction and a synthetic model peptide (Ala-Gly)15, both of which can be converted either into silk I by dialysis from 9 M LiBr or into silk II by treatment with formic acid. Our results demonstrate that silk II fibers are intrinsically heterogeneous, consisting of beta-sheets, distorted beta-turns, and distorted beta-sheets. This higher-order heterogeneity is revealed by the 13C-NMR Cbeta-peak of Ala, indicating that the Ala side chains are stacked partially in parallel and partially face-to-face, at a ratio of 1:2.     

Synthesis of light-sensitive arenesulfonylazido polycarboxylic acids

Several light-sensitive arenesulfonylazido polycarboxylic acids were synthesized by a one-step reaction of a polymeric anhydride with a light-sensitive monofunctional alcohol, or by the reverse reaction, i.e., reaction of a polymeric alcohol with a light-sensitive monofunctional anhydride. The polycarboxylic acids are soluble in polar organic solvents and in aqueous base. Neutralization of part of the carboxyl groups gives rise to the formation of water-soluble polymers. Coreaction of poly(maleic anhydride-co-methyl vinyl ether) with 2-hydroxyethyl 4-sulfonylazidocarbanilate and 2-hydroxyethyl trans-2,5-dimethoxystilbene-4′-carbamate produces a light-sensitive polycarboxylic acid with “built-in” sensitizer.     

NONLINEAR LASERSPECTROSCOPIC INVESTIGATIONS OF THE PIGMENT-PIGMENT INTERACTION WITHIN THE LIGHT-HARVESTING COMPLEX OF PHOTOSYSTEM II

Using a pump and test beam technique in the frequency domain with pump pulses in the nanosecond time range, the nonlinear transmission properties were investigated at room temperature in photosystem (PS) II membrane fragments and isolated light-harvesting chlorophyll a/b-protein preparations (LHC II preparations). In LHC II preparations and PS II membrane fragments, respectively, pump pulses of 620 nm and 647 nm cause a transmission decrease limited to a wavelength region in the nearest vicinity of the pump pulse wavelength (full width at half maximum ' 0.24 nm). In contrast, at 670 nm neither a transmission decrease nor a narrow band feature were observed. The data obtained for PS II membrane fragments and LHC II preparations at shorter wavelengths (620 nm, 647 nm) were interpreted in terms of excited state absorption of whole pigment-protein clusters within the light-harvesting antenna of photosystem II. The interpretation of the small transmission changes as homogeneously broadened lines led to a transversal relaxation time for chlorophyll in the clusters of about 4 ps.     

Beta-diiminate platinum complexes for alkane dehydrogenation

New five-coordinate Pt(IV) complexes [{(o-R2-p-R'-C6H2)NC(R' ')}2CH]PtMe3 (R, R', R' ' = alkyl or H) are reported. The complex with R = Me, R' = tBu, R' ' = Me generates unsaturated Pt(II) species capable of alkane C-H bond activation and stoichiometric dehydrogenation.     

Über Glasbildung und Eigenschaften von Chalkogenidsystemen. IX. Zur Struktur der GeS/GeS2- und GeSe/GeSe2-Gläser

Glass Formation and Properties of Chalcogenide Systems. IX. On the Structure of GeS/GeS₂ and GeSe/GeSe₂ Glasses Mole volume, glass transition temperature, and coefficient of thermal expansion were measured on glasses prepared by standardized conditions. Near the composition Ge₂S₃ and Ge₂Se₃ the properties show a singularity or point of inflexion, respectively. Random models of structure are inconsistent with such a behaviour. A model of higher chemical order is proposed which can explain the observed dependence of the glass properties on composition and is in accordance with radial distribution studies.     

Polarizing a hydrophobic cavity for the efficient binding of organic guests: the case of calix[6]tren, a highly efficient and versatile receptor for neutral or cationic species

The host-guest properties of calix[6]tren 1 have been evaluated. The receptor is based on a calix[6]arene that is covalently capped at the narrow rim by a tren unit. As a result, the system presents a concave hydrophobic cavity with, at its bottom, a grid-like nitrogenous core. Despite its well-defined cavity and opening to the outside at the large rim, 1 did not behave as a good receptor for neutral molecules in chloroform. However, it exhibited efficient endo-complexation of ammonium guests. By contrast, the per-protonated host, 1.4H(+), behaved as a remarkable receptor for small organic molecules. The complexation is driven by a strong charge-dipole interaction and hydrogen bonds between the polar guest and the tetracationic cap of the calixarene. Finally, coordination of Zn(2+) to the tren core led to the asymmetrization of calixarene cavity and to the strong but selective endo-binding of neutral ligands. This study emphasizes the efficiency of a receptor presenting a concave hydrophobic cavity that is polarized at its bottom. The resulting combination of charge-dipole, hydrogen bonding, CH-pi, and van der Waals interactions highly stabilizes the supramolecular architectures. Also, importantly, the tren cap allows the tuning of the polarization, offering either a basic (1), a highly charged and acidic (1.4H(+)), or a coordination (1.Zn(2+)) site. As a result, the system proved to be highly versatile, tunable, and interconvertible in solution by simple addition of protons, bases, or metal ions.     

Study of ion chromatographic behavior of inorganic and organic antimony species by using inductively coupled plasma mass spectrometric (ICP-MS) detection

An on-line method for the analysis of Sb(III), Sb(V) and trimethylstiboxide (TMSbO) is presented. The separation is performed using ion chromatography (IC) on a strong anion-exchange column with phthalic acid plus 2% acteone at pH 5 as mobile phase. The chromatographic system is coupled to an ICP-MS as detector. The influence of different complexing agents on the chromatographic behavior of the antimony species is studied. Rather stable complexes of Sb(III) seem to be formed with citrate and tartrate under the experimental conditions. TMSbO forms a dianionic species with citrate in contrast to the otherwise monoanionic complex. Received: 31 Juli 1997 / Revised: 8 December 1997 / Accepted: 11 December 1997     

Acidity functions of aqueous fluorinated acid solutions by differential pulse polarography

The acidity functions of aqueous trifluoroacetic and trifluoromethanesulphonic acid mixtures, and aqueous hexafluoropropane-2, 2-diol solutions, have been determined by differential pulse polarography. The apparent shift of the half-wave potential for the ferrocene—ferricinium couple, as the solvent composition is changed, is used to indicate the change in potential of a glass electrode; acidity is measured as the H_GF acidity function. The densities of two of these solvent systems as a function of composition are also reported. Trifluoromethanesulphonic acid—water mixtures represent the strongest aqueous acid solvent system so far studied.     

Novel Access to Furan-3-thiols and Derivatives,Impact Meat-Flavor Compounds

A versatile process for the preparation of a number of 3-thio-substituted furans 1–4 is described. These products have very low odor thresholds and are thus potent flavor compounds. Fur-3-yl thiocyanates 10a , b as well as other S-containing analogues ( 2b , 7a , b , and 8 ) were prepared by a Michael-type addition of thiocyanic acid, thioacetic acid, alakanethiols, and sodium thiosulfate to alkynones 6 or 15 , followed by cyclization (Schemes 3 and 4). The thiocyanates 10a, b were converted to mixed disulfides 3 , symmetric disulfides 4 , thioethers 2 , and thiols 1 , using ‘hard’ or ‘soft’ nucleophiles or reducing agents, respectively (Scheme 6).