Photochemical Z-->E isomerization of a hemithioindigo/hemistilbene omega-amino acid.

The molecule HTI, which combines hemithioindigo and hemistilbene molecular parts, allows reversible switching between two isomeric states. Photochromic behaviour of the HTI molecule is observed by irradiation with UV/Vis light. The photochemical reaction, a Z/E isomerization around the central double bond connecting the two molecular parts, is investigated by transient absorption and emission spectroscopy. For a special HTI molecule, namely, an omega-amino acid, the Z-->E isomerization process occurs on a timescale of 30 ps. In the course of the reaction fast processes on the 1-10 ps timescale are observed which point to motions of the molecule on the potential-energy surface of the excited state. The combination of transient absorption experiments in the visible spectral range with time-resolved fluorescence and infrared measurements reveal a photochemical pathway with three intermediate states. Together with a theoretical modelling procedure the experiments point to a sequential reaction scheme and give indications of the nature of the involved intermediates.     

Additionsreaktionen von 3-Dimethylamino-2,2-dimethyl-2H-azirin an Isothiocyanate

Addition Reactions of 3-Dimethylamino-2, 2-dimethyl- 2 H-azirine and Isothiocyanates. The title azirine readily reacts with two molecules of benzyl- or methylisothiocyanate to form the zwitterionic 1:2 addition compounds 4 and 13 , respectively (Scheme 2). The presumed 1:1 addition products, which are intermediates in the formation of 4 and 13 , cannot be detected. The structure of 4 and 13 follows from their spectroscopic and chemical properties. With water they give the thiourea derivates 5 and 14 , respectively; treatment with aqueous acid leads to the Δ²-1, 3-thiazolin-5-on-derivates 7 and 15 , respectively. With sodium borohydride compounds 8 and 16 , respectively, are obtained (Scheme 2). The zwitterionic compounds 4 and 13 are able to react further with one molecule of the isothiocyanates to give, in high-yield, triazines 9 and 18 , respectively (Scheme 3). The structure of these compounds was again derived from their spectroscopic data. The mechanism for the formation of 9 and 18 is given in Scheme 3. Acid catalysed hydrolysis of 9 and 18 lead to the trithiocyanuric acid derivates 12 and 20 , and to the spiro compounds 11 and 19 , respectively (Sceme 6). Reaction of 4 with one molecule of phenylisocyanate gives triazine 10 (Scheme 5). According to the X-ray analysis of the methyl compound 18 , there are strong steric interactions in this molecule which are due to the side chain. This is demonstrated by the small distances between C(2) … C(13), N(7) … C(11), and C(8) … C(11) (Table 4). These steric interactions, in addition, cause widening of the bond angles N(1)? C(2)? N(7) and C(9)? N(10)? C(11) (Fig.2). Furthermore, the triazine ring is no longer planar. This deformation of the ring diminishes repulsion between the methyl groups C(13) and C(15).     

Sequence Analysis of Polypeptides and Proteins by Combined Gas Chromatography-Mass Spectrometry

A new method for determining the amino acid sequence of polypeptides consists in initial partial hydrolysis to yield a complex mixture of oligopeptides. After derivatization to enhance its volatility, the mixture is analyzed by combined gas chromatography and mass spectrometry. The sequence of the polypeptide is established by a computer from the identified oligopeptides. So far polypeptides having up to 40 amino acids have been analyzed by this method. The advantages and disadvantages of the new method compared with the stepwise procedure of the Edman degradation are considered. Since the two methods are based on fundamentally different principles they may prove to be complementary.     

Gas phase molecular structure of bis(trifluoromethyl). mercury (CF3)2 Hg

The molecular structure of bis(trifluoromethyl)mercury has been determined by electron diffraction of gases. The best agreement between experiment and model was obtained for freely rotating CF₃ groups and the following geometric parameters (r°_α values): C-F = 1.345(3) Å, Hg-C = 2.101(5) Å and <FCF = 106.8°(0.2). The effect of CH₃/CF₃ substitution on the Hg-C bond length is discussed.     

Production of certified reference materials for mycotoxins: IRMM’s view on the assessment of uncertainties

Analytical difficulty and the economic importance of controlling mycotoxin levels in food and feed led the Community Bureau of Reference (BCR) to prepare a series of certified reference materials (CRM) for various mycotoxins. Because of the wide acceptance of these CRM and the need to ensure the comparability and traceability of measurements in the future it is necessary to prepare and certify new batches of mycotoxin reference materials (RM). In the following text two different approaches for evaluation of the characterisation uncertainty of CRM will be compared using the certification of aflatoxin M₁ (AfM₁) in milk powder as an example. The conventional approach is based on evaluation of characterisation exercise data; the alternative approach is based on measurement uncertainties of the employed analytical methods. Because laboratories are using totally different approaches to estimate the measurement uncertainties, combination of the uncertainties obtained from the participating laboratories was not recommended. Therefore, a new integrated approach for assessment of the measurement uncertainties of the analytical methods on the basis of additional data collected during the characterisation exercise will be described. The conventional approach was found to be the most appropriate and economical approach to evaluate the characterisation uncertainty as a characterisation exercise must be performed anyway to establish the property values of candidate (C)RM, irrespective of whether or not reliable measurement uncertainties can be provided by the laboratories. An integrated approach for assessment of measurement uncertainties based on additional characterisation data as applied here to enable use of an uncertainty-based approach provides more information but is too time-consuming and cost-intensive to become common practice.     

Reactions of 2‐Unsubstituted 1H‐Imidazole 3‐Oxides with 2,2‐Bis(trifluoromethyl)ethene‐1,1‐dicarbonitrile: A Stepwise 1,3‐Dipolar Cycloaddition

The reaction of 1,4,5‐trisubstituted 1H‐imidazole‐3‐oxides 1 with 2,2‐bis(trifluoromethyl)ethene‐1,1‐dicarbonitrile ( 7 , BTF) yielded the corresponding 1,3‐dihydro‐2H‐imidazol‐2‐ones 10 and 2‐(1,3‐dihydro‐2H‐imidazol‐2‐ylidene)malononitriles 11 , respectively, depending on the solvent used. In one example, a 1 : 1 complex, 12 , of the 1H‐imidazole 3‐oxide and hexafluoroacetone hydrate was isolated as a second product. The formation of the products is explained by a stepwise 1,3‐dipolar cycloaddition and subsequent fragmentation. The structures of 11d and 12 were established by X‐ray crystallography.     

Novel urea-linked cinchona-calixarene hybrid-type receptors for efficient chromatographic enantiomer separation of carbamate-protected cyclic amino acids

Two novel diastereomeric cinchona-calixarene hybrid-type receptors (SOs) were synthesized by inter-linking 9-amino(9-deoxy)-quinine (AQN)/9-amino(9-deoxy)-epiquinine (eAQN) and a calix[4]arene scaffold via an urea functional unit. Silica-supported chiral stationary phases (CSPs) derived from these SOs revealed, for N-protected amino acids, complementary chiral recognition profiles in terms of elution order and substrate specificity. The AQN-derived CSP showed narrow-scoped enantioselectivity for open-chained amino acids bearing pi-acidic aromatic protecting groups, preferentially binding the (S)-enantiomers. In contrast, the eAQN congener exhibited broad chiral recognition capacity for open-chained as well as cyclic amino acids, and preferential binding of the (R)-enantiomers. Exceedingly strong retention due to nonenantioselective hydrophobic analyte-calixarene interactions observed with hydro-organic mobile phases could be largely suppressed with organic mobile phases containing small amounts of acetic acid as acidic modifier. With the eAQN-calixarene hybrid-type CSP particularly high levels of enantioselectivity could be achieved for tert-butoxycarbonyl (Boc)-, benzyloxycarbonyl (Z)- and fluorenylmethoxycarbonyl (Fmoc)-protected cyclic amino acids using chloroform as mobile phase, e.g. an enantioselectivty factor alpha >5.0 for Boc-proline. Increasing amounts of acetic acid compromised enantioselectivity, indicating the crucial contributions of hydrogen bonding to chiral recognition. Comparison of the performance characteristics of the urea-linked eAQN-calixarene hybrid-type CSP with those of structurally closely related mutants provided evidence for the active involvement of the urea and calixarene units in the chiral recognition process. The urea linker motif was shown to contribute to analyte binding via multiple hydrogen bonding interactions, while the calixarene module is believed to support stereodiscrimination by enhancing the shape complementarity of the SO binding site.     

Beiträge zur Chemie der Pyrrolpigmente, 77. Mitt. Synthese und Struktur vonb-Nor-bilatrienen-abc undb-Nor-biladienen-ac bzw. Bi-9,9′-dipyrrinonylidenen und Bi-9,9′-dipyrrinonylenen

B-Nor-biladienes-ac are prepared by oxidative coupling of dipyrrinones in nearly quantitative yields. Further oxidation of these derivatives yieldsb-nor-bilatrienes-abc. Constitutions, tautomerism, configurations at exocyclic double bonds, and conformations at exocyclic chromophore single bonds are determined for the crystalline state and their states of solution using X-ray structural analysis and spectroscopic methods, respectively.B-nor-biladienes-ac are found to be of (4Z,15Z)5sp,9sc,14sp geometry,b-nor-bilatrienes-abc are planar systems of (4Z,9E,15Z) configuration in the crystal, whereas in solutions there are indications of a more twisted conformation at their exocyclic chromophore single bonds.

Herr Dipl.-Ing.Heinz Flödl hat uns am 11. April 1988 für immer verlassen — wir trauern um ihn.     

Über den Einfluss des Donor-Teilchromophors bei Dimethinmerocyanin-Farbstoffen auf das Aggregationsverhalten in wässriger Lösung

On the Influence of the Donor Part of the Chromophor of Dimethinmerocyanine Dyes upon the Aggregation Tendency in Aqueous Solution A series of novel long-chain alkyl-dimethinmerocyanine dyes containing the (rhodaninyl)benzenesulfonate or (cyanoacetoxy)alkanesulfonate group is synthesized in good yield and their aggregation tendency in H₂O and/or in various organic solvents/H₂O mixtures without addition of salt investigated. Except for the merocyanine 2h , all dyes give J-aggregation in H₂O or in organic solvents containing H₂O. The Lambert-Beer rule is proved for the solution of 3a (4) in H₂O. The degree of aggregation of 3a (4) in H₂O is investigated by ultracentrifugation. The kind of the aggregation (J- or H-type) can be found out very quickly by dissolving the dye in H₂O with the help of ultra-sound.     

Structural determination of a recalcitrant molecule (S2F4)

The structure determination of S₂F₄ (or SF₃SF), the dimer of SF₂, is made difficult by the large variety of possible conformers and the instability of the compound. An electron diffraction and microwave study succeeded only with the help of a molecular model derived from ab initio calculations, after initial experimental attempts had failed. The force field required for a joint electron diffraction and microwave spectroscopy analysis was calculated by ab initio methods and adjusted to the experimental vibrational frequencies. The structure of S₂F₄ is a trigonal bipyramid with the electron lone pair, the SF group and one fluorine atom in equatorial positions. The SF₃ group is strongly distorted with the two axial SF bonds differing by 0.10 Å and bond angles between axial bonds and equatorial plane of about 77° and 92°, respectively. The geometric structure of S₂F₄ in combination with the ab initio calculations allows one to visualize the dissociation process SF₃SF → 2 SF₂ more clearly.