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  • 1
    Electronic Resource
    Electronic Resource
    Variable-temperature NMR studies of thyroid hormone conformations (1988)
    Chichester : Wiley-Blackwell
    Organic Magnetic Resonance 26 (1988), S. 275-280 
    Details
    ISSN: 0749-1581
    Keywords: Thyroxine ; Triiodothyronine ; Variable-temperature ; 1H NMR ; Activation barriers ; Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: 1H NMR spectra at 300 and 400 MHz have been recorded for the thyroid hormones thyroxine (T4) and triiodothyronine (T3) over a range of temperatures from 175 to 298 K in MeOH-d4. Above 200 K the aromatic region of the NMR spectrum of T4 consists of two peaks, one for the H-2′, H-6′ protons and one for H-2,H-6. As the temperature is lowered each of these resonances first broadens and then splits into two peaks of equal intensity at low temperature. This is consistent with slowed exchange between two equally populated conformers. Similar behaviour is noted for T3, although in this case the spectrum is complicated by the presence of spin-spin splitting. Consideration of coalescence temperatures and a line-shape analysis yielded activation barriers in the range 35-38 kJ mol-1. It is concluded that the observed spectral changes are due to dynamic interconversion between proximal and distal forms of the hormones, and that the interconversion takes place by cooperative rotations around the bonds between each phenyl ring and the linking oxygen atom.
    Additional Material: 7 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/mrc.1260260402
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  • 2
    Electronic Resource
    Electronic Resource
    Variable-temperature and pH studies of a DNA minor groove binder, Hoechst 32985, by 1H NMR spectroscopy (1994)
    Chichester : Wiley-Blackwell
    Organic Magnetic Resonance 32 (1994), S. 509-516 
    Details
    ISSN: 0749-1581
    Keywords: NMR ; 1H NMR ; 13C NMR ; Hoechst 32985 ; Dynamics of piperazine rings ; pKa ; Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: 1H NMR spectra of the dipiperazine-based DNA minor groove binding compound Hoechst 32985 indicate that at room temperature and low pH (2.90) a single conformation is detected. This is attributed to the form in which both piperazine rings are protonated at the N-methyl and the bulky methyl and aromatic groups are in an equatorial arrangement. The aliphatic regions of the spectra broaden considerably with an increase in either temperature or pH, indicating the presence of a dynamic exchange process. This process results in interchange of axial and equatorial environments of the piperazine ring protons and is attributed to combined ring and nitrogen inversions. Analysis of the variable-temperature spectra for a solution at pH 2.90 allowed a Gibb's free energy of activation (ΔG
    Additional Material: 8 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/mrc.1260320903
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  • 3
    Electronic Resource
    Electronic Resource
    1H and 13C NMR studies of aminoglycoside antibiotics (1993)
    Chichester : Wiley-Blackwell
    Organic Magnetic Resonance 31 (1993), S. 1077-1084 
    Details
    ISSN: 0749-1581
    Keywords: 1H NMR ; 13C NMR ; Aminoglycosides ; Antibiotics ; Clindamycin ; Clinformational analysis ; Coupling constants ; Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: 1H NMR spectroscopy was used to determine the conformations of the aminoglycoside antibiotic N-demethylclindamycin and two of its cyclic derivatives. The conformational features of these systems were determined by consideration of vicinal coupling constants and, in some cases, nuclear Overhauser enhancement (NOE) effects. 1H and 13C chemical shifts are reported and compared with previous results for the related antibiotic lincomycin. The stability of the two cyclized derivatives in aqueous solution was examined. Both cyclizations involved formation of a 4-imidazolidinone ring. It was found that the ring system based on cyclization with formaldehyde was stable in aqueous solution, whereas that based on benzaldehyde was not.
    Additional Material: 8 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/mrc.1260311209
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  • 4
    Electronic Resource
    Electronic Resource
    13C and 1H NMR studies of nitrogen inversion and molecular flexibility in the tricyclic antidepressant drug trimipramine maleate (1995)
    Chichester : Wiley-Blackwell
    Organic Magnetic Resonance 33 (1995), S. 367-374 
    Details
    ISSN: 0749-1581
    Keywords: NMR ; 1H NMR ; 13C NMR ; antidepressants ; trimipramine maleate ; nitrogen inversion ; molecular dynamics ; Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The solution conformations and dynamics of the antidepressant drug trimipramine maleate were investigated by 1H and 13C NMR spectroscopy. Of particular interest was an observed non-equivalence of the N-methyl groups in the aliphatic side-chain of the drug, which is present in both 1H and 13C spectra under a range of aqueous and non-aqueous solvent conditions when the terminal nitrogen is protonated. Although the inequivalence was first observed for the maleate salt, the nature of the counter ion is not a determining factor. The solution concentration of the drug does, however, modulate the observed inequivalence, with the two peaks coalescing at higher drug concentrations. The diastereotopic environments of the two methyl groups arise because of a chiral centre in the aliphatic side-chain. Inversion of the terminal nitrogen produces exchange between the two environments, thereby destroying the inequivalence and leading to a single averaged resonance for the two methyl groups. The energy barrier for the exchange process was determined from variable-temperature NMR experiments to be 16.3 kcal ml-1 (1 kcal = 4.184 kJ). This is higher than would be expected for simple inversion of a tertiary nitrogen, but reflects the fact that inversion can only occur for the small fraction of molecules which are not protonated. The barrier determined from the variable-temperature experiments was in agreement with rates of interchange between the two methyl environments determined from saturation transfer experiments. These rates were measured for a series of different protonation states of the nitrogen, with the rate contant for interchange of environments decreasing with an increasing degree of nitrogen protonation. 13C spin-lattice relaxation times and nuclear Overhauser enhancement effects were also measured to determine the degree of mobility in the aliphatic side-chain of trimipramine maleate.
    Additional Material: 7 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/mrc.1260330508
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  • 5
    Electronic Resource
    Electronic Resource
    NMR conformational studies of fenamate non-steroidal anti-inflammatory drugs (1994)
    Chichester : Wiley-Blackwell
    Organic Magnetic Resonance 32 (1994), S. 335-342 
    Details
    ISSN: 0749-1581
    Keywords: NMR ; 1H NMR ; 13C NMR ; Conformational analysis ; Coupling constants ; NOE ; Non-steroidal anti-inflammatory drugs ; Mefenamic acid ; Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: 1H and 13C NMR spectra of the non-steroidal anti-inflammatory drugs mefenamic acid, meclofenamic acid and flufenamic acid were assigned and the solution dynamics and conformations of the compounds were probed by the analysis of NMR chemical shifts, temperature coefficients, 3J(C,H) coupled constants, nuclear Overhauser enhancement (NOE) effects and saturation transfer experiments. At low concentrations the anthranilic acid derivatives (the fenamates) exist as monomer units with intramolecular hydrogen bonding between the amine proton and the carboxylic acid group. At concentrations of 10 mM and above, solution aggregates exist. The intermolecular association does not disrupt the intramolecular N—H…O=C hydrogen bonding, resulting in a similar conformation for mefenamic and flufenamic acids at both low and high concentrations. Meclofenamic acid has an additional intramolecular hydrogen bond, resulting in a slightly different conformation to the other two fenamates. The results from this conformational study are of potential value in molecular and pharmacophore modelling studies on the bioactive conformations of these drugs.
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/mrc.1260320605
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