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  • 1
    Digitale Medien
    Digitale Medien
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 115 (2001), S. 5220-5227 
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The reaction field of a water molecule in liquid water has been computed with the help of continuum, discrete-continuum, and discrete models, using density functional theory calculations. In the continuum model, the liquid is simply described by a polarizable dielectric medium. The solute is placed in a cavity defined by a scaled van der Waals surface. Two different sets of van der Waals radii have been used for the atomic spheres. The discrete-continuum model consists of a quantum molecule surrounded by four classical molecules, the resulting aggregate being embedded in a dielectric continuum. Finally, in the discrete model, a molecular dynamics simulation is carried out for a quantum molecule in a box containing 215 classical molecules with periodic boundary conditions. The reaction field and the induced dipole moment in the standard continuum model are substantially underestimated. However, the use of optimized van der Waals radii for the atomic spheres produces a notable improvement. The discrete-continuum and discrete models lead to close results that are in good agreement with experimental data and previous theoretical estimations. For instance, the induced dipole moment (0.80 and 0.82 D for discrete-continuum and discrete models, respectively) compares well with the experimental estimate (0.75 D) and with Car–Parrinello simulations (1.08 D). The reaction field potential is analyzed in terms of multipole moment contributions. The role of the first shell and bulk solvent are also examined. © 2001 American Institute of Physics.
    Materialart: Digitale Medien
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  • 2
    ISSN: 1520-6904
    Quelle: ACS Legacy Archives
    Thema: Chemie und Pharmazie
    Materialart: Digitale Medien
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  • 3
    Digitale Medien
    Digitale Medien
    s.l. : American Chemical Society
    Journal of the American Chemical Society 115 (1993), S. 3722-3730 
    ISSN: 1520-5126
    Quelle: ACS Legacy Archives
    Thema: Chemie und Pharmazie
    Materialart: Digitale Medien
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  • 4
    ISSN: 1432-0886
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Biologie , Medizin
    Notizen: Abstract Multiple interactions of members of the hsp70 family with cellular components have already been described. We present, however, the first evidence that upon heat shock treatment hsp70 molecules interact with specific chromosomal subdivisions of the polytene chromosomes ofDrosophila hydei. After a heat shock treatment of 20 min the protein binds to subdivision 3-58D1 and to the heat shock inducible subdivisions 2-48B3–6 and 2-48C1–2. Hsp70 molecules were also observed in subdivision 3-58D1 during recovery at 25°C but not in subdivisions 2-48B3–6 and 2-48C1–2. Our data suggest that this interaction is stress specific. DNase and RNase experiments suggest, moreover, that the hsp70 molecules bind to RNA from ribonucleoproteins (RNPs) in subdivisions 2-48B3–6 and 2-48C1–2 and to DNA in subdivision 3-58D1. The DNA sequences in subdivision 3-58D1 seem to have the potential to adopt the Z-DNA conformation.
    Materialart: Digitale Medien
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  • 5
    ISSN: 1432-072X
    Schlagwort(e): Bradyrhizobium japonicum ; Nitrate ; Membrane-bound nitrate reductase ; Nitrate reductase mutants ; Microaerobiosis
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Biologie
    Notizen: Abstract Native PAGE of Triton x-100-solubilized membranes from Bradyrhizobium japonicum strain PJ17 grown microaerobically (2% O2, v/v) in defined nitrate-containing medium resolved two catalytically active nitrate reductase (NR) species with apparent molecular masses of 160 kDa (NRI) and 200 kDa (NRII). NRI and NRII were also found in membranes from cells of strain PJ17 that were first grown in defined medium with glutamate and further incubated microaerobically in the presence of 5 mmol/l KNO3. However, only NRI was detected in cell membranes of strain PJ17 when nitrate was omitted from the microaerobic incubation medium. Four mutants unable to grow at low O2 tension in the presence of nitrate were isolated after transposon Tn5 mutagenesis. Membranes from mutants GRF110 and GRF116 showed mainly NRI, while the other two mutants, GRF3 and GRF4, expressed mostly NRII. These results indicate that the ability of B. japonicum PJ17 to grow under microaerobic conditions depends upon the presence of two membrane-bound NR enzymes whose synthesis seem to be independently induced by microaerobiosis (NRI) or by both microaerobiosis and nitrate (NRII).
    Materialart: Digitale Medien
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  • 6
    Digitale Medien
    Digitale Medien
    Springer
    Journal of molecular modeling 6 (2000), S. 608-617 
    ISSN: 0948-5023
    Schlagwort(e): Ozonolysis ; Ozone ; Reaction mechanisms ; Solvent effects ; Theoretical calculations
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Chemie und Pharmazie
    Notizen: Abstract Electrostatic solvent effects on the ozonolysis of ethylene have been investigated using correlated ab initio and density functional approaches. We use a simple polarizable continuum model for the solvent. It allows us to evaluate the medium effect on both the electronic and nuclear structure of the chemical species involved in the reaction. The computations confirm that basically the reaction proceeds through the Criegee mechanism. However, formation of the van der Waals complexes ethyl-ene/ozone and carbonyl oxide/formaldehyde also appears to play a role. All the calculated species are stabilized with respect to the reactants except the transition state corresponding to the primary ozonide formation. In general, electrostatic solvent effects are relatively small for activation barriers of single reaction steps and more substantial for the corresponding reaction energies. Moreover, the medium significantly modifies the structure of some species for which polarization effects are crucial.
    Materialart: Digitale Medien
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  • 7
    Digitale Medien
    Digitale Medien
    Springer
    Journal of molecular modeling 6 (2000), S. 527-538 
    ISSN: 0948-5023
    Schlagwort(e): Hybrid methods ; QM/MM ; LSCF ; Zinc metalloproteases ; Thermolysin ; Peptide hydrolysis ; Enzymatic mechanism
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Chemie und Pharmazie
    Notizen: Abstract The hydrolysis by thermolysin of a Gly-Phe-Leu peptide, considered as a model substrate of the enkephalin family, has been studied with a mixed QM/MM method with the AM1/AMBER parameterization. This study is based on the mechanism proposed by Matthews in which the Glu-143 residue plays the role of a proton shuttle in the course of the reaction. The study focused on the description of every step of the process, reaction intermediates and transition states, and on the influence, both energetical and structural, of the whole protein on these stationary points. The overall mechanism appears to be quite realistic, but the study shows that some reaction steps that were assumed to be concerted should occur in two phases. Analysis of the role of the amino-acids surrounding the active site has shown their important participation in the fluctuations of the energy. In particular, the major role of His-231 on the overall mechanism has been confirmed. This study shows that modeling reaction mechanisms for enzymes is quite feasible and opens the way for computer experiments that may be helpful in devising and interpreting detailed experimental investigations.
    Materialart: Digitale Medien
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  • 8
    ISSN: 1432-2234
    Schlagwort(e): Key words: Proton transfer – Serine – Continuum solvent model – Nonequilibrium effects
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Chemie und Pharmazie
    Notizen: Abstract. Serine amino acid in aqueous solution is theoretically studied at the B3PW91/6-31+G** level using a dielectric continuum solvent model. Neutral and zwitterionic structures in the gas phase and in solution are described and the proton-transfer mechanism is discussed. A neutral conformation in which the carboxyl hydrogen atom is already oriented toward the amino group seems to be the absolute energy minimum in the gas phase and the most stable neutral form in solution. The absolute energy minimum in solution is a zwitterionic form. The energy barrier for proton transfer is predicted to be very small, in particular when zero-point-energy contributions are added. Our calculations allow the dynamic aspects of the ionization mechanism to be discussed by incorporating nonequilibrium effects.
    Materialart: Digitale Medien
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  • 9
    ISSN: 1573-5192
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Biologie , Medizin
    Notizen: Abstract During 1987 and the first months of 1988, several carnivores were surveyed for ecological studies by means of radio-tracking techniques and in order to identify the ischnoceran species parasitising these animals. The hosts belonged to the following species: Felis pardina, Felis silvestris, Herpestes ichneumon, Genetta genetta, Vulpes vulpes and Meles meles. While no lice were found on the two first species, the remaining ones were parasitised by Felicola (Felicola) inaequalis, Lorisicola (Paradoxuroecus) genettae n. comb., Felicola (Suricatoecus) vulpis and Trichodectes (Trichodectes) melis, respectively. Descriptions of these species are included.
    Materialart: Digitale Medien
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  • 10
    Digitale Medien
    Digitale Medien
    Springer
    Journal of computer aided molecular design 7 (1993), S. 183-198 
    ISSN: 1573-4951
    Schlagwort(e): QSAR ; Conformational analysis ; Prostaglandin synthase ; Antiinflammatory ; Amfenac
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Chemie und Pharmazie
    Notizen: Summary The new nonsteroidal antiinflammatory drug (NSAID) arylacetic amfenac (2-amino-3-benzoylphenylacetic acid) and 19 substituted derivatives were studied in order to correlate the biological activities with the structure-related parameters. The geometry of amfenac in neutral and anionic form was totally optimized, starting from standard geometries and crystallographic data, using semiempirical AM1 and MNDO quantum-mechanical methods. Conformational analysis shows the existence of a rigid structure for rotations of the acetic acid chain (α°) and the central carbonyl group (γ°) around the bonds with the phenylamine ring, whereas the carboxyl group (β°) and the phenyl ring of the benzoyl group (δ°) can rotate almost freely. Electrostatic potential maps were analyzed and showed that the electrostatic orientation effect seems to make an important contribution to the binding of the active compounds to prostaglandin synthase. An electrostatic orientation model of the binding site is proposed. The frontier orbital charge distribution was also described for each compound. On the other hand, steric, electronic and hydrophobic (log P) parameters were calculated and QSAR analysis showed that the most significant parameter for the antiinflammatory activity was the π-electron density of the HOMO orbital in the second aromatic ring. These results suggest a possible electronic charge transfer between the aromatic fragments and the receptor.
    Materialart: Digitale Medien
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