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  • 1
    Electronic Resource
    Electronic Resource
    Theoretical mechanistic study on the ion–molecule reactions of CCN+/CNC+ with H2O and HCO+/HOC+ with HCN/HNC (2002)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 116 (2002), S. 1892-1910 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A detailed [C2H2NO+] potential-energy surface in singlet, including 48 minimum isomers and 73 transition states, is built up at the B3LYP/6-311G(d,p) and CCSD(T)/6-311G(2df,p) (single-point) levels in order to explore the mechanisms of the important ion–molecule reactions between CCN+/CNC+ and H2O and between HCO+/HOC+ and HCN/HNC. For the reactions of both CCN+ and CNC+ towards H2O, product HCO++HNC may be the most abundant followed by the much less HCO++HCN and then HCNH++CO. Significant discrepancies on the product distributions are found between our calculated results and two previous experimental findings. On the other hand, for the HCO++HCN/HNC reactions, the barrierless association may lead to the stable adducts OC(H)NCH+/OC(H)CNH+, while the proton-transfer may barrierlessly lead to product HCNH++CO via the hydrogen-bound complexes OC...HNCH+/OC...HCNH+. For the HOC++HCN/HNC reactions, both the barrierless proton-transfer and association-elimination processes can lead to HCNH++CO via the complexes CO...HNCH+/CO...HCNH+ and unstable adducts HOCNCH+/HOCCNH+, respectively. The computations reported in this paper may represent the first theoretical study on the chemical reactivity of the C2N+ ion, and may thus provide a useful guide for understanding the mechanisms of the other analogous reactions such as those of C2N+ with CH4, NH3, H2S and CH3OH, etc. The present calculations may also provide useful information for future laboratory investigations on the HCO+/HOC++HCN/HCN reactions that have not been previously studied. Interstellar implications of the title reactions are discussed. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1431272
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  • 2
    Electronic Resource
    Electronic Resource
    The reorganization of the lamellar structure of a single polyethylene chain during heating: Molecular dynamics simulation (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 115 (2001), S. 10001-10006 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Molecular dynamics simulation starting from a lamellar crystal model of a single polyethylene chain is performed to investigate the lamellar reorganization during heating at the molecular level. It is shown that three stages are involved in the process of the reorganization: at temperatures 300 K〈T≤370 K the lamella thickens, in the region of 370 K〈T≤500 K the boundary premelting of the lamella occurs, and when T〉500 K the lamella starts to melt and becomes a random coil in the end. Particularly, the thickening process is investigated in our simulations. It is found that the lamellar thickening occurs in discrete steps and is driven mainly by the van der Waal attraction between the chain segments. There are two mechanisms for the lamellar thickening. At lower temperature the thickening occurs by the sliding diffusion of adjacent chain segments, while at higher temperature the recrystallization after the partial melting of thinner stems leads to the thickening. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1415343
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  • 3
    Electronic Resource
    Electronic Resource
    Molecular dynamics simulation of the linear low-density polyethylene crystallization (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 115 (2001), S. 3916-3922 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: By means of molecular dynamics simulation, the crystallization process of the copolymer (ethylene-co-propene) with different branch distributions is studied at the molecular level. It is shown that subglobules formed at the branch sites along the copolymer chains and subsequently coalesced into a single globule and then developed to a lamellar structure in the end. This process can be considered as crystal nucleation and growth at the early stage of the copolymer crystallization. In the nucleation the branch acts as a nucleating seed and in the crystal growth the branch is rejected from the crystal region as a defect. The driving force for the relaxation process is the attractive van der Waals interaction between the chain segments. Furthermore, it is found that the branch distribution is an important factor in determining the crystallinity of the copolymer, when the comonomer composition and the molecular weight of the copolymers are fixed, as a blocky type of copolymer will show nearly unhampered crystallization and a true random copolymer will show a hindered one. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1389856
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  • 4
    Electronic Resource
    Electronic Resource
    C4N: The first CnN radical with stable cyclic isomers (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 114 (2001), S. 5170-5179 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The potential-energy surface of the interstellar molecule C4N is explored at the B3LYP/6-311G(d) level of theory. Thirteen isomers including the linear, three-membered ring, four-membered ring, A-like, Y-like, and cage-like structures are located as minima connected by 23 interconversion transition states. The structures of the most relevant isomers and transition states are further optimized at the QCISD/6-311G(d) level followed by single-point energy calculations at the MP4SDTQ, CCSD(T), and QCISD(T) levels with the 6-311G(2df) basis set. At the CCSD(T)/6-311G(2df)//QCISD/6-311G(d) level, the lowest-lying isomer is a linear structure CCCCN 1 followed by a CCC three-membered ring structure 4 with exocyclic CCN bonding that lies only 2.8 kcal/mol higher. The third and fourth low-lying isomers possess a CCC three-membered ring structure 5 with exocyclic CNC bonding at 21.4 kcal/mol and a linear structure CCCNC 2 at 23.4 kcal/mol, respectively. All the four isomers 1, 2, 4, and 5 and another high-lying isomer 3 with a linear CCNCC structure at 62.5 kcal/mol are shown to have considerable kinetic stability towards isomerization and dissociation. Thus, all the five isomers may be experimentally observable. Possible formation of these five stable C4N isomers in interstellar space is discussed. Finally, our calculations show that the Møller–Plesset methods fail to predict even qualitatively the energetic properties between the four low-lying isomers 1, 2, 4, and 5, in comparison with the QCISD and CCSD results. This paper indicates that C4N may be the first interstellar molecule with stable low-lying cyclic isomers among the CnN radical series to be detectable in near future. The results presented in this paper may provide useful information for future laboratory and interstellar identification of various C4N isomers. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1351884
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  • 5
    Electronic Resource
    Electronic Resource
    CCNN: The last kinetically stable isomer of cyanogen (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 1745-1754 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The dissociation and isomerization stability of the linear cyanogen isomer CCNN, a formally charge-transfer species comprising C2− and N2+, is theoretically investigated by means of ab initio methods, including the Hartree–Fock (HF), the Møller–Plesset perturbation theory from second through fourth order (MP2, MP4SDQ, MP4SDTQ), the configuration interaction with singles and doubles (CISD), the quadratic configuration interaction with singles and doubles (QCISD) as well as with triples [QCISD(T)], and the density functional theory (DFT) including Beck's three parameter hybrid methods with the Lee–Yang–Parr correlation functional (B3LYP) and with the Perdew–Wang 91 correlation functional (B3PW91) methods. At the QCISD(T)/6-311G(3df)//QCISD(T)/6-311G(d) level with QCISD/6-311G(d) zero-point vibrational energy (ZPVE) correction, the barriers from CCNN to the NNC three-membered ring structure with exocyclic C–C bonding and to the dissociation products C2 and N2 are predicted to be 42.1 and 51.8 kcal/mol, respectively. The QCISD(T)/6-311G(3df) potential energy surface of C2N2 indicates that CCNN may be kinetically more stable than the other two well-studied isomers CNCN and CNNC. Thus, although CCNN is thermodynamically less favorable due to its high energy, it is still experimentally observable. Moreover, a barrier of 5.5 kcal/mol is predicted for the reverse association reaction between C2 and N2 to produce CCNN at the QCISD(T)/6-311G(3df)//QCISD(T)/6-311G(d)+ZPVE level, while both B3LYP and B3PW91 methods erroneously predict a barrierless association process. Finally, the possible strategy for the formation of CCNN in laboratory and in interstellar space is discussed in detail. The calculated results may provide a useful guide for future laboratory and interstellar identification of the last kinetically stable isomer of cyanogen, CCNN. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.481977
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  • 6
    Electronic Resource
    Electronic Resource
    FROM VACUOLAR GS-X PUMPS TO MULTISPECIFIC ABC TRANSPORTERS (1998)
    Palo Alto, Calif. : Annual Reviews
    Annual Review of Plant Physiology and Plant Molecular Biology 49 (1998), S. 727-760 
    Details
    ISSN: 1040-2519
    Source: Annual Reviews Electronic Back Volume Collection 1932-2001ff
    Topics: Biology
    Notes: Abstract While the concept of H+-coupling has dominated studies of energy-dependent organic solute transport in plants for over two decades, recent studies have demonstrated the existence of a group of organic solute transporters, belonging to the ATP-binding cassette (ABC) superfamily, that are directly energized by MgATP rather than by a transmembrane H+-electrochemical potential difference. Originally identified in microbial and animal cells, the ABC superfamily is one of the largest and most widespread protein families known. Competent in the transport of a broad range of substances including sugars, peptides, alkaloids, inorganic anions, and lipids, all ABC transporters are constituted of one or two copies each of an integral membrane sector and cytosolically oriented ATP-binding domain. To date, two major subclasses, the multidrug resistance-associated proteins (MRPs) and multidrug resistance proteins (MDRs) (so named because of the phenotypes conferred by their animal prototypes), have been identified molecularly in plants. However, only the MRPs have been defined functionally. This review therefore focuses on the functional capabilities, energetics, organization, and regulation of the plant MRPs. Otherwise known as GS-X pumps, or glutathione-conjugate or multispecific organic anion Mg2+-ATPases, the MRPs are considered to participate in the transport of exogenous and endogenous amphipathic anions and glutathionated compounds from the cytosol into the vacuole. Encoded by a multigene family and possessing a unique domain organization, the types of processes that likely converge and depend on plant MRPs include herbicide detoxification, cell pigmentation, the alleviation of oxidative damage, and the storage of antimicrobial compounds. Additional functional capabilities might include channel regulation or activity, and/or the transport of heavy metal chelates. The identification of the MRPs, in particular, and the demonstration of a central role for ABC transporters, in general, in plant function not only provide fresh insights into the molecular basis of energy-dependent solute transport but also offer the prospect for manipulating and investigating many fundamental processes that have hitherto evaded analysis at the transport level.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1146/annurev.arplant.49.1.727
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  • 7
    Electronic Resource
    Electronic Resource
    Insect resistance conferred by 283-kDa Photorhabdus luminescens protein TcdA in Arabidopsis thaliana (2003)
    [s.l.] : Nature Publishing Group
    Nature biotechnology 21 (2003), S. 1222-1228 
    Details
    ISSN: 1546-1696
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: [Auszug] The tcdA gene of Photorhabdus luminescens encodes a 283-kDa protein, toxin A, that is highly toxic to a variety of insects, including some agriculturally important pests. We tested the efficacy of transgenic toxin A in Arabidopsis thaliana for control of feeding insects. Plants with toxin A ...
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1038/nbt866
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  • 8
    Electronic Resource
    Electronic Resource
    The GS-X Pump in Plant, Yeast, and Animal Cells: Structure, Function, and Gene Expression (1997)
    Springer
    Bioscience reports 17 (1997), S. 189-207 
    Details
    ISSN: 1573-4935
    Keywords: ABC-transporter ; GS-X pump ; multidrug resistance associated protein (MRP) ; canalicular membrane multispecific organic anion transporter (cMOAT) ; yeast cadmium factor 1 (YCF1) ; γ-glutamylcystaine synthetase ; glutathione
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Abstract This review addresses the recent molecular identification of several members of the glutathione S-conjugate (GS-X) pump family, a new class of ATP-binding cassette (ABC) transporters responsible for the elimination and/or sequestration of pharmacologically and agronomically important compounds in mammalian, yeast and plant cells. The molecular structure and function of GS-X pumps encoded by MRP, cMOAT, YCF1. and AtMRP genes, have been conserved throughout molecular evolution. The physiologic function of GS-X pumps is closely related with cellular detoxification, oxidative stress, inflammation, and cancer drug resistance. Coordinated expression of GS-X pump genes, e.g., MRP1 and YCF1, and γ-glutamylcystaine synthetase, a rate-limiting enzyme of cellular glutathione (GSH) biosynthesis, has been frequently observed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1027385513483
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  • 9
    Electronic Resource
    Electronic Resource
    On geminals and symplectic bases in quantum chemistry (1997)
    Springer
    Journal of mathematical chemistry 21 (1997), S. 159-182 
    Details
    ISSN: 1572-8897
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mathematics
    Notes: Abstract The construction of a symplectic basis set with 〉N electrons is exhibited by means of three kinds of units, the first kind geminal, the second kind geminal and the one‐particle operators. The optimization procedure of the variation method is extended to the coefficients in the linear sum of the symplectic bases, the parameters in the geminals, and the orbitals. For practical use, these bases are expanded explicitly as a linear sum of the Slater determinants. For illustration, the LiH molecule, which is taken as an example, is calculated by using some symplectic bases.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1019118318475
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  • 10
    Electronic Resource
    Electronic Resource
    The kth radius of gyration for a polycondensation reaction with identical structural units AaBb (1992)
    Basel : Wiley-Blackwell
    Die Makromolekulare Chemie, Theory and Simulations 1 (1992), S. 91-97 
    Details
    ISSN: 1018-5054
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: For a polycondensation reaction with identical structural units AaBb (i.e. each unit has a functional groups of type A and b of type B) the kth radius of gyration \documentclass{article}\pagestyle{empty}\begin{document}$ \begin{array}{*{20}c} {\left\langle {R^2 } \right\rangle _k = \sum\limits_n {n^k \cdot \left\langle {R_n^2 } \right\rangle P_n } } & {({\rm sum over all }n{\rm - mers; see Eqs}{\rm . (11) and (17))}{\rm }} \\ \end{array} $\end{document} is investigated. By means of a differential method, a recursion formula that holds true for both the pre-gel and the post-gel state is obtained for the evaluation of the kth radius.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/mats.1992.040010204
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