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31.
Arisaka K Auerbach LB Axelrod S Belz J Biery KA Buchholz P Chapman MD Cousins RD Diwan MV Eckhause M Ginkel JF Guss C Hancock AD Heinson AP Highland VL Hoffmann GW Horvath J Irwin GM Joyce D Kaarsberg T Kane JR Kenney CJ Kettell SH Kinnison WW Knibbe P Konigsberg J Kuang Y Lang K Lee DM Margulies J Mathiazhagan C McFarlane WK McKee RJ Melese P Milner EC Molzon WR Ouimette DA Riley PJ Ritchie JL Rubin P Sanders GH Schwartz AJ Sivertz M Slater WE Urheim J Vulcan WF Wagner DL Welsh RE Whyley RJ 《Physical review letters》1993,71(24):3910-3913
32.
Beltrami I Bylsma BG DeBonte R Gan KK Koltick D Loeffler FJ Low EH McIlwain RL Miller DH Ng CR Ong PP Rangan LK Shibata EI Wilson RJ Derrick M Fernandez E Fries R Hyman L Kooijman P Loos JS Musgrave B Price LE Schlereth J Sugano K Weiss JM Wood DE Baranko G Baringer P Blockus D Brabson B Forden GE Gray SW Jung C Neal H Ogren H Rust DR Valdata-Nappi M Akerlof C Bonvicini G Chapman J Errede D Harnew N Kesten P Kooijman S Meyer DI Nitz D Rubin D Seidl AA Thun R Trinko T Willutzky W Cork B Keller L 《Physical review letters》1985,54(16):1775-1778
33.
Several procedures for synthesis of stereoregular poly(methacrylic acid) have been examined and the polymer characterized by (13C)NMR. Using d6 DMSO as solvent for spectroscopy gives better spectra than those previously obtained using aqueous solutions and stereochemical splittings can be resolved in the methyl signals. Free-radical polymerization in toluene solution is a Bernouilli process giving mainly heterotactic/syndiotactic polymer. Polymers produced with free-radical initiation in aqueous solution have a higher, and pH dependent, content of syndiotactic triads. A previously described procedure for producing regular polymers by hydrolysis of poly(trimethylsilyl methacrylate) requires modification to produce isotactic contents of above 90% and does not give truly syndiotactic polymer. In contrast, polymerization with γ-radiation can produce polymers with close to 90% of syndiotactic triads. 相似文献
34.
Poon T Turro NJ Chapman J Lakshminarasimhan P Lei X Jockusch S Franz R Washington I Adam W Bosio SG 《Organic letters》2003,5(26):4951-4953
Oxazolidinone-substituted enecarbamates represent a mechanistically rich system for the study of stereoelectronic, steric, and conformational effects on stereoselectivity and mode selectivity in (1)O(2) [2 + 2] and ene reactions. Photooxygenation of these enecarbamates with (1)O(2) leads to diastereomerically pure dioxetanes that decompose to yield an oxazolidinone carbaldehyde and one of the two enantiomers of methyldesoxybenzoin in enantiomeric excess. Stereoselectivity originates at the allylic stereocenter, a result supported by quenching studies, computational analysis, and deuterium solvent isotope effects. [reaction: see text] 相似文献
35.
Toby M. Chapman Ramazan Benrashid 《Journal of polymer science. Part A, Polymer chemistry》1990,28(13):3685-3699
Block copolyurethanes with N-alkylated polyurethanes as nonself-associating soft segments are prepared. The polymers compare well in hydrolytic stability with block copoly(N-alkylamide urethane)s prepared in an earlier study but have better dimensional stability on exposure to moist heat. They show comparable hydrolytic stability and better oxidative stability than a poly(ether urethane) from the earlier study. The effect of different alkyl substituents on tensile properties and thermostability is studied. The tensile properties are also examined as a function of soft-segment molecular weight at constant weight-percent of hard-block. It is shown that yield stress and ultimate stress increase and elongations decrease as the soft-block M?n increases. DSC measurements do not generally show well defined endotherms with few exceptions but dynamic mechanical testing of some samples does show transitions between 14 and 23°C. 相似文献
36.
Abstract— Conformational changes of bovine lens crystallins in a photodynamic system generating singlet oxygen, have been investigated. The formation of intersubunit crosslinks was observed in all three classes (α-, β and γ-) of crystallins by irradiation in the presence of the photosensitizer methylene blue. Near-UV circular dichroism (CD) spectra of the crystallins were significantly altered by irradiation under these conditions, indicating changes in tertiary structure but the far-UV CD remained unchanged suggesting that the secondary structure ((β-sheet conformation) remains unchanged. Significant changes in the absorption and fluorescence spectra were also observed. Measurement of total sulfhydryl content showed a decrease of 27%, 50% and 37% for α-, β- and γ-crystallins respectively, after irradiation. Fluorescence lifetime measurements of N-iodoacetyl-N'-(5-sulfo-l-naphthyl)ethylenediamine-labeled crystallins showed a significant decrease of the lifetime of the major decay components of the label bound to sulfhydryl groups of α- and γ-crystallins, but showed no change in the microenvironment of the sulfhydryl groups of β-crystallin. The results are consistent with the microenvironments of the tryptophan and sulfhydryl groups predicted from sequence studies. 相似文献
37.
Jelica Strauch James McDonald Bogdan E. Chapman Philip W. Kuchel Brian S. Hawkett G. Evan Roberts Matthew P. Tonge Robert G. Gilbert 《Journal of polymer science. Part A, Polymer chemistry》2003,41(16):2491-2501
The diffusion coefficients are reported of rubbery ternary systems consisting of the polymer, its monomer analogue (i.e., the saturated equivalent of the monomer), and trace quantities of oligomers (dimer, trimer, tetramer and hexamer) for 2‐hydroxyethyl methacrylate (HEMA). These have been obtained with pulsed‐field‐gradient NMR spectroscopy with a polymer weight fraction (fp) of 0 ≤ fp ≤ 0.4. The oligomers are macromonomers synthesized with a cobalt catalytic chain‐transfer agent. The diffusion coefficients are about an order of magnitude smaller than those for monomers such as methyl methacrylate; this effect is ascribed to hydrogen bonding in HEMA. The diffusion coefficient Di of an i‐meric oligomer has been fitted with moderate accuracy by an empirical universal scaling relation, Di(fp)/D1(fp) ≈ i, previously found to provide an adequate fit to corresponding data for styrene and for methyl and butyl methacrylates. The approximate empirical scaling relation seems to hold for a remarkably wide range of types of monomer/polymer systems. These results are of use in modeling rates and molecular weight distributions in free‐radical polymerization, particularly for termination (which is chain‐length‐dependent and is controlled by the diffusion coefficient of chains of the low degrees of polymerization studied here). © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 2491–2501, 2003 相似文献
38.
Lynch CL Hale JJ Budhu RJ Gentry AL Finke PE Caldwell CG Mills SG MacCoss M Shen DM Chapman KT Malkowitz L Springer MS Gould SL DeMartino JA Siciliano SJ Cascieri MA Carella A Carver G Holmes K Schleif WA Danzeisen R Hazuda D Kessler J Lineberger J Miller M Emini E 《Organic letters》2003,5(14):2473-2475
[reaction: see text] A novel approach to alpha,alpha-disubstituted-beta-amino acids (beta(2,2)-amino acids) was employed in the synthesis of a series of 3-(pyrrolidin-1-yl)propionic acids possessing high affinity for the CCR5 receptor and potent anti-HIV activity. The rat pharmacokinetics for these new analogues featured higher bioavailabilities and lower rates of clearance as compared to cyclopentane 1. 相似文献
39.
Douglas A. Chapman Szczepan Roszak Phillip B. Keegstra P. C. Hariharan Joyce J. Kaufman Robert S. Buenker 《International journal of quantum chemistry》1991,39(4):541-560
Recently we extended our strategy for MRD-CI (multireference double excitation-configuration interaction) calculations, based on localized/local orbitals and an “effective” CI Hamiltonian, for molecular decompositions of large molecules to breaking a chemical bond in a molecule in a crystalline or other solid environment. Our technique begins with an explicit quantum chemical SCF calculation for a reference molecule surrounded by a number of other molecules in the multipole environment of more distant neighbors. The resulting canonical molecular orbitals are then localized, and the localized occupied and virtual orbitals in the region of interest are included explicitly in the MRD-CI with the remainder of the occupied localized orbitals being folded into an “effective” CI Hamiltonian. The MRD-CI calculations are then carried out for breaking a bond in the reference molecule. This method is completely general in that the space treated explicitly, as well as the surrounding space, may contain voids, defects, deformations, dislocations, impurities, dopants, edges and surfaces, boundaries, etc. Dimethylnitramine is the smallest prototype of the energetic R2N—NO2 nitramines, such as the 6-member ring RDX or the 8-member ring HMX. Decomposition of energetic compounds is initiated in the solid by a breaking of the target bond. Thus, it is crucial to know the difference in energy between breaking a bond in an isolated energetic molecule versus in the molecule in a solid. In the present study, we have carried out MRD-CI calculations for the Me2N—NO2 dissociation of dimethylnitramine in a dimethylnitramine crystal. The cases we investigated were one dimethylnitramine molecule (surrounded by 53 and 685 neighboring dimethylnitramine molecules represented by multipoles), three dimethylnitramine molecules, and three dimethylnitramine molecules (surrounded by 683 neighbors). All multipoles were cumulative atomic multipoles up through quadrupoles. The MRD-CI calculations on dimethylnitramine required large numbers of reference configurations from which were allowed all single and double excitations. 相似文献
40.