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  • 1
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 14 (1978), S. 675-694 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Theoretical issues that must be considered for calculating rates of intramolecular electron transfer are discussed. The process itself is well defined only if the ground state is well approximated by a localized electronic function (Robin-Day class I or II). Under those conditions, the linear-response formalism yields the rate as the Fourier transform of the particle-hole propagator. We discuss the validity of electron propagators as approximations to particle-hole propagators, and find that they are valid, for simple models, within the stability range of the Hartree-Fock procedure. The vibronic nature of the transfer process is stressed, and formal schemes are given for calculating direct and through-bridge transfer rates. Outstanding difficulties, and applications to biological systems, are very briefly noted.
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 19 (1981), S. 131-137 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A compact and efficient scaled single-zeta basis set has been developed for use in conjunction with the coreless Hartree-Fock silicon effective potential. The scale factors were determined by minimizing the electronic energy of the disilane molecule. Based upon a generalized valence bond computation using this basis, we conclude that the classical concept of localized σ bonds is adequate to fully explain the electronic structure of disilane in analogy to the ethane molecule.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 34 (1988), S. 707-720 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: This article focuses on the effects of specific solvation on electron transfer (ET) reactions and illustrates how differing configurations of solvent molecules can change the ET rate. The evolution of the ET system is given by a recently developed dynamical model for ET [1]. The dynamical model allows for evolution of the ET system dynamically coupled to the outer solvent and the intramolecular degrees of freedom. The evolution of the ET system is followed in direct time using time-dependent statistical density operators. The initial state of the ET system is selected by a variational scheme, thereby determining an appropriate time-dependent statistical density operator for the initial state. The electronic subsystem is treated by a full ab-initio electronic Hamiltonian and allows for a detailed description of the electronic structure. The ET system consists of a donor, an acceptor, and a selection of solvent/bridge molecules; this encounter is surrounded by a dielectric medium [1, 2]. The actual calculations concern an ET system consisting of benzene anion radical as donor, benzene as acceptor, and three water molecules in the first solvation shell. These calculations show how important specific solvation of the donor-acceptor complex is for the ET process. The ET probability can be increased or decreased depending on the configuration of the water molecules.
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 14 (1978), S. 767-777 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Ab initio molecular-orbital theory with the STO-3G and 4-31 G basis sets has been used to study the 1,3-sigmatropic hydrogen rearrangements: propene→propene, formic acid→formic acid, and vinyl alcohol→acetaldehyde, and the1,5-shifts:1,3-pentadiene→ 1,3-pentadiene and β-hydroxyacrolein→ β-hydroxyacrolein. Transition states have been determined using gradient procedures. Improved descriptions of the energies of the reactions have been obtained using 3 × 3 configuration interaction. In accord with expectations based on orbital-symmetry considerations, the calculated barriers are considerably greater for 1,3- than for 1,5-shifts. The forbidden pathway for the degenerate 1,3-shift in propene is predicted to require less activation energy than the allowed pathway, a result that can be rationalized in terms of interactions with subjacent and superjacent orbitals.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 26 (1984), S. 195-211 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Current experimental efforts, both with isolated binuclear metal complexes and with modified proteins, are focusing on the role of distance and of bridging groups in intramolecular electron transfer. After a brief overview both of standard and nonadiabatic electron transfer rates (in Hopfield's formulation) and of the current experiments. We consider the possiblility of bridge-assisted intramolecular electron transfer in three specific situations. For the bridged binuclear transition-metal complexes studied by Taube and his students, the tunneling integral T can be deduced by study of the optical intervalence transfer band. Here we discuss semiempirical calculations which suggest that bridge assistance occurs, and determine the size of T. The assistance is via a superexchange-type mechanism, and we suggest that hole-type or electron-type superexchange should dominate in saturated or unsaturated bridges, respectively. For the very strongly coupled Creutz-Taube ion, involving a pyrazine bridge, the electronic structure study of Ondrechen et al. requires the invoking of a three-site model to understand the optical and EPR data; this involves bridge-assisted transfer with a vengeance. Finally, for an oxide-bridged phthalocyanine dimer, the transfer takes place via ring π-π overlap, and no bridge assistance occurs. Thus bridge assistance in T will depend on the geometry and energetics of the specific case under study. One generally suspects, however, that bridge assistance will be found in very long-range (〉6 Å) transfer.
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
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  • 6
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 32 (1987), S. 341-354 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A new treatment for studying electron transfer reactions is considered. The treatment allows for a detailed description of the electronic structure and is therefore appropriate for a complete description of weakly interacting electron transfer reactions. The initial state for the electron transfer system is selected by a variational scheme. The electron transfer is pictured to take place in an encounter complex consisting of the donor, the acceptor, and (optionally) a solvent/bridge molecule. The electronic structure of the encounter complex is dynamically coupled to the surrounding solvent, and the direct time evolution of the electron transfer system is followed by a nonlinear time-dependent Hartree-Fock method. We present examples with benzeneanion radical as the donor, pyridine as the acceptor, and water as the bridge/solvent molecule. The general formalism appears to be a useful approach for studying electron transfer reactions in solution.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
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  • 7
    ISSN: 0362-2525
    Keywords: Life and Medical Sciences ; Cell & Developmental Biology
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Biology , Medicine
    Notes: In the first half of this century, several workers observed small, seemingly glandular structures attached to the ampullate glands of spiders. Hence, they were termed accessory ampullate glands. In juvenile Araneus cavaticus, two pairs of these structures are present (starting at least with third instars), one pair attached to the major ampullate (MaA) glands and the other pair attached to the minor ampullate (MiA) glands. In adults, two pairs of accessory MaA glands and two pairs of accessory MiA glands are present. The two latter-formed pairs of accessory ampullate glands are clearly the remnants of those ampullate glands which atrophy shortly after adulthood is reached. Morphological similarities between these accessory ampullate glands and those present in juveniles provide an indication that the latter also have their origin in functional ampullate glands.A reduction in the number of ampullate glands following the last molt occurs in many spiders. The reason(s) for these reductions is unknown. In penultimate spiders close to ecdysis, we have observed that while the larger pairs of MaA and MiA glands (those that are retained in the adult) are undergoing molt-related changes which apparently render them nonfunctional, their smaller counterparts are seemingly unaffected and functional. This raises the possibility that the principal role of the smaller ampullate glands may be to assume functions during the pre-ecdysial period which are normally in the domain of the larger ampullate glands. If true, then their degeneration after the last molt would make economic sense.The presence of cylindrical spigots in juvenile females starting with fourth instars is documented.
    Additional Material: 32 Ill.
    Type of Medium: Electronic Resource
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  • 8
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    Cell Motility and the Cytoskeleton 25 (1993), S. 345-357 
    ISSN: 0886-1544
    Keywords: fluorescent analogue cytochemistry ; cytoskeletal transport ; photobleach technology ; Life and Medical Sciences ; Cell & Developmental Biology
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Biology , Medicine
    Notes: We have examined the effects of various means of photobleaching on the recovery of fluorescene, movement, and morphology of the microtubules in the neurites of rhodamine-tubulin-injected PC12 cells. We find that, depending on power of and time of exposure to the bleaching beam, we can generate at least three different patterns of fluorescence recovery in regenerating PC12 neurites. If bleaching is performed with a relatively low-power beam for an extended period, fluorescence in polymer recovers very little after 1 hours. Under these conditions, however, tubulin immunostaining is seen extending through the bleach zone, and microtubules are present through the bleached zone in thin section electron micrographs. If bleaching is performed with a high-power laster, for 0.5-5 seconds, fluorescence recovery also is quite slow, but electron microscopic observations reveal that no microtubules extend through the bleached region of the neurite, and the uranyl acetate-stained cytoplasm appears more electron lucent than in the unbleached neurite. Finally, if bleaching is performed by very brief exposure to a high-intensity laser beam, resulting in an incomplete reduction of fluorescence intensity through the bleach zone, fluorescence recovery occurs within 20-30 minutes, and immunostained microtubules appear intact through the bleach zone; electron microscopy confirms that microtubules extended through the bleached zone of such neurites. In all three cases, movement of the bleach zone is observed in approximately half of the experimental neurites. These results indicate that highly variable microtubule behaviors can be obtained with photobleach technology, presumably due to different levels and pathways of photodamage generated by different bleach protocols. Nevertheless, it is clear that both turnover and movement of microtubules occur in FC12 neurites, and both are likely to be involved in neurite maintenance and growth. © 1993 Wiley-Liss, Inc.
    Additional Material: 8 Ill.
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  • 9
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    Cell Motility and the Cytoskeleton 22 (1992), S. 117-126 
    ISSN: 0886-1544
    Keywords: algae ; cell division ; cytokinesis ; mitosis ; Life and Medical Sciences ; Cell & Developmental Biology
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Biology , Medicine
    Notes: We have used two monoclonal antibodies to demonstrate the presence and localization of actin in interphase and mitotic vegetative cells of the green alga Chlamydomonas reinhardtii. Commercially available monoclonal antibodies raised against smooth muscle actin (Lessard: Cell Motil. Cytoskeleton 10:349-362, 1988; Lin: Proc. Natl. Acad. Sci. USA 78:2335-2339, 1981) identify Chlamydomonasactin as a ∼43,000-Mr protein by Western immunoblot procedures. In an earlier study, Detmers and coworkers (Cell Motil. 5:415-430, 1985) first identified Chlamydomonas actin using NBD-phallacidin and an antibody raised against Dictyostelium actin; they demonstrated that F-actin is localized in the fertilization tubule of mating gametes. Here, we show by immunofluorescence that vegetative Chlamydomonas cells have an array of actin that surrounds the nucleus in interphase cells and undergoes dramatic reorganization during mitosis and cytokinesis. This includes the following: reorganization of actin to the ante- rior of the cell during preprophase; the formation of a cruciate actin band in prophase; reorganization to a single anterior actin band in metaphase; rearrange- ment forming a focus of actin anterior to the metaphase plate; reextension of the actin band in anaphase; presence of actin in the forming cleavage furrow during telophase and cytokinesis; and finally reestablishment of the interphase actin array. The studies presented here do not allow us to discriminate between G and F-actin. None the less, our observations, demonstrating dynamic reorganization of actin during the cell cycle, suggest a role for actin that may include the movement of basal bodies toward the spindle poles in mitosis and the formation of the cleavage furrow during cytokinesis. © 1992 Wiley-Liss, Inc.
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
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  • 10
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    Cell Motility and the Cytoskeleton 4 (1984), S. 197-213 
    ISSN: 0886-1544
    Keywords: gelation ; actin ; filamin ; cytoplasm ; Life and Medical Sciences ; Cell & Developmental Biology
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Biology , Medicine
    Notes: We have compared the meniscus depletion assay and falling ball viscometry, two means of assessing the extent of gelation in actin-based systems using mixtures of actin and the actin-binding protein filamin. We examined the effect of varying the concentrations of actin and filamin in both assays. The interaction of actin and filamin was detected only above a threshold concentration of filamin. This threshold concentration was lower for falling ball viscometry than for the meniscus depletion assay at equal actin concentrations. At constant concentrations of filamin, an increase in actin concentration caused an increase in apparent viscosity measured by the falling ball assay, but a decrease in sedimentability detected by the meniscus depletion assay. The rate of sedimentation of actin was dependent on the molar ratio of actin to filamin. At each molar ratio, the sedimentation of actin was not dependent on the specific concentrations of actin and filamin used. The apparent viscosity was dependent on both the molar ratio and the specific concentrations of actin and filamin. To relate the present results to earlier studies, we examined mixtures of actin and filamin using a macroscopic assay of gelation (tube tipping assay), and polarized light microscopy. The effect of increasing filamin concentration in the four assays was compared at three actin concentrations. Mixtures of actin and filamin whose apparent viscosities were low enough to be estimated by falling ball viscometry were optically isotropic fluids that flowed out of inverted test tubes. Mixtures of actin and filamin in the range of sensitivity of the meniscus depletion assay were either viscous fluids or gels, and were either optically isotropic or anisotropic. Thus, the four assays provide different estimates of gelation. Both the meniscus depletion assay and falling ball viscometry can be used to determine relative gelation activity, but neither can be used as a quantitative assay of gelation.
    Additional Material: 7 Ill.
    Type of Medium: Electronic Resource
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