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  • 1990-1994  (9)
  • 1840-1849
  • 1994  (9)
  • 1
    Electronic Resource
    Electronic Resource
    Large fluctuations and optimal paths in chemical kinetics (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 100 (1994), S. 5735-5750 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The eikonal approximation (instanton technique) is applied to the problem of large fluctuations of the number of species in spatially homogeneous chemical reactions with the probability density distribution described by a master equation. For both autocatalytic and nonautocatalytic reactions, the analysis of the distribution about a stable stationary state and of the transitions between coexisting stable states comes, to logarithmic accuracy, to the analysis of Hamiltonian dynamics of an auxiliary dynamical system. The latter can be done explicitly in a few cases, including one-species systems, systems with detailed balance, and systems close to the bifurcation points where the number of the stable states changes. In the last case, the fluctuations display universal features, and, for saddle-node bifurcation points, the logarithm of the probability of escape from the metastable state (per unit time) is proportional to the distance to the bifurcation point (in the parameter space) raised to the power 3/2. We compare the eikonal approximation for the stationary distribution of a master equation to Monte Carlo numerical solutions for two chemical two-variable systems with multiple stationary states, where none of the cited restrictions exists. For one of the systems in the pattern of optimal paths we observe caustics emanating from the saddle point.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.467139
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  • 2
    Electronic Resource
    Electronic Resource
    Random paths and fluctuation–dissipation dynamics for one-variable chemical systems far from equilibrium (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 100 (1994), S. 7279-7294 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A random path integral representation of the Ross–Hunt–Hunt thermodynamic and stochastic theory is given for chemical reactions far from equilibrium in the case of constant-step and one-variable processes. An explicit analytical expression for the chemical Lagrangian is presented. A connection is made between the thermodynamic fluctuation–dissipation regimes characteristic to the process and the chemical Lagrangian. The path integral formalism is used to prove the validity of fluctuation regression hypothesis and to derive two variational principles for the most probable and average paths, respectively. The most probable path corresponds to the absolute maximum of the Lagrangian and the average path corresponds to the minimum value of the information gain obtained by observing a certain average path. For nonlinear regimes these two variational principles generally give distinct results; they are identical only in the vicinity of a stable steady state. An eikonal approximation is suggested for evaluating time-dependent probability distributions which reduces the integration of Master Equations to two quadratures. The suggested eikonal approximation leads to a proportionality between the species-specific free energy of the system and the extremal value of the time integral of the chemical Lagrangian. This relationship is similar to the expression of the mechanical action in terms of the Lagrangian in classical mechanics. Most results derived in this paper for one variable can be extended to multivariable systems. Finally a comparison is made with other stochastic approaches to nonequilibrium thermodynamics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.466874
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    Paper (German National Licenses)
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  • 3
    Electronic Resource
    Electronic Resource
    Fluctuation–dissipation relations for chemical systems far from equilibrium (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 100 (1994), S. 7268-7278 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A new fluctuation–dissipation relation is suggested for constant step, one intermediate chemical processes far from equilibrium. It establishes a relationship between the net reaction rate t˜(x), the probability diffusion coefficient D(x) in the composition space, and the species-specific affinity A(x): t˜(x)=2D(x)tanh[−A(x)/2kT], where x is the concentration of the active intermediate, k is Boltzmann's constant, and T is the absolute temperature. The theory is valid for nonlinear fluctuations of arbitrary size. For macroscopic systems the fluctuation–dissipation relation may be viewed as a force-flux relationship. We distinguish four fluctuation–dissipation regimes which correspond to the decrease of the absolute value of the species-specific affinity. The passage from high ||A(x)|| to small ||A(x)|| corresponds to a crossover from a linear dependence of the species-specific dissipation rate φ(overdot)(x) on ||A(x||)||, φ(overdot)(x)∼−||A(x)||, to a square one: φ(overdot)(x)∼−A2(x). A main feature of the fluctuation–dissipation relation is its symmetry with respect to the contributions of the forward and backward chemical processes to fluctuation and relaxation. Two new physical interpretations of the probability diffusion coefficient are given: one corresponds to a measure of the strength of fluctuations at a steady state, and the other to a measure of the instability of a given fluctuation state. The dispersion of the number q of reaction events in a given time interval is given by a generalized Einstein relation: 〈Δq2〉=2VD(x)t, where V is the volume of the system. The diffusion coefficient D(x) is proportional to the reciprocal value of the mean age 〈τ(x)〉 of a fluctuation state characterized by the concentration x: D(x)=1/[2V〈τ(x)〉]. These interpretations are not related to the use of a Fokker–Planck approximation of the chemical Master Equation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.466873
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  • 4
    Electronic Resource
    Electronic Resource
    Thermodynamic approach to nonequilibrium chemical fluctuations (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 100 (1994), S. 7295-7309 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A generalized thermodynamic description of one-variable complex chemical systems is suggested on the basis of the Ross, Hunt, and Hunt (RHH) theory of nonequilibrium processes. Starting from the stationary solution of a chemical Master Equation, two complimentary, related sets of generalized state functions are introduced. The first set of functions is derived from a generalized free energy FX, and is used to compute the moments of stationary and non-Gaussian concentration fluctuations. Exact expressions for the cumulants of concentration are derived; a connection is made between the cumulants and the fluctuation–dissipation relations of the RHH theory. The second set of functions is derived from an excess free energy φ(x); it is used to express the conditions of existence and stability of nonequilibrium steady states. Although mathematically distinct, the formalisms based on the FX and φ(x) functions are physically equivalent: both lead to the same type of differential expressions and to similar global equations. A comparison is made between the RHH and Keizer's theory of nonequilibrium processes. An appropriate choice of the integration constants occurring in Keizer's theory is made for one-variable systems. The main differences between the two theories are: the constraints for the two theories are different; the stochastic and thermodynamic descriptions are global in RHH, whereas Keizer's theory is local. However, both theories share some common features. Keizer's fluctuation–dissipation relation can be recovered by using the RHH approach; it is valid even if the fluctuations are nonlinear.If the thermodynamic constraints are the same, then Keizer's theory is a first-order approximation of RHH; this approximation corresponds to a Gaussian description of the probability of concentration fluctuations. Keizer's theory is a good approximation of RHH in the vicinity of a stable steady state: near a steady state the thermodynamic functions of the two theories are almost identical; the chemical potential in the stationary state is of the equilibrium form in both theories. Keizer's theory gives a very good estimate of the absolute values of the peaks of the stationary probability density of RHH theory. Away from steady states the predictions of the two theories are different; the differences do not vanish in the thermodynamic limit. The shapes of the tails of the stationary probability distributions are different; and hence the predictions concerning the relative stability are different for the two theories.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.467265
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    Paper (German National Licenses)
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  • 5
    Electronic Resource
    Electronic Resource
    Toward a thermodynamic theory of hydrodynamics: The Lorenz equations (1994)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 6 (1994), S. 550-563 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Earlier work on the thermodynamics of nonlinear systems is extended to the Lorenz model in a first attempt to apply the theory to hydrodynamics. An excess work, Φ, related to the work necessary for displacement of the system from stationary states is defined. The excess work Φ is shown to have the following properties: (1) The differential of Φ is expressed in terms of thermodynamic functions: the energy for viscous flow and the entropy for thermal conduction when taken separately; (2) Φ is an extremum at all stationary states, a minimum (maximum) at stable (unstable) stationary states, and thus yields necessary and sufficient criteria for stability; (3) Φ describes the bifurcation from homogeneous to inhomogeneous stationary states; (4) Φ is a Lyapunov function with physical significance parallel to that of the Gibbs free energy change (maximum work) on relaxation to an equilibrium state; (5) Φ is the thermodynamic "driving force'' (potential) toward stable stationary states; (6) Φ(overdot) is a component of the total dissipation during the relaxation toward a stable stationary state; (7) for constraints leading to equilibrium Φ reduces to the known thermodynamic function, which is the work of displacing the system from the equilibrium for those given constraints; and (8) Φ qualitatively explains the positive energy release in both the destruction and formation of a convective structure in a Rayleigh–Bénard experiment.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.868351
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    Paper (German National Licenses)
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  • 6
    Electronic Resource
    Electronic Resource
    Selective suppression of the magnocellular visual pathway during saccadic eye movements (1994)
    [s.l.] : Nature Publishing Group
    Nature 371 (1994), S. 511-513 
    Details
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] We measured contrast sensitivity to horizontal gratings of various spatial frequencies, briefly presented during large voluntary horizontal saccades. As the saccades ran parallel to the bars of the gratings, they produced no effective image motion. The gratings were modulated sinusoidally either in ...
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1038/371511a0
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  • 7
    Electronic Resource
    Electronic Resource
    Gibberellins and phytochrome regulation of stem elongation in pea (1994)
    Springer
    Planta 192 (1994), S. 489-496 
    Details
    ISSN: 1432-2048
    Keywords: Gibberellin (levels, response) ; Light (phytochrome B) ; Mutant (photomorphogenic, gibberellin) ; Pisum (mutants) ; Stem elongation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract In garden pea (Pisum sativum L.) neither etiolation nor the phytochrome B (phyB)-response mutation lv substantially alters the level of the major active endogenous gibberellin, GA1 in the apical portion of young seedlings. The phyB-controlled responses to continuous red light and end-of-day far-red light are retained even in a GA-overproducing mutant (sln). Comparison of the effects of the lv mutation and GA1 application on seedling development shows important differences in rate of node development, cell extension and division, and leaf development. These results suggest that in pea the control of stem elongation by light in general and phyB in particular is not mediated by changes in GA1 content. Instead, the increased elongation of dark-grown and lv plants appears to result from increased responsiveness of the plant to its endogenous levels of GA1. Three GA1-deficient mutants, na, ls and le have been used to investigate these changes in responsiveness, and study of these and the double mutants na lv, ls lv and le lv has demonstrated that the relative magnitude of the change in responsiveness is dependent on GA1 level. The difference in pleiotropic effects of GA1 application and the lv mutation suggest that light and GA1 interact late in their respective transduction pathways. A model for the relationship between light, GA1 level and elongation in pea is presented and discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00203586
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  • 8
    Electronic Resource
    Electronic Resource
    Recent advances in the study of gibberellin mutants (1994)
    Springer
    Plant growth regulation 15 (1994), S. 193-206 
    Details
    ISSN: 1573-5087
    Keywords: elongated ; gibberellins ; mutants ; response ; synthesis
    Source: Springer Online Journal Archives 1860-2000
    Topics: Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: Abstract Recent advances in the study of GA mutants are reviewed. Endogenous GAs in the vast majority of GA synthesis and response mutants have now been quantified by physicochemical means, and the implications of the results are discussed. In recent papers the effects of synthesis mutations on processes other than stem elongation have received increased attention, as has the advent of mutants with reportedly elevated GA levels. The ‘feedback theory’ has been formulated, explaining paradoxical observations on endogenous GA levels in certain response mutants. In a significant breakthrough, a GA biosynthesis gene has been cloned, paving the way for a combined approach to future GA research, involving GA mutants, physicochemical analyses, and molecular techniques.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00029892
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  • 9
    Electronic Resource
    Electronic Resource
    Large scale production and semi-purification of kedarcidin in a 1000-L fermentor (1994)
    Springer
    Journal of industrial microbiology and biotechnology 13 (1994), S. 356-360 
    Details
    ISSN: 1476-5535
    Keywords: Kedarcidin ; Antitumor antibiotic ; Chromoprotein
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Summary Actinomycete strain ATCC 53650 was grown in a 1000-L fermentor containing 680 L of medium and the production of kedarcidin was monitored by HPLC. The titers of kedarcidin in the fermentor cultures were 0.49–0.53 mg ml−1. A quick and efficient purification method involving the use of anion exchange resin DE23 (batch adsorption-desorption) and an ultrafiltration system yielded high recovery (65% yield) of kedarcidin from the fermentor culture. Over 200 grams of lyophilized kedarcidin of 70% purity was recovered from each of two 1000-L fermentor cultures using this process.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01577219
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