813 research outputs found
A cooperative Pd-Cu system for direct C-H bond arylation
The authors are grateful to the Royal Society (University Research Fellowship to CSJC) for financial support.A novel and efficient method for C-H arylation using well-defined Pd- and Cu-NHC systems has been developed. This process promotes the challenging construction of C-C bonds from arenes or heteroarenes using aryl bromides and chlorides. Mechanistic studies show that [Cu(OH)(NHC)] plays a key role in the C-H activation and is involved in the transmetallation with the Pd-NHC co-catalyst.Publisher PDFPeer reviewe
Large-scale bottleneck effect in two-dimensional turbulence
The bottleneck phenomenon in three-dimensional turbulence is generally
associated with the dissipation range of the energy spectrum. In the present
work, it is shown by using a two-point closure theory, that in two-dimensional
turbulence it is possible to observe a bottleneck at the large scales, due to
the effect of friction on the inverse energy cascade. This large-scale
bottleneck is directly related to the process of energy condensation, the
pile-up of energy at wavenumbers corresponding to the domain size. The link
between the use of friction and the creation of space-filling structures is
discussed and it is concluded that the careless use of hypofriction might
reduce the inertial range of the energy spectrum
Non-equilibrium temperatures in steady-state systems with conserved energy
We study a class of non-equilibrium lattice models describing local
redistributions of a globally conserved quantity, which is interpreted as an
energy. A particular subclass can be solved exactly, allowing to define a
statistical temperature T_{th} along the same lines as in the equilibrium
microcanonical ensemble. We compute the response function and find that when
the fluctuation-dissipation relation is linear, the slope T_{FD}^{-1} of this
relation differs from the inverse temperature T_{th}^{-1}. We argue that T_{th}
is physically more relevant than T_{FD}, since in the steady-state regime, it
takes equal values in two subsystems of a large isolated system. Finally, a
numerical renormalization group procedure suggests that all models within the
class behave similarly at a coarse-grained level, leading to a new parameter
which describes the deviation from equilibrium. Quantitative predictions
concerning this parameter are obtained within a mean-field framework.Comment: 16 pages, 2 figures, submitted to Phys. Rev.
Enstrophy dissipation in two-dimensional turbulence
Insight into the problem of two-dimensional turbulence can be obtained by an
analogy with a heat conduction network. It allows the identification of an
entropy function associated to the enstrophy dissipation and that fluctuates
around a positive (mean) value. While the corresponding enstrophy network is
highly nonlocal, the direction of the enstrophy current follows from the Second
Law of Thermodynamics. An essential parameter is the ratio of the intensity of driving as a function of
wavenumber , to the dissipation strength , where is the
viscosity. The enstrophy current flows from higher to lower values of ,
similar to a heat current from higher to lower temperature. Our probabilistic
analysis of the enstrophy dissipation and the analogy with heat conduction thus
complements and visualizes the more traditional spectral arguments for the
direct enstrophy cascade. We also show a fluctuation symmetry in the
distribution of the total entropy production which relates the probabilities of
direct and inverse enstrophy cascades.Comment: 8 pages, revtex
Helicity cascades in rotating turbulence
The effect of helicity (velocity-vorticity correlations) is studied in direct
numerical simulations of rotating turbulence down to Rossby numbers of 0.02.
The results suggest that the presence of net helicity plays an important role
in the dynamics of the flow. In particular, at small Rossby number, the energy
cascades to large scales, as expected, but helicity then can dominate the
cascade to small scales. A phenomenological interpretation in terms of a direct
cascade of helicity slowed down by wave-eddy interactions leads to the
prediction of new inertial indices for the small-scale energy and helicity
spectra.Comment: 7 pages, 8 figure
Positively charged amino acids are essential for electron transfer and protein-protein interactions in the soluble methane monooxygenase complex from methylococcus capsulatus (Bath)
The soluble methane monooxygenase (sMMO) complex from Methylococcus capsulatus (Bath) catalyses oxygen- and NAD(P)H-dependent oxygenation of methane, propene and other substrates. Whole-complex sMMO oxygenase activity requires all three sMMO components: the hydroxylase, the reductase and protein B. Also, in the presence of hydrogen peroxide, the hydroxylase alone catalyses substrate oxygenation via the peroxide shunt reaction. We investigated the effect of amine cross-linking on hydroxylase activity in order to probe the role of a gross conformational change that occurs in the hydroxylase upon binding of the other protein components. The cross-linker inhibited hydroxylase activity in the whole complex but this effect was due to covalent modification of primary amine groups rather than cross-linking. Covalent modification of arginine side-chains on the hydroxylase had a similar effect but, most remarkably, neither form of modification affected the activity of the hydroxylase via the peroxide shunt reaction. It was shown that covalent modification of positively charged groups on the hydroxylase, which occurred at multiple sites, interfered with its physical and functional interactions with protein B and with the passage of electrons from the reductase. These results indicate that protein B and the reductase of the sMMO complex interact via positively charged groups on the surface of the hydroxylase to induce a conformational change that is necessary for delivery of electrons into the active site of the hydroxylase. Modification of positively charged groups on protein B had no effect on its function, consistent with the hypothesis that positively charged groups on the hydroxylase interact with negative charges on protein B. Thus, we have discovered a means of specifically inactivating the interactions between the sMMO complex while preserving the catalytic activity of the hydroxylase active site which provides a new method of studying intercomponent interactions within sMMO.</p
Field theory of the inverse cascade in two-dimensional turbulence
A two-dimensional fluid, stirred at high wavenumbers and damped by both
viscosity and linear friction, is modeled by a statistical field theory. The
fluid's long-distance behavior is studied using renormalization-group (RG)
methods, as begun by Forster, Nelson, and Stephen [Phys. Rev. A 16, 732
(1977)]. With friction, which dissipates energy at low wavenumbers, one expects
a stationary inverse energy cascade for strong enough stirring. While such
developed turbulence is beyond the quantitative reach of perturbation theory, a
combination of exact and perturbative results suggests a coherent picture of
the inverse cascade. The zero-friction fluctuation-dissipation theorem (FDT) is
derived from a generalized time-reversal symmetry and implies zero anomalous
dimension for the velocity even when friction is present. Thus the Kolmogorov
scaling of the inverse cascade cannot be explained by any RG fixed point. The
beta function for the dimensionless coupling ghat is computed through two
loops; the ghat^3 term is positive, as already known, but the ghat^5 term is
negative. An ideal cascade requires a linear beta function for large ghat,
consistent with a Pad\'e approximant to the Borel transform. The conjecture
that the Kolmogorov spectrum arises from an RG flow through large ghat is
compatible with other results, but the accurate k^{-5/3} scaling is not
explained and the Kolmogorov constant is not estimated. The lack of scale
invariance should produce intermittency in high-order structure functions, as
observed in some but not all numerical simulations of the inverse cascade. When
analogous RG methods are applied to the one-dimensional Burgers equation using
an FDT-preserving dimensional continuation, equipartition is obtained instead
of a cascade--in agreement with simulations.Comment: 16 pages, 3 figures, REVTeX 4. Material added on energy flux,
intermittency, and comparison with Burgers equatio
Intermittency in the large N-limit of a spherical shell model for turbulence
A spherical shell model for turbulence, obtained by coupling replicas of
the Gledzer, Okhitani and Yamada shell model, is considered. Conservation of
energy and of an helicity-like invariant is imposed in the inviscid limit. In
the limit this model is analytically soluble and is remarkably
similar to the random coupling model version of shell dynamics. We have studied
numerically the convergence of the scaling exponents toward the value predicted
by Kolmogorov theory (K41). We have found that the rate of convergence to the
K41 solution is linear in 1/N. The restoring of Kolmogorov law has been related
to the behaviour of the probability distribution functions of the instantaneous
scaling exponent.Comment: 10 pages, Latex, 3 Postscript figures, to be published on Europhys.
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Non-local interactions in hydrodynamic turbulence at high Reynolds numbers: the slow emergence of scaling laws
We analyze the data stemming from a forced incompressible hydrodynamic
simulation on a grid of 2048^3 regularly spaced points, with a Taylor Reynolds
number of Re~1300. The forcing is given by the Taylor-Green flow, which shares
similarities with the flow in several laboratory experiments, and the
computation is run for ten turnover times in the turbulent steady state. At
this Reynolds number the anisotropic large scale flow pattern, the inertial
range, the bottleneck, and the dissipative range are clearly visible, thus
providing a good test case for the study of turbulence as it appears in nature.
Triadic interactions, the locality of energy fluxes, and structure functions of
the velocity increments are computed. A comparison with runs at lower Reynolds
numbers is performed, and shows the emergence of scaling laws for the relative
amplitude of local and non-local interactions in spectral space. The scalings
of the Kolmogorov constant, and of skewness and flatness of velocity
increments, performed as well and are consistent with previous experimental
results. Furthermore, the accumulation of energy in the small-scales associated
with the bottleneck seems to occur on a span of wavenumbers that is independent
of the Reynolds number, possibly ruling out an inertial range explanation for
it. Finally, intermittency exponents seem to depart from standard models at
high Re, leaving the interpretation of intermittency an open problem.Comment: 8 pages, 8 figure
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