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261.
J Shearer I Y Kung S Lovell W Kaminsky J A Kovacs 《Journal of the American Chemical Society》2001,123(3):463-468
To determine how a substitutionally inert metal can play a catalytic role in the metalloenzyme nitrile hydratase (NHase), a reactive five-coordinate Co(III) thiolate complex ([Co(III)(S(2)(Me2)N(3)(Pr,Pr))](PF(6)) (1)) that resembles the active site of cobalt containing nitrile hydratase (Co NHase) was prepared. This was screened for reactivity, by using low-temperature electronic absorption spectroscopy, toward a number of biologically relevant "substrates". It was determined 1 will react with azide, thiocyanate, and ammonia, but is unreactive toward nitriles, NO, and butyrate. Substrate-bound 1 has similar spectroscopic and structural properties as [Co(III)(ADIT(2))](PF(6)) (2). Complex 2 is a six-coordinate Co(III) complex containing cis-thiolates and imine nitrogens, and has properties similar to the cobalt center of Co NHase. Substrate binding to 1 is reversible and temperature-dependent, allowing for the determination of the thermodynamic parameters of azide and thiocyanate binding and the rates of ligand dissociation. Azide and thiocyanate bind trans to a thiolate, and with similar entropies and enthalpies (thiocyanate: DeltaH = -7.5 +/- 1.1 kcal/mol, DeltaS = -17.2 +/- 3.2 eu; azide: DeltaH = -6.5 +/- 1.0 kcal/mol, DeltaS = -12.6 +/- 2.4 eu). The rates of azide and thiocyanate displacement from the metal center are also comparable to one another (k(d) = (7.22 +/- 0.04) x 10(-)(1) s(-)(1) for thiocyanate and k(d) = (2.14 +/- 0.50) x 10(-)(2) s(-)(1) for azide), and are considerably faster than one would expect for a low-spin d(6) six-coordinate Co(III) complex. These rates are comparable to those of an analogous Fe(III) complex, demonstrating that Co(III) and Fe(III) react at comparable rates when in this ligand environment. This study therefore indicates that ligand displacement from a low-spin Co(III) center in a ligand environment that resembles NHase is not prohibitively slow so as to disallow catalytic action in nonredox active cobalt metalloenzymes. 相似文献
262.
Iron-sulfur clusters are prevalent in biological systems. Through studies of iron-sulfur proteins and synthetic model clusters, it was realized early on that these clusters functioned as facile electron transfer agents. Until recently it was widely thought that they served exclusively in that capacity. However, in the last decade, it has become clear that their reactivities and biological functions are much more diverse. It is now apparent that these clusters can serve as the active sites of enzymes, as well as in the regulation of enzymatic activity. Synthetic clusters, which have been shown to undergo a variety of core rearrangements or structural changes, have provided insight into possible mechanisms of cluster formation or activity regulation in enzymes. Rigid tripodal ligands have been constructed which capture synthetic iron-sulfur clusters in a cavity which permits controlled reactivity studies. In this article, we review these recent developments and suggest some future directions the field may take. 相似文献
263.
The Crystal Structure of tBu2P? P?P(Br)tBu2 tBu2P? P?P(Br)tBu2 1 crystallizes in the monoclinic space group P21/c with a = 2 888.9(3), b = 972.16(10), c = 1 534.04(14) pm, β = 105.129(8)° and 8 formula units in the unit cell. The two independent P3-units in 1 form angles of 105.77° or 105.98°, resp. One P? P distance (220,4 pm) corresponds to a single bond, the other one (207.9 pm) to a double bond. 相似文献
264.
Summary Generalized conjugate gradient algorithms which are invariant to a nonlinear scaling of a strictly convex quadratic function are described. The algorithms when applied to scaled quadratic functionsfR
n
R
1 of the formf(x)=h(F(x)) withF(x) strictly convex quadratic andhC
1(R
1) an arbitrary strictly monotone functionh generate the same direction vectors as for the functionF without perfect steps. 相似文献
265.
tBu2P? P?P(X)tBu2 Ylides (X = Cl, Br, I) by Halogenation of [tBu2P]2P? SiMe3 [tBu2P]2P? SiMe3 1 with halogenating agents as Br2, I2, Br-succinimide, CCl4, CBr4, CI4 or C2Cl6 via cleavage of the Si? P bond in 1 produces the ylides tBu2P? P?P(X)tBu2 (X = Cl, Br, I). This proceeds independent from the formerly known pathway – [tBu2P]2PLi + 1,2-dibromoethane – and shows that the Li-phosphide must not be present as a necessary requirement for the formation of ylides. 相似文献
266.
Reactions of tBu(Me3Si)P? P(Li)? P(tBu)2 with CH3Cl and 1,2-Dibromoethane tBu(Me3Si)P? P(Li)? P(tBu)2 · 0.95 THF 1 with CH3Cl (?70°C) yields tBu(Me3Si)P? P = P(Me)(tBu)2 2 at ?70°C, with 1,2-Dibromoethane tBu(Me3Si)P? PBr? P(tBu)2 3 (main product) and tBu(Me3Si)P? P?P(Br)tBu2 4. 3 eliminates Me3SiBr yielding the cyclotetraphosphane {tBuP? P[P(tBu)2]}2 5 . 相似文献
267.
Tibor Cserhati Eva Fenyvesi Jozsef Slejtli 《Journal of inclusion phenomena and macrocyclic chemistry》1992,14(2):181-188
The modification of the hydrophobicity of some ethoxylated nonylphenol and tributylphenol surfactants with various soluble -cyclodextrin polymers has been studied by reversed-phase chromatography. Stepwise regression analysis proved that the complex forming capacity of surfactants decreases with increasing diameter of the hydrophobic moiety of surfactants, the properties of the crosslinking agent used for preparation of the polymers has no significant effect on the host-guest interaction, the presence of carboxyl groups in the polymer considerably improved the complex stability. 相似文献
268.
In the present note we show asynchronous exponential growth for important age- and size-structured population models. 相似文献
269.
270.
Mihaly Kovacs 《Semigroup Forum》2005,71(3):462-470
For a generator $A$ of a $C_0$-semigroup $T(\cdot)$ on a Banach space $X$ we consider the semi-norm $M^{k}_x:=\limsup_{t\to
0+}\|t^{-1}(T(t)-I)A^{k-1}x\|$ on the Favard space ${\cal F}_{k}$ of order $k$ associated with $A$. The use of this semi-norm
is motivated by the functional analytic treatment of time-discretization methods of linear evolution equations. We show that
sharp inequalities for bounded linear operators on ${\cal D}(A^k)$ can be extended to the larger space ${\cal F}_{k}$ by using
the semi-norm $M^{k}_{(\cdot)}$. We also show that $M^{k}_{(\cdot)}$ is a norm equivalent to the norms that are usually considered
in the literature if A has a bounded inverse. 相似文献