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541.
542.
Catalytic properties of two glucoamylases, AmyC and AmyD, without starch binding domains from Rhizopus oryzae strain 99-880 are determined using heterologously expressed enzyme purified to homogeneity. AmyC and AmyD demonstrate pH optima of 5.5 and 6.0, respectively, nearly one unit higher than the Rhizopus AmyA glucoamylase enzyme. Optimal initial activities are at 60 and 50 °C for AmyC and AmyD, respectively. Inactivation of both enzymes occurs at 50 °C following 30 min pre-incubation. The two enzymes demonstrate substantially slower catalytic rates toward soluble starch relative to AmyA. AmyC has similar k cat and K m for oligosaccharides to other Rhizopus and Aspergillus glucoamylases; however, the enzyme has a 2-fold lower K m maltose . AmyD has a 3-fold higher K m and lower k cat for maltooligosaccharides than AmyC and other glucoamylases. AmyC (but not AmyD) exhibits substrate inhibition. K i for substrate inhibition decreases with increasing length of the oligosaccharides. Data from pre-steady-state binding of AmyC to maltose and maltotriose and pre-steady-state to steady-state catalytic turnover experiments of AmyC acting on maltotriose were used to interrogate models of substrate inhibition. In the preferred model, AmyC accumulates an enzyme-maltose-maltotriose dead-end complex in the steady state.  相似文献   
543.
Verena T. Trepohl 《Tetrahedron》2009,65(33):6510-6518
The Rh(I)-catalyzed conjugate phosphinyl transfer from an Si-P reagent to an electron-deficient acceptor requires individual protocols for cyclic and acyclic α,β-unsaturated carbonyls and carboxyls. While 1,4-addition to cyclic acceptors is catalyzed by a Rh(I)-phosphine complex, a Rh(I)-carbene complex is needed to promote conjugate phosphination of acyclic acceptors. General procedures for both systems are reported. Aside from monophosphine-derived Si-P reagents as phosphinide sources, dppe- as well as dppp-derived reagent having two Si-P units do also participate in this reaction. The mechanism of this Rh(I)-catalyzed activation of the Si-P reagent is still under debate. Control experiments with enantiopure silicon-stereogenic and racemic phosphorus-stereogenic Si-P reagents support a catalysis commencing with transmetalation rather than oxidative addition. Preparation and full characterization data of the Si-P compounds used in this investigation is included.  相似文献   
544.
To solve linear programming problems by interior point methods an approximately centered interior point has to be known. Such a point can be found by an algorithmic approach – a so-called phase 1 algorithm or centering algorithm. For random linear programming problems distributed according to the rotation symmetry model, especially with normal distribution, we present probabilistic results on the quality of the origin as starting point and the average number of steps of a centering algorithm.  相似文献   
545.
546.
547.
A total synthesis of (-)-centrolobine (1) based on a diastereoselective ring rearrangement metathesis-double bond isomerisation sequence and a one-pot cross metathesis-hydrogenation procedure is described.  相似文献   
548.
Cyclocondensation reactions of 3,5-dioxopimelates, new and versatile synthetic building blocks, with hydrazines, urea, thiourea and phenylene-1,2-diamines allow for a convenient synthesis of various heterocycles containing two ester-substituted side-chains.  相似文献   
549.
This paper generalizes Shelah’s generic pair conjecture (now theorem) for the measurable cardinal case from first order theories to finite diagrams. We use homogeneous models in the place of saturated models.  相似文献   
550.
We investigate the dynamics of a lattice soliton on a monatomic chain in the presence of damping and external forces. We consider Stokes and hydrodynamical damping. In the quasi-continuum limit the discrete system leads to a damped and forced Boussinesq equation. By using a multiple-scale perturbation expansion up to second order in the framework of the quasi-continuum approach we derive a general expression for the first-order velocity correction which improves previous results. We compare the soliton position and shape predicted by the theory with simulations carried out on the level of the monatomic chain system as well as on the level of the quasi-continuum limit system. For this purpose we restrict ourselves to specific examples, namely potentials with cubic and quartic anharmonicities as well as the truncated Morse potential, without taking into account external forces. For both types of damping we find a good agreement with the numerical simulations both for the soliton position and for the tail which appears at the rear of the soliton. Moreover we clarify why the quasi-continuum approximation is better in the hydrodynamical damping case than in the Stokes damping case. Received 22 August 2001 and Received in final form 7 December 2001  相似文献   
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