Insertion kinetics measurements

The function of the B domain has been confirmed by subcloning and preliminary kinetic measurements. We subcloned the AB domain of E. coli II, residues 348-637, after inserting a restriction site at a position corresponding to residue 348. The purified protein restored mannitol phosphorylation activity when measured with the A domain assay in Fig. 4A, and the B domain assay in Fig. 4B [42]. The B domain... 

The metalloalkyne complex Ru ( )-CH=CH(CH2)4C CH Cl(CO)(P,Pr3)2 exhibits behavior similar to that of cyclohexylacetylene (Scheme 10).40 Thus, it reacts with OsHCl(CO)(P Pr3)2 to give the hydride-vinylidene derivative (P Pr3)2 (CO)ClRu ( )-CH=CH(CH2)4CH=C OsHCl(CO)(P,Pr3)2, which evolves in toluene into the heterodinuclear-pi-bisalkenyl complex (P Pr3)2(CO)ClRu (is)-CH=CH(CH2)4CH=CH-( ) OsCl(CO)(P,Pr3)2. Kinetic measurements between 303 and 343 K yield first-order rate constants, which afford activation parameters ofAH = 22.1 1.5, kcal-mol-1 andAS = -6.1 2.3 cal-K 1-mol 1. The slightly negative value of the activation entropy suggests that the insertion of the vinylidene ligand into the Os—H bond is an intramolecular process, which occurs by a concerted mechanism with a geometrically highly oriented transition state. 

Two-dimensional heteronuclear ( H- N) nuclear magnetic relaxation studies indicate that the dihydrofolate reductase-folate complex exhibits a diverse range of backbone fluctuations on the time-scale of picoseconds to nanoseconds To assess whether these dynamical features influence Michaelis complex formation, Miller et al used mutagenesis and kinetic measurements to assess the role of a strictly conserved residue, namely Gly-121, which displays large-amplitude backbone motions on the nanosecond time scale. Deletion of Gly-121 dramatically reduces the hydride transfer rate by 550 times there is also a 20-times decrease in NADPH cofactor binding affinity and a 7-fold decrease for NADP+ relative to wild-type. Insertion mutations significantly decreased both... 

Despite the short lifetimes of most silylenes, improvements in flash photolysis techniques for their generation and time-resolved spectroscopic detection methods in the past decade have made possible direct kinetic measurements on the reactions of silylenes. The purpose of these kinetic studies has been to elucidate the mechanisms of silylene reactions. While considerable work remains to be done, transition state structures and activation barriers are emerging from these experiments, and aspects of silylene insertion and addition mechanisms have been revealed that were not uncovered by product studies and were, indeed, unexpected. 

The increase in length of the polymer chain occurs by insertion of the monomer in a metal-carbon bond of the active complex. Dyachkovski and co-workers (149) believe, based on kinetic measurements, that the insertion takes place on a titanium cation. An ion of the type (C5H5)2Ti+—R, derived from complexing and dissociation,... 

No precise information about the olefin polymerisation mechanism has been obtained from kinetic measurements in systems with heterogeneous catalysts analysis of kinetic data has not yet afforded consistent indications either concerning monomer adsorption on the catalyst surface or concerning the existence of two steps, i.e. monomer coordination and insertion of the coordinated monomer, in the polymerisation [scheme (2) in chapter 2], Note that, under suitable conditions, each step can be, in principle, the polymerisation rate determining step [241]. Furthermore, no % complexes have been directly identified in the polymerisation process. Indirect indications, however, may favour particular steps [242]. Actually, no general olefin polymerisation mechanism that may be operating in the presence of Ziegler-Natta catalysts exists, but rather the reaction pathway depends on the type of catalyst, the kind of monomer and the polymerisation conditions. 

To extend the comparison between homogeneous and supported systems, we have carried out kinetic measurements on the reactions of polymer-supported [Rh(CO)2I2] complexes with Mel. In order to monitor the reactions in situ, polymer films were inserted between the windows of a conventional infrared liquid cell, fitted with a thermostatted jacket. The cell was then filled with neat Mel or a solution of Mel in CH2C12, and transmission spectra recorded directly through the polymer film immersed in the Mel solution. An example of a series of spectra recorded in this way is shown in Figure 5. The absorption bands of [Rh(CO)2I2] are replaced cleanly by those of the product acetyl complex, [Rh(CO)(COMe)I3], with the high frequency band of the reactant almost coinciding with the terminal v(CO) band of the product. 

The higher content of linear aldehydes in the reaction mixture is explained by the lower steric hindrance of the CO insertion reaction for a linear acyl complex compared to the branched isomer. However, kinetic measurements showed that the reaction rate is inversely proportional to the CO partial pressure, which can be explained by the equilibrium reactions (2-97) and (2-98), the latter preceding oxidative addition of hydrogen. 

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