Rate of dissociation

The preexponent for the rate of dissociation of molecules adsorbed at a surface can be a few orders of magnitude less than thatfor desorption. 

Then the transition state for dissociation is a tight transition state. 

TABLE 4.9. Pre-Exponential Factors for Dissociation of Adsorbed Molecules  

When the (p.fff of the dissociating molecule corresponds to that of the tight transition-state complex 

The frequencies of the complex are much higher than kT/h. Then 

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Leu G-H, Huang C-L, Lee S-H, Lee Y-C and Chen l-C 1998 Vibrational levels of the transition state and rate of dissociation of triplet acetaldehyde J. Chem. Phys. 109 9340-50... 

Tetraneopentyltitanium [36945-13-8] Np Ti, forms from the reaction of TiCl and neopentyllithium ia hexane at —80° C ia modest yield only because of extensive reduction of Ti(IV). Tetranorbomyltitanium [36333-76-3] can be prepared similarly. When exposed to oxygen, (NpO)4Ti forms. If it is boiled ia ben2ene, it decomposes to neopentane. When dissolved ia monomers, eg, a-olefins or dienes, styrene, or methyl methacrylate, it initiates a slow polymerisation (211,212). Results from copolymerisation studies iadicate a radical mechanism (212). Ultraviolet light iacreases the rate of dissociation to... 

The absolute rate of dissociation of the radical anion of /i-nitrobenzyl chloride has been measured as 4 x 10 s . The w-nitro isomer does not undergo a corresponding reaction. This is because the meta nitro group provides no resonance stabilization of the benzylic radical. 

The rate of evaporation is given by the intrinsic rate of dissociation of bound molecules from the surface... 

In general, the kinetics of most allosteric modulators have been shown to be faster than the kinetics of binding of the tracer ligand. This is an initial assumption for this experimental approach. Under these circumstances, the rate of dissociation of the tracer ligand (pA t) n the presence of the allosteric ligand is given by [11, 12]... 

Overall reaction rates for dual substrates are the sum of the rates of dissociation of two substrates. 

The curve for the conversion of the unsubstituted ( -benzenediazoate in Figure 5-3 is consistent with the (ii)-diazoate-diazohydroxide pre-equilibrium followed by the slow and pH-independent elimination of the hydroxide ion from the ( )-diazohydroxide (rate constant 6 in Scheme 5-14) as found by Lewis and Suhr. Below pH 3 the acid-catalyzed dissociation of ( >diazohydroxide (k 6) is observable. Electron-withdrawing substituents such as N02 in the 2- or 4-position reduce the rate of dissociation of diazohydroxides and increase the rate of (E) (Z)... 

Broxton and Roper measured the rate of dissociation (A 3) of the (ii)-diazo ether, A 2, and the rate of the protection reaction (A p), i.e., the transformation of the (Z)-into the (ii)-ether ( protection because the diazo ether is protected against dediazoniation almost completely if present as the ( >isomer). Rate constants kx and k are known from Ritchie and Virtanen s work (1972). The results demonstrate firstly that the initial reaction of the diazonium ion takes place in such a way that almost exclusively the (Z)-ether is formed directly (ki/k3 = 120). The protection rate constant kp is a simple function of the intrinsic rate constants as shown in Scheme 6-4. 

Ethane, C2H , dissociates into methyl radicals by a first-order reaction at elevated temperatures. If 250. mg of ethane is confined to a 500.-mL reaction vessel and heated to 700°C, what is the initial rate of ethane decomposition if k = 5.5 X 10 4 s-1 in the rate law (for the rate of dissociation of C2H6) ... 

The rate of dissociation increases rapidly above 2000°C. It also increases with decreasing pressure.P" ] The rate of recombination (i.e., the formation of the molecule) is rapid since the mean-free-path dependent half-life of atomic hydrogen is only 0.3 sec. 

The simplest mechanism to explain the much faster rate of dissociation of actomyosin-S-1 by ATP than that of ATP cleavage is that actin activates the myosin ATPase by accelerating the rate at which ADP and Pj are released. That is when ATP is added to actomyosin-S-1, ATP rapidly binds and dissociates actomyosin, myosin ATPase then hydrolyzes ATP to form myosin-ADP.Pj, this state then reattaches to actin and phosphate is released much faster from actomyosin. ADP.Pj than it is from myosin.ADP.Pj, as shown in the scheme below ... 

The volcano curve is bounded by the rate of dissociative adsorption of CO and hydrogenation of adsorbed carbon. This is illustrated in Figure 1.9. 

Studying the kinetics of the interaction of N2 with the Ru catalysts revealed that the Cs promoter enhances both the rate of dissociative chemisorption and the rate of recombinative desorption. Ru catalysts were found to be rather inactive for NH3 synthesis without alkali... 

Thus, as described by Equation (2.1), the equilibrium dissociation constant depends on the rate of encounter between the enzyme and substrate and on the rate of dissociation of the binary ES complex. Table 2.1 illustrates how the combination of these two rate constants can influence the overall value of Kd (in general) for any equilibrium binding process. One may think that association between the enzyme and substrate (or other ligands) is exclusively rate-limited by diffusion. However, as described further in Chapter 6, this is not always the case. Sometimes conformational adjustments of the enzyme s active site must occur prior to productive ligand binding, and these conformational adjustments may occur on a time scale slower that diffusion. Likewise the rate of dissociation of the ES complex back to the free... 

Until now our discussions of enzyme inhibition have dealt with compounds that interact with binding pockets on the enzyme molecule through reversible forces. Hence inhibition by these compounds is always reversed by dissociation of the inhibitor from the binary enzyme-inhibitor complex. Even for very tight binding inhibitors, the interactions that stabilize the enzyme-inhibitor complex are mediated by reversible forces, and therefore the El complex has some, nonzero rate of dissociation—even if this rate is too slow to be experimentally measured. In this chapter we turn our attention to compounds that interact with an enzyme molecule in such a way as to permanendy ablate enzyme function. We refer to such compounds as enzyme inactivators to stress the mechanistic distinctions between these molecules and reversible enzyme inhibitors. 

Here k, is the rate constant for this dissociation. By the law of mass action, we know that the rate of dissociation will be directly proportional to the concentration of El complex, with -k, being the constant of proportionality (the minus sign denotes the fact that the concentration of El is diminishing over time). Thus the rate equation for this dissociation reaction is given by... 

Because it is the rate of dissociation of the M-X bond that determines the rate of substitution, the rate law involves only the concentration of the starting complex, ML X. 

It has also been observed that the ionic strength of the solution or the presence of SDS micelles will affect the rate of dissociation of iron-siderophore complexes (22,181). 

The reaction with oxygen has been used to measure the rate of dissociation of the ethane.7... 

The rate of dissociation has been measured by oxygen uptake in the presence of an inhibitor of chain reactions as in the case of hexaaryl-ethanes. Since the uptake of oxygen obeys the same kinetic law, it is a reasonable extrapolation to suppose that here too the rate-determining step is a dissociation into radicals. When one of the phenyl groups in triphenylmethyl is replaced by a cyclohexyl group, the rate of dissociation of the ethane is reduced by a factor of 170.38 Some dissociation rate parameters are given in Tables III A and B. 

Most protein serine-threonine kinases undergo autophosphorylation. The autophosphorylation of most protein kinases is associated with an increase in kinase activity [4, 10]. In some instances, such as with the RII subunit of PKA, autophosphorylation represents a positive feedback mechanism for kinase activation, in this case by enhancing the rate of dissociation of the RII and C subunits. In the case of CaMKII, autophosphorylation causes the catalytic activity of the enzyme to become independent of Ca2+ and calmodulin. This means that the enzyme, activated originally in response to elevated cellular Ca2+, remains active after Ca2+ concentrations have returned to baseline. By this mechanism, neurotransmitters that activate CaMKII can produce relatively long-lived alterations in neuronal function. In other instances, such as with the receptor-associated protein tyrosine kinases (discussed in Ch. 24), autophosphorylation is an obligatory step in the sequence of molecular events through which those kinases are activated and produce physiological effects. 

The above reactions are rather slow because the insertion reactivity is directly related to the rate of dissociation of the THF ligand, which is also present as the solvent. The most reactive cationic complexes are therefore base-free, i. e. they do not contain coordinating solvents or ligands. Logically, anion coordination is a problem in such systems. Ion pairing reduces the reactivity of the cationic zirconocenes, but in the presence of B(C6F5)4, RB(C6F5)3 (R = H, Me), or BARF", this interaction is only weak. 

At any rate these results prove that the degree and the rate of dissociation of the HpHb and Hp-globin complexes are extremely low. The results are also in agreement with findings made in the investigation of the exchange between free and Hp-bound Hb with labeled (Fe59) dog Hb (N3). 

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