Constants dissociation rate constant

A covalent bond (or particular nomial mode) in the van der Waals molecule (e.g. the I2 bond in l2-He) can be selectively excited, and what is usually observed experimentally is that the unimolecular dissociation rate constant is orders of magnitude smaller than the RRKM prediction. This is thought to result from weak coupling between the excited high-frequency intramolecular mode and the low-frequency van der Waals intemiolecular modes [83]. This coupling may be highly mode specific. Exciting the two different HE stretch modes in the (HF)2 dimer with one quantum results in lifetimes which differ by a factor of 24 [84]. Other van der Waals molecules studied include (NO)2 [85], NO-HF [ ], and (C2i J )2 [87]. 

Quasiequilibrium statistical theory was applied to the negative ion mass spectra of diphenylisoxazoles. Electron capture by the isoxazole leads to molecular ions having excited vibrations of the ring and of bonds attached to it. The dissociation rate constants were also calculated (77MI41615, 75MI416U). 

Phenomenological evidence for the participation of ionic precursors in radiolytic product formation and the applicability of mass spectral information on fragmentation patterns and ion-molecule reactions to radiolysis conditions are reviewed. Specific application of the methods in the ethylene system indicates the formation of the primary ions, C2H4+, C2i/3+, and C2H2+, with yields of ca. 1.5, 1.0, and 0.8 ions/100 e.v., respectively. The primary ions form intermediate collision complexes with ethylene. Intermediates [C4iZ8 + ] and [CJH7 + ] are stable (<dissociation rate constants <107 sec.-1) and form C6 intermediates which dissociate rate constants <109 sec. l). The transmission coefficient for the third-order ion-molecule reactions appears to be less than 0.02, and such inefficient steps are held responsible for the absence of ionic polymerization. 

Now ku < 0.8 X 109 sec.-1, only slightly smaller than the upper limit 9 < 1.1 X 109 sec.-1 Apparently the unimolecular dissociation rate constants of all secondary complexes are less than ca. 5 X 107 sec.-1, those of the tertiary complexes less than 109 sec.-1, and those of the quaternary complexes probably of the order of 1010 sec.-1 These conclusions substantiate the view 16) that the mass spectrometrically observed tertiary ions arise predominantly from dissociation of the intermediate addition complexes C6Hi2+, C6Hn+, and C6Hi0+. Higher order ions, however, should arise principally from reactions of the dissociation products of the above complexes 62). 

B. Studies of Equilibria and Reactions.—N.m.r. spectroscopy is being increasingly employed to study the mode and course of reactions. Thus n.m.r. has been used to unravel the mechanism of the reaction of phosphorus trichloride and ammonium chloride to give phosphazenes, and to follow the kinetics of alcoholysis of phosphoramidites. Its use in the study of the interaction of nucleotides and enzymes has obtained valuable information on binding sites and conformations and work on the line-widths of the P resonance has enabled the calculation of dissociation rate-constants and activation energies to be performed. 

To distinguish between simple, reversible slow binding (scheme B) and an enzyme isomerization mechanism (scheme C), one can examine the dependence of kobs on inhibitor concentration. If the slow onset of inhibition merely reflects inherently slow binding and/or dissociation, then the term kobs in Equations (6.1) and (6.2) will depend only on the association and dissociation rate constants k3 and k4 as follows ... 

To measure the dissociation rate constant, all that is necessary, in principle, is first to secure a satisfactory occupancy of the receptors by the radioligand and then to prevent further association, either by adding a competing agent in sufficient concentration or by lowering [L] substantially by... 

Channel blockers are often classified as slow, intermediate, or fast blockers, based on the very wide range of values that have been found for the microscopic dissociation rate constant of different... 

The interaction between the receptor and the G-protein is transient and rapidly reversible. This is indicated, for example, by the fact that a single light-activated rhodopsin molecule may activate 500 to 1000 transducin molecules during its 1 to 3 sec lifetime. Hence, the interaction should, in the endpoint, be governed by the normal laws of chemical interaction and expressible in terms of association and dissociation rate constants and binding affinity. The question then arises as to whether the affinity of different receptors for different G-proteins varies. That is, is there specificity in receptor-G-protein coupling, and, if so, what determines this ... 

Numerous quantum mechanic calculations have been carried out to better understand the bonding of nitrogen oxide on transition metal surfaces. For instance, the group of Sautet et al have reported a comparative density-functional theory (DFT) study of the chemisorption and dissociation of NO molecules on the close-packed (111), the more open (100), and the stepped (511) surfaces of palladium and rhodium to estimate both energetics and kinetics of the reaction pathways [75], The structure sensitivity of the adsorption was found to correlate well with catalytic activity, as estimated from the calculated dissociation rate constants at 300 K. The latter were found to agree with numerous experimental observations, with (111) facets rather inactive towards NO dissociation and stepped surfaces far more active, and to follow the sequence Rh(100) > terraces in Rh(511) > steps in Rh(511) > steps in Pd(511) > Rh(lll) > Pd(100) > terraces in Pd (511) > Pd (111). The effect of the steps on activity was found to be clearly favorable on the Pd(511) surface but unfavorable on the Rh(511) surface, perhaps explaining the difference in activity between the two metals. The influence of... 

Stone CL, Bosron WF, Dunn MF. Amino acid substitutions at position 47 of human beta 1 beta 1 and beta 2 beta 2 alcohol dehydrogenases affect hydride transfer and coenzyme dissociation rate constants. J Biol Chem 1993 268 892-899... 

Clifford et ah (1987a,b) considered acid spurs (primary radicals H and OH) and computed the evolution of radical and molecular products by the master equation (ME) and IRT methods. Reasonable values were assumed for initial yields, diffusion constants, and rate constants, and a distribution of spur size was included. To be consistent with experimental yields at 100 ns, however, they found it necessary that the spur radius be small—for example, the radius of H distribution (standard deviation in a gaussian distribution) for a spur of one dissociation was only in the 0.4—0.75 nm range. Since in acid spurs H atoms inherit the distribution of eh, this is considered too low. This preliminary finding has later been revised in favor of spurs of much greater radius. 

In the preceding chapter, thermodynamic aspects of macrocycle complexation were treated in some detail. In this chapter, kinetic aspects are discussed. Of course, kinetic and thermodynamic factors are interrelated. Thus, in terms of a simple complexation reaction of the type given below (charges not shown), the stability constant (/CML) may be expressed directly as the ratio of the second-order formation constant (kf) to the first-order dissociation rate constant (kd) ... 

Shuman and Michael [10] applied a rotating disk electrode to the measurement of copper complex dissociation rate constants in marine coastal waters. An operational definition for labile and non-labile metal complexes was established on kinetic criteria. Samples collected off the mid-Atlantic coast of USA showed varying degrees of copper chelation. It is suggested that the technique should be useful for metal toxicity studies because of its ability to measure both equilibrium concentrations and kinetic availability of soluble metal. 

The dynamics of a supramolecular system are defined by the association and dissociation rate constants of the various components of the system. The time-scale for the dynamic events is influenced by the size (length-scale) and by the complexity of the system. The fastest time for an event to occur in solution is limited by the diffusion of the various components to form encounter complexes. This diffusion limit provides an estimate for the shortest time scale required for kinetic measurements. The diffusion of a small molecule in water over a distance of 1 nm, which is the length-scale for the size of small host systems such as CDs or calixarenes, is 3 ns at room temperature. In general terms, one can define that mobility within host systems can occur on time scales shorter than nanoseconds, while the association/dissociation processes are expected to occur in nanoseconds or on longer time scales. The complexity of a system also influences its dynamics, since various kinetic events can occur over different time scales. An increase in complexity can be related to an increase in the number of building blocks within the system, or complexity can be related to the presence of more than one binding site. 

In a typical SPR experiment real-time kinetic study, solution flows over the surface, so desorption of the guest immobilized on the surface due to this flow must be avoided.72 In the first stage of a typical experiment the mobile reactant is introduced at a constant concentration ([H]0) into the buffer flowing above the surface-bound reactant. This favors complex association, and the progress of complex formation at the surface is monitored. The initial phase is then followed by a dissociation phase where the reactant is removed from the solution flowing above the surface, and only buffer is passed over the surface to favor dissociation of the complex.72 74 The obtained binding curves (sensograms) contain information on the equilibrium constant of the interaction and the association and dissociation rate constants for complex formation (Fig. 9). 

Table 1 Association (k+) and dissociation rate constants (k ) for the binding of ethidium bromide to DNA assuming a 1 1 complexation stoichiometry...

The values for k+ and k were compared for temperature jump and stopped-flow conditions for DNA concentrations where the decay followed a mono-exponential function and no migration between DNA molecules occurred (see below).94 This report shows the importance of detecting fluorescence signals at the magic angle, which eliminated the fast components in the kinetics due to artifacts. The values for the association and dissociation rate constants obtained by both techniques are similar. 

Table 5 Association and dissociation rate constants for anthraquinone derivatives with ct-DNA and polydeoxynucleotides from ref. 101...

Surface plasmon resonance studies were employed to measure the equilibrium constants and association and dissociation rate constants of bisnaphthalimide derivatives (20, 21) with hairpin DNA immobilized on the metal surface.123 The equilibrium constants were higher and the dynamics slower for compounds 20 and 21 when compared to the equilibrium constants and dynamics of the model monomer (19). The values for ka and kd were determined from the change in the surface plasmon resonance signal when, respectively, the ligand solution was flowed over the... 

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