Complexation reactions dissociation, rate constants, estimation

We can now make sensible guesses as to the order of rate constant for water replacement from coordination complexes of the metals tabulated. (With the formation of fused rings these relationships may no longer apply. Consider, for example, the slow reactions of metal ions with porphyrine derivatives (20) or with tetrasulfonated phthalocyanine, where the rate determining step in the incorporation of metal ion is the dissociation of the pyrrole N-H bond (164).) The reason for many earlier (mostly qualitative) observations on the behavior of complex ions can now be understood. The relative reaction rates of cations with the anion of thenoyltrifluoroacetone (113) and metal-aqua water exchange data from NMR studies (69) are much as expected. The rapid exchange of CN " with Hg(CN)4 2 or Zn(CN)4-2 or the very slow Hg(CN)+, Hg+2 isotopic exchange can be understood, when the dissociative rate constants are estimated. Reactions of the type M+a + L b = ML+(a "b) can be justifiably assumed rapid in the proposed mechanisms for the redox reactions of iron(III) with iodide (47) or thiosulfate (93) ions or when copper(II) reacts with cyanide ions (9). Finally relations between kinetic and thermodynamic parameters are shown by a variety of complex ions since the dissociation rate constant dominates the thermodynamic stability constant of the complex (127). A recently observed linear relation between the rate constant for dissociation of nickel complexes with a variety of pyridine bases and the acidity constant of the base arises from the constancy of the formation rate constant for these complexes (87). 

The resulting hydroperoxo-ferriheme complex is nonetheless unstable, as the peroxide ligand weakly ligates the heme iron , and the dissociation rate constant for the peroxide anion as the sixth ligand in Fe-microperoxidase-8 (Fe-MP8) and Mn-MP8 is in the neighborhood of 10-20 s (ref [60]). Similar estimates can be made from other kinetic studies" which measured the and values for the formation of Compound I in reaction of HRP with hydrogen peroxide. The reported millimolar values and the observed cat order of 10 M s suggest that... 

For the evaluation of the dissociation rate constant k is the modified equation for the preceding reaction (77) has been applied. The diffusion-controlled peak height, Ip, was estimated in the absence of ligand in the same medium. The equilibrium constant K was expressed in terms of the subsequent stability constants and P2 of the Cu(II)-pro-line complexes present in the solution ... 

Relative reactivies of the species Zn2+, Zn(OH)+, Zn(OH) s, and Zn(OH)4 have been established for the reaction of zinc(II) with tetrad-methyl-4-pyridyl)porphyrins in basic solution (319). The rate constant for reaction of a typical zinc finger peptide with Zn2q has been estimated as 2.8 x 107 M-1 s 1, for dissociation of this complex 1.6 x 104 s 1 (282). 

Remember that this reaction-rate constant was derived for a single molecule of A. A similar procedure can be used to estimate the reverse-rate constant, except that the boundary conditions on the diffusion equation must be modified instead of Cg = 0 at the complex surface and Cg = cb,oo far from that surface (as was used to find Equation 4-64), the reverse reaction starts with a single B molecule bound in the encounter complex (of volume 4na /3), which must subsequently diffuse into an unbounded fluid in which its concentration is negligible. The rate of dissociation of the complex is equal to the rate of diffusion of B molecules away from the complex surface ... 

From this plot the half-lives (rate constants) for the inactivation by each concentration of inactivator can be calculated from the slopes of the individual lines. These half-life values are then plotted along the y-axis versus 1/[I] plotted along the x-axis. This plot is also known as a Kitz-Wilson plot (Fig. 4.12). In the case of a saturation reaction, (i.e., at infinite inactivator concentration there is a finite half-life) the point where the plotted line intersects the y-axis is equal to 0.693/kinact where A inact is the rate of inactivation and represents a complex mixture of 2 3 and 4 (see Scheme 4.5). The dissociation constant for the enzyme-mechanism-based inactivator complex (Ki) can also be estimated from this plot as the x-intercept of the line represents —l/Ki (Fig. 4.12). 

The Michaelis-Menten kinetics (75) provide a facile estimate of the altered reaction dynamics and the energetics of the nanoconfined enzyme systems. The well-known Michaelis constant Km ( k i/ki) measures the dissociation of the enzyme-substrate complex and in turn serves as an estimate of its stability. An increased value of the Michaelis constant implies that the equilibrium of the enzyme substrate complex is shifted towards the left i.e. towards the free enzyme and substrate, and suggests a relatively weaker enzyme-substrate complex. Another parameter, kcat ( k2), called the turnover number, estimates the rate of formation of the product from Ae enzyme-substrate complex. A facilitated product formation will result in increased turnover number (i.e. increased k<. The ratio kcat/ m consequently, represents the apparent rate constant for combination of a substrate with the free enzyme. The... 

While reduction of [Fe(CN)6]" by [ Ru(bipy)3] is diffusion controlled, " reduction by [ Os(5-Clphen)3] is slower and data comparisons lead to an estimate of the rate of electron transfer within the ion pairs of 1.6xl0 s . Unexpectedly, this value increases with increasing ionic strength. Reduction of [Fe(CN)e] by MV is diffusion controlled with a rate constant of 7.6 x 10 A/ s at 23 C. The reverse reaction involves rate-determining successor complex dissociation and is an order of magnitude slower than the rate of electron transfer derived from optical data. 

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