Formation rate constants

The concentration of a metabolite (Cm), too, follows such a bi-exponential kinetics depending on the metabolite formation rate constant (Kmet). 

In the M. trichosporium OB3b system, a third intermediate, T, with kmax at 325 nm (e = 6000 M-1cm 1) was observed in the presence of the substrate nitrobenzene (70). This species was assigned as the product, 4-nitrophenol, bound to the dinuclear iron site, and its absorption was attributed primarily to the 4-nitrophenol moiety. No analogous intermediate was found with the M. capsulatus (Bath) system in the presence of nitrobenzene. For both systems, addition of methane accelerated the rate of disappearance of the optical spectrum of Q (k > 0.065 s-1) without appreciatively affecting its formation rate constant (51, 70). In the absence of substrate, Q decayed slowly (k 0.065 s-1). This decay may be accompanied by oxidation of a protein side chain. 

For such a mechanism, the overall second-order formation rate constant is given by the product of the first-order constant ktx and the equilibrium constant Kos. The characteristic solvent exchange rates are thus often useful for estimating the rates of formation of complexes of simple monodentate ligands but, as mentioned already, in some cases the situation for macrocyclic and other polydentate ligands is not so straightforward. 

Ionophores such as A-23187 and X-14885A are flexible, so despite the need for conformational change, established for A-23187 by a 1H and 13C NMR solution study (556), their complex formation reactions can take place quite quickly as they can change their conformations rapidly as required for sequential bonding to the cation, and thus proceed in a series of energetically not-too-demanding steps. Formation rate constants for the Ca2+ complexes of A-23187 and X-14885A are 6 x 105 and... 

Ka Dimensionless complex formation rate constant None (68)... 

Under these conditions, the formation rate constant, k, can be estimated from the product of the outer sphere stability constant, Kos, and the water loss rate constant, h2o, (equation (28) Table 2). The outer sphere stability constant can be estimated from the free energy of electrostatic interaction between M(H20)q+ and L and the ionic strength of the medium [5,164,172,173]. Consequently, Kos does not depend on the chemical nature of the ligand. A similar mechanism will also apply to a coordination complex with polydentate ligands, if the rate-limiting step is the formation of the first metal-ligand bond [5]. Values for the dissociation rate constants, k, are usually estimated from the thermodynamic equilibrium constant, using calculated values of kf ... 

Table 2. Representative values of the dehydration rate constant and the complex formation rate constant for a singly and a doubly charged anionic ligand for several metals in aqueous solutions. An ionic strength of 0.01 mol dm 3 was assumed. T] is the half dissociation time (equivalent to O J/ka) for a singly charged anionic ligand with a stability constant of 106 dm3 mol. Based on [5,164,172]...

Figure 4. Copper complexation by a pond fulvic acid at pH 8 as a function of the logarithm of [Cu2+]. On the x-axis, complex stability constants and kinetic formation rate constants are given by assuming that the Eigen-Wilkens mechanism is valid at all [M]b/[L]t. The shaded zone represents the range of concentrations that are most often found in natural waters. The + represent experimental data for the complexation of Cu by a soil-derived fulvic acid at various metakligand ratios. An average line, based on equations (26) and (30) is employed to fit the experimental data. Data are from Shuman et al. [2,184]...

For complexation/dissociation reactions, ji corresponds with the average distance that M can travel following dissociation of ML (and prior to reassociation) [40,46]. Complexes are dynamic when M frequently changes from its free to complexed state during its diffusion time to the membrane surface or, in other words, if the first-order dissociation rate constant, k(, and the pseudo first-order formation rate constant, kf[L], are much larger than their effective diffusion rate constants (D/<52) [325,326]. Thus, for conditions of planar diffusion, complexes are labile if ... 

In other words, the complex formation rate constant kf [M1 s 1] depends on the outer sphere electrostatic encounter and on the water exchange rate of the aquo metal ion k w... 

Melanin formation rate constant Rate determining Catalyst pH 7 pH 4 (1/mol sec) step... 

Nickel(III) peptide complexes have a tetragonally-distorted octahedral geometry as shown by electron spin resonance studies (19) and by reaction entropies for the Ni(III,II) redox couple (17). Axial substitutions for Ni(III)-peptide complexes are very fast with formation rate constants for imidazole greater... 

Ligand Stability Constant, Formation Rate Constant,... 

Factors Affecting Formation Rate Constants of Chelates... 

Some Factors which Affect the Formation Rate Constant (k ) in Substitution Reactions... 

Figure 6. Formation rate constants for ligands reacting with Ni(phen)(HtO)A2 showing the effect of increasing stacking interaction with increasing number of...

Non-statistical successive binding of O2 and CO to the four heme centers of hemoglobin ( cooperativity ) has been thoroughly documented. It is difficult to test for a similar effect for NO since the equilibrium constants are very large ( 10 M ) and therefore difficult to measure accurately. It is found that the four successive formation rate constants for binding NO to hemoglobin are identical. In contrast, the rate constant for dissociation of the first NO from Hb(NO)4 is at least 80 times less than that for removal of NO from the singly bound entity Hb(NO). This demonstrates cooperativity for the system, and shows that it resides in the dissociation process. The thermodynamic implications of any kinetic data should therefore always be assessed. 

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