Reaction rate water exchange

Activation Parameters and Rate Constants for The Water-Exchange Reaction... 

Water exchange reaction mechanism 332 Water NMRD in diamagnetic systems 33-9 Water protein relaxation rate 149 Wigner rotation matrices 65, 67 Wild type azurin 122... 

A number of important structural aspects of zinc complexes as found in enzymes are introduced in this section to serve as background information for the subsequent sections. Aquated Zn(II) ions exist as octahedral [Zn(H20)6] + complexes in aqueous solution. The coordinated water molecules are loosely bound to the Zn + metal center and exchange rapidly with water molecules in the second coordination sphere (see Figure 1) with a rate constant of ca 10 s at 25 °C extrapolated from complex-formation rate constants of Zn + ions with a series of nucleophiles. The mechanism of the water exchange reaction on Zn(II) was studied theoretically, from which it was concluded that the reaction follows a dissociative mechanism as outlined in Figure 2. ... 

The above mechanistic interpretation is in contrast with the one appearing in the coordination chemistry of NO on the very labile Fe(III) porphyrins and hemoproteins, which show water substitution-controlled kinetics at the iron(III) center (22,25). The latter Fe(III) moieties are, however, high-spin systems, whilst the cyano-complexes are low-spin. There is strong experimental evidence to support the dissociative mechanism with the Fe(III)-porphyrins, because the rates are of the same order as the water-exchange reactions measured in these systems (22d). Besides, the Fe(III) centers are less oxidizing than [Fein(CN)5H20]2- (21,25). 

In the case of Gd3+, there is a rapid water exchange with respect to the relaxation 7im [28]. For water exchange reaction of Gd3+ aquo ion the water tumbling time is 7 x 10 11 s. When Gd3+ is bound to a macromolecule, part of the hydration sphere is substituted by a protein molecule, the effective correlation changes (i.e.) effectively it becomes the electron relaxation time [29] which is about 10-9 s. By virtue of binding to a macromolecule, a net enhancement in proton relaxation rate, Eq is observed which is characteristic of the Gd3+ complex and depends on the resonance frequency and temperature. Some data on the enhancements obtained for Gd3+ protein complexes are given in Table 11.5. 

The Idnetic rate constants for CO2 hydration determined in the laboratory in sterile seawater (Table 4.6) are known sufficiently well that this value should create little uncertainty in the above calculation. However, in natural waters the reaction rates may be enzymatically catalyzed. Carbon dioxide hydration catalysis by carbonic anhydrase (CA) is the most powerful enzyme reaction known (see the discussion in Section 9.3). The catal5dic turnover number (the number of moles of substrate reacted, divided by the number of moles of enz5mie present) is 8 x 10 min for CA (Table 9.7), and marine diatoms are loiown to produce carbonic anhydrase (Morel et al, 1994). The calculations presented in Fig. 10.14 indicate that increasing the CO2 hydration rate constant by 10-fold should increase the gas exchange rate of CO2 in the ocean by 10%-50%. 

Rates of ligand replacement reactions of metal ion complexes are relevant in a number of situations. Assigning a lability to a particular metal based on the rate of a representative ligand replacement reaction is an attractive prospect, but one that has important limitations. In this regard, rates of water-exchange reactions have been examined ... 

The tetrahedrally [77-81] coordinated beryllium(II) and the octahedrally [82-87] coordinated magnesium] 11) show relatively slow water exchange rates from the first coordination sphere and can be measured directly by NMR techniques (Table 4.5). The water exchange reaction on beryllium(ll) is characterized by the most negative activation volume observed for a water exchange process (AV = -13.6... 

In the case where X is H20, that is, for the water-exchange reaction, the pressure-dependence of the rate has been measured and the volume of activation found to be +1.2 ml per mole. This result definitely excludes a predominantly SN2 mechanism, but it does not agree satisfactorily with an extreme 5N1 mechanism either. It is most consistent with a transition state in which the initial Co—OH2 bond is stretched quite far while formation of a new Co—bond is only beginning to occur, that is, a predominantly dissociative mechanism. 

Fig. 1. Rate constants for water-exchange reactions on Mn -, Fe -and Fe° (L)H20 complexes plotted on a logarithmic scale.

Another factor that must be considered to rationalize the product stereochemistry is the isomerization rates of the starting complexes (as well as those of the products). Table 12.11 lists data for [Co(en)2(H20)X]" isomerization and water exchange reactions. 

Rates of water exchange. The rates of water exchange between the inner coordination sphere of lanthanide ions and bulk water for the reaction... 

The most fundamental substitution reaction in aqueous solution, water exchange, reaction (22), has been studied for a variety of metal ions (Figure 6.3). Exchange of water in the coordination sphere of a metal with bulk solvent water occurs very rapidly for most metal ions, and therefore the rates of these reactions were studied primarily by relaxation techniques. In these methods, a system at equilibrium is disturbed, for example, by a very sudden increase in temperature. Under the new condition— higher temperature—the system will no longer be at equilibrium. The rate of equilibration can then be measured. If one can change the temperature of a solution in 10 s, then one can measure the rates of reactions that take longer than 10 s. 

Table 5 Rate constants of water exchange reactions of hydrated cations and thermodynamic parameters of the reaction at 25°C (for details see Ohtaki and Radnai, 1993 and references therein).

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