Metal ions rate enhancement

The kinetics and salt effects for the oxidation of octacyanomolyb-date(IV) (69, 73, 74) and octacyanotungstate(IV) (75, 76) by peroxydi-sulphate ions have been studied extensively. These reactions are first order with respect to [S20g ], [M(CN)8 l, and alkali metal ion concentrations. The alkali metal ion rate enhancement follows the order Li first-order dependence in alkali metal ion concentrations should function in a mechanism (74). The magnitude for the association constants in equilibria, such as... 

It should be noted that the metal ion may enhance the oxidation rate of the substrate without being a real catalyst. In this case the oxidation of the substrate becomes faster because of the formation of reactive species in Eqs. (3) and (4), but Eq. (5) does not occur and the metal ion is not recycled to its original oxidation state. Consequently, the catalyst quickly loses its activity over the course of the reaction. 

While keeping in mind the general picture of nuclear relaxation in paramagnetic systems as described in Section 3.1, it is appropriate to consider first the simple case of dipolar coupling between two point-dipoles as if the unpaired electrons were localized on the metal ion. The enhancement of the nuclear longitudinal relaxation rate Rim due to dipolar coupling with unpaired electrons can be calculated starting from the Hamiltonian for the system ... 

Treatment of 2,6-f>w(bromomethyl)pyridine with 2-(methylamino)ethanol afforded 2,6-6is[(2-hydroxyethyl)methylaminomethyl]pyridine, which with 4-nitrophenylphosphoro dichloride under high dilution conditions gave rise to the desired cyclic phosphotriester 24 divalent metal ions dramatically enhance the rate of hydrolysis [95TL4031],... 

Just as metal ions can bind nucleic acids both specifically and nonspecifically, they can promote both specific and nonspecific cleavage of nucleic acids. The rate and specificity of the cleavage reactions varies markedly with the identity of both metal ions and nucleic acid structures, as well as with the couditions of experimeuts. Higher pH, temperature, and concentrations of metal ions could enhance the rate of the polynucleotide cleavage but would decrease the specificity. 

A different reaction of this type, which has been the subject of considerable investigation, is the hydrolysis of amino acid esters in the presence of divalent metal ions which enhance the rate of hydrolysis (P, 10y 40j 70). Kroll 40) was the first to demonstrate that the rate of hydrolysis of ethyl... 

Paramagnetic metal ions usually enhance longitudinal and transverse relaxation rates (1/Ti and l/7 2) of nuclear spins of ligand nuclei. The relaxation rates of a nucleus that experiences chemical exchange between the bulk of solution and a paramagnetic center are described by the... 

Also the presence of foreign metallic ions can enhance the dehydrogenation activity. Freidlin and Levit (86) observed that addition of 5 to 30% K2C03 to silica promotes this reaction. Dalmai et al. (6) showed that not only potassium, but also lithium and magnesium ions act as promoters. Also on ZnO the addition of alkali (Li or Na) increases the rate of dehydrogenation [Shabrowa (94)]. 

In addition, metal ions can enhance oxidation rates by acting as electron acceptors in themselves. Litter points out that metal ions can be either beneficial or detrimental [22]. Examples are Cu " ", Fe +, and 7 " ", all of which can have beneficial effects in certain concentration ranges and detrimental effects at higher concentrations. Metal ions can be present in water that is being photocatalyticaUy treated and the effects must be taken into account. Finally, Litter points out that the addition of metal ions is generally too expensive in large-scale apphcations. 

A quantitative correlation between rate and equilibrium constants for the different metal ions is absent. The observed rate enhancements are a result of catalysis by the metal ions and are clearly not a result of protonation of the pyridyl group, since the pH s of all solutions were within the region where the rate constant is independent of the pH (Figure 2.1). 

Noncnzymc-Catalyzcd Reactions The variable-time method has also been used to determine the concentration of nonenzymatic catalysts. Because a trace amount of catalyst can substantially enhance a reaction s rate, a kinetic determination of a catalyst s concentration is capable of providing an excellent detection limit. One of the most commonly used reactions is the reduction of H2O2 by reducing agents, such as thiosulfate, iodide, and hydroquinone. These reactions are catalyzed by trace levels of selected metal ions. Eor example the reduction of H2O2 by U... 

Metal ions, in particular Zn, Ni, and Cu enhance the rate of general base-catalyzed enolization of 2-acetylpyridine by several orders of magnitude. Account for this effect. 

An artificial metalloenzyme (26) was designed by Breslow et al. 24). It was the first example of a complete artificial enzyme, having a substrate binding cyclodextrin cavity and a Ni2+ ion-chelated nucleophilic group for catalysis. Metalloenzyme (26) behaves a real catalyst, exhibiting turnover, and enhances the rate of hydrolysis of p-nitrophenyl acetate more than 103 fold. The catalytic group of 26 is a -Ni2+ complex which itself is active toward the substrate 1, but not toward such a substrate having no metal ion affinity at a low catalyst concentration. It is appearent that the metal ion in 26 activates the oximate anion by chelation, but not the substrate directly as believed in carboxypeptidase. 

In contrast to 1, isomeric p-nitrophenyl nicotinate shows almost no catalysis. Thus, it is clear that substrate coordination to the metal ion complex plays the critical role for an enormous rate enhancement. The lipophilic ester (R = C5Hn) also undergoes a large rate enhancement indicating the importance of substrate binding into the micellar phase by hydrophobic interaction. A large rate enhancement can also be seen in lipophilic esters which lack the metal coordination site as given below with the enantioselective micellar reactions (Table 9, 10). 

Table 9 indicates that the rate enhancement (kL/ko) is relatively small when Zn2 + ions or a ligand is used separately for both 50 and 52 substrates. A large rate enhancement is obtained only when a ligand and the metal ion are used together as in the previous examples (Table 1, 3, 4, 7). Ligands L-45 and L-46 are relatively inactive as compared to other ligands having the imidazole moiety. The ligand activation by metal ion is the order of Zn2+ > Co2+ > Ni2+ in all the cases, the same as in non-micellar reactions (Table 1). Rate-enhancing effects (kL/ko) of L-47-Zn2 +, L-48-Zn2 +, and L,L-49-Zn2+ ion complexes are remarkably large in view of the consideration...

In addition to the standard constraints introduced previously, structural constraints obtainable from the effects of the paramagnetic center(s) on the NMR properties of the nuclei of the protein can be used (24, 103). In iron-sulfur proteins, both nuclear relaxation rates and hyperfine shifts can be employed for this purpose. The paramagnetic enhancement of nuclear relaxation rates [Eqs. (1) and (2)] depends on the sixth power of the nucleus-metal distance (note that this is analogous to the case of NOEs, where there is a dependence on the sixth power of the nucleus-nucleus distance). It is thus possible to estimate such distances from nuclear relaxation rate measurements, which can be converted into upper (and lower) distance limits. When there is more than one metal ion, the individual contributions of all metal ions must be summed up (101, 104-108). If all the metal ions are equivalent (as in reduced HiPIPs), the global paramagnetic contribution to the 7th nuclear relaxation rate is given by... 

Increase the oxidation rate of polymers, e.g. metal ions which increase the hydroperoxide decomposition rate. Photodegradation and thermal degradation are enhanced by transition metal ion containing pro-oxidants, such as iron dithiocarbamate (as opposed to nickel dithiocarba-mate, which acts as a photo-antioxidant). 

In the absence of catalytically active transition-metal ions, micelles impede the reaction. In contrast to SDS, CTAB, and C12E7, Cu(DS)2 micelles catalyze the Diels-Alder reaction with extremely high efficiency, leading to rate enhancements up to 1.8 x 106 compared to... 

Metal-deactivating antioxidants. Transition metal compounds decompose hydroperoxides with the formation of free radicals, thereby increasing the rate of oxidation. Such an enhanced oxidation can be slowed down by the addition of a compound that interacts with metal ions to form complexes that are inactive with respect to hydroperoxides. Diamines, hydroxy acids, and other bifunctional compounds exemplify this type of antioxidants. 

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