Reactivity interchange mechanism

Ru(edta)(H20)] reacts very rapidly with nitric oxide (171). Reaction is much more rapid at pH 5 than at low and high pHs. The pH/rate profile for this reaction is very similar to those established earlier for reaction of this ruthenium(III) complex with azide and with dimethylthiourea. Such behavior may be interpreted in terms of the protonation equilibria between [Ru(edtaH)(H20)], [Ru(edta)(H20)], and [Ru(edta)(OH)]2- the [Ru(edta)(H20)] species is always the most reactive. The apparent relative slowness of the reaction of [Ru(edta)(H20)] with nitric oxide in acetate buffer is attributable to rapid formation of less reactive [Ru(edta)(OAc)] [Ru(edta)(H20)] also reacts relatively slowly with nitrite. Laser flash photolysis studies of [Ru(edta)(NO)]-show a complicated kinetic pattern, from which it is possible to extract activation parameters both for dissociation of this complex and for its formation from [Ru(edta)(H20)] . Values of AS = —76 J K-1 mol-1 and A V = —12.8 cm3 mol-1 for the latter are compatible with AS values between —76 and —107 J K-1mol-1 and AV values between —7 and —12 cm3 mol-1 for other complex-formation reactions of [Ru(edta) (H20)]- (168) and with an associative mechanism. In contrast, activation parameters for dissociation of [Ru(edta)(NO)] (AS = —4JK-1mol-1 A V = +10 cm3 mol-1) suggest a dissociative interchange mechanism (172). 

Cobalamin and Cobaloxime Substitution Reactions.— The substitution kinetics of vitamin Biaa with 8-azaguanine and 6-mercaptopurine have been investigated in aqueous media at 25 °C and ionic strength 0.1 M. The complex pH dependences are interpreted in terms of reactions of the base on aquocobalamin with protonated and deprotonated forms of the ligands. Rate constants are listed in Table 7 and are consistent with a dissociative interchange mechanism. A similar mechanism is likely in the substitution reaction of cyanocobalamin with [Fe(CN)e] . Both base on and base off forms of the coenzyme are reactive. 

The modest variation of with nucleophile suggests an interchange mechanism, la for the weaker nucleophiles becoming more with the more reactive bases. 

Cobalt(m).— Anation of [Co(NH3)6(OH2>] + by sulphate or chloride proceeds by a dissociative interchange mechanism, in which the reactive species are ion-pairs and ion-triplets, The activation enthalpy for anation of [Co(NH3)6(OHa)] + by glycine is 29 kcal mol again a dissociative mechanism is operative. An earlier study of oxalate anation of [Co(en)a(OH2)2] in acidic solution is complemented by a study of oxalate anation of [Co(en)a(OH2)(OH)] + in basic solution. Several examples of anation reactions of bridged dicobalt complexes have been mentioned in the section on aquation of these complexes. ... 

In what appears, initially, to be a closely similar reaction, acid chlorides react with alkyl halides under solidtliquid two-phase conditions using sodium hydrogen carbonate in the presence of sodium iodide and tetra-n-butylammonium bromide [45]. Although the mechanism is not clear, it has been proposed that the acid chloride is initially converted into the carboxylate anion. It is also probable that the halogen interchange between the sodium iodide and the alkyl halides enhances their reactivity. Although the yields are high, the availability of the alkyl halides and alcohols are usually similar and there appears to be little to commend this process over the catalysed reaction of the acid chlorides with the alcohols. 

The mechanism and thermodynamics of transesterification of acetate-ester enolates in the gas phase have been investigated. The catalytic effect of alkali-metal t-butoxide clusters on the rate of ester interchange for several pairs of esters has been determined in non-polar and weakly polar solvents. Reactivities increase in the order (Li+ < Na+ < K+ < Rb+ < Cs+) with the fastest rates reaching lO catalytic... 

After having described the mechanism of electron interchange between the catalyst and the reactive gas molecules, we will show in the following chapters how the electron disorder and the space-charge effects are expected to influence the rate of reaction. The electron interchange between the catalyst and the different species of molecules involves a different mechanism of potentially higher reaction rates than is the case in the homogeneous gas phase reactions. 

Further structure-activity relationship (S AR) analyses of other cytoprotective enzyme inducers revealed the fact that all inducers can react with thiol/disulfide groups by alkylation, oxidoreduction, or thiol-disulfide interchange [Dinkova-Kostova and Talalay, 1999]. In fact, the capability of enzyme inducers to induce cytoprotective enzymes is well correlated with their reactivity with thiols. These results suggested a cellular sensor of inducers with highly reactive sulfhydryl groups, possibly reactive thiols in cysteine residues of a sensor protein. Nevertheless, the initial search for the sensor protein by using radioactively labeled inducers was not successful due to the abundance of thiol groups presented in many proteins in cells [Holtzclaw et al., 2004]. The molecular mechanism by which cytoprotective enzymes are induced remained to be elucidated. 

There is another phenomenon, regarded as a deteriorative change in the protein of soy milk, caused also by the evaporation of water. This is a film formation on the surface of soy milk, which occurs when heated soy milk is kept open to the air. This phenomenon is observed not only in heated soy milk but also in heated cow s milk. Film formation of soy milk occurs only when the soy milk is heated above 60°C and there is evaporation of water from the surface of the soy milk. The mechanism of protein insolubilization is basically the same as that of soy milk powder produced from heated soy milk (10. When water is removed from the surface of heated soy milk by evaporation, the molecular concentration of protein near the surface increases locally and the exposed reactive groups of the denatured molecules come close enough to interact intermolecularly both by hydrophobic interactions and through the sulfhydryl/disulfide interchange reaction to form a polymerization (film) on the surface. The upper side of the film contains more hydrophobic amino acids because of orientation of the hydrophobic portions of the unfolded molecules to the atmosphere rather than into the aqueous solution. 

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