Mechanism zinc-hydroxide’ reaction

Figure 12.5 Zinc-hydroxide reaction mechanism for peptide hydrolysis by carboxypeptidase A. (Reprinted with permission from Lipscomb and Strater, 1996. Copyright (1996) American Chemical Society.)...

The cell uses a replaceable anode cassette made up of a slurry of elec-trochemically generated zinc particles in a potassium hydroxide solution with a collection frame and a separator envelope with two sides of oxygen reduction cathodes that extract oxygen from the air for the zinc-oxidation reaction. The discharged zinc-air module is refueled or mechanically recharged by exchanging spent cassettes with fresh cassettes. 

The carbonic anhydrase (Cam) in M. thermophila cells is elevated several fold when the energy source is shifted to acetate, suggesting a role for this enzyme in the acetate-fermentation pathway. It is proposed that Cam functions outside the cell membrane to convert CO2 to a charged species (reaction A4) thereby facilitating removal of product from the cytoplasm. Cam is the prototype of a new class (y) of carbonic anhydrases, independently evolved from the other two classes (a and P). The crystal structure of Cam reveals a novel left-handed parallel P-helix fold (Kisker et al. 1996). Apart from the histidines ligating zinc, the activesite residues of Cam have no recognizable analogs in the active sites of the a- and P-classes. Kinetic analyses establish that the enzyme has a zinc-hydroxide mechanism similar to that of Cab (Alber et al. 1999). 

Although, as explained above, zinc is commonly deposited from cyano baths, it may also be deposited from strongly alkaline baths ( alkaline non-cyanide bath ). Typically a zinc hydroxide ratio of 1 10 would be employed and the major solution species would be the tetrahedral zincate anion [Zn(OH)4]2" as evidenced from Raman studies.5 Careful studies have been carried out to investigate the electrode reaction.6 An electrokinetic study gave data consistent with a four-step mechanism for the overall process... 

A fully concerted mechanism for reaction 299 has been eliminated as inconsistent with 14C and 15N KIEs and also with the observed inverse solvent D2O effect. The reaction path for the deamination of AMP has been formulated613 as a stepwise conversion involving the formation of tetrahedral intermediate 515 characterized by full-bonded hydroxyl and amino groups (equation 300). The TS for slow formation of 515, resulting from the attack of the hydroxyl from enzyme zinc-activated water at the C(6), is characterized by the C(6) OH bond order of 0.8 0.1 (late TS) and fully bonded NH2, that is by the nearly complete conversion to sp3 at C(6), and by nearly complete protonation of Nq), 516, The protonation of NH2 (in 515) and departure of NH3 (with TS 517) take place in the subsequent rapid steps as shown in equation 300, Zinc hydroxide is formed prior to attack514 at C(6). Enzymatic degradation of [6-14C]AMP has been carried out to prove the position of the radiolabel in 513 (equation 301). No radioactivity in the allantoin... 

It has been established that the actual anode reaction is more complicated and it takes place through the following mechanism First, zinc is oxidized and reacts with hydroxide ions OH to form a transient intermediate zinc hydroxide Zn(OH)2, and then Zn(OH)2 undergoes a dissolution process in the concentrated alkali solution to form zincate Zn(OH)4, namely. 

Overall, [([12]aneN3)Zn(0H2)](C104)2 exhibits pH-dependent catalytic behavior for the hydration of C02 and dehydration of HCO. The rate-determining step of the C02 hydration reaction is the uptake of C02 by the zinc hydroxide complex. The rate-determining step in bicarbonate dehydration is substitution of the labile zinc-bound water molecule by the bicarbonate anion. The overall catalytic mechanism for both reactions is shown in Scheme 4. In considering this mechanism, it is... 

Kinetic studies of reactions of three tris(pyrazolyl)borate-ligated zinc hydroxide complexes ([(TpR,Me)Zn-OH], Fig. 42, top) with triorganophosphate ester substrates in chloroform provided evidence for a concerted or hybrid-type mechanism.96 The reactions under study in this case are stoichiometric (Fig. 42, bottom) and involve generation of 4-nitrophenol (Rr// = 0C6H4-/>-N02). Notably, this acidic phenol rapidly undergoes reaction with the starting [(TpR,Me)Zn-OH] complex to yield [(TpR,Me)Zn-0C6H4-/ -N02] derivatives. This second reaction is at least... 

Kinetic studies of the reaction of a mononuclear N2S(thiolate)-ligated zinc hydroxide complex (PATH)Zn-OH with tris(4-nitrophenyl) phosphate in 33% ethanol-water and 7=0.10 (NaN03) also point to a hybrid-type mechanism (Fig. 43).228 Overall, this reaction is second order and a pH-rate profile indicates that the zinc hydroxide species (PATH)Zn-OH is involved in the reaction. The maximum rate constant for this reaction (16.1(7) M-1 s-1) is higher than that reported for free hydroxide ion (10.7 +0.2 M 1 s-1).225 This implies that a simple mechanism involving nucleophilic attack is not operative, as free OH- is a better nucleophile. Studies of the temperature dependence of the second-order rate constants for this reaction yielded activation parameters of A77 = 36.9(1) kJ mol-1 and AS = —106.7(4) JmolK-1. The negative entropy is consistent with considerable order in the transition state and a hybrid-type mechanism (Fig. 43, bottom). 

Binuclear zinc complexes wherein each metal center is supported by a tripodal tetradentate ligand environment involving a bridging phenolate donor are reactive toward tris(4-nitrophenyl) phosphate and bis(4-nitrophenyl) phosphate in ethanol/ water.234 The pH-rate profile studies of these reactions are consistent with a mechanism that involves the formation of a reactive mononuclear zinc hydroxide species via cleavage of the binuclear solid-state structure. This mononuclear Zn-OH species is proposed to act as the nucleophile toward the phosphate ester substrate. 

The carbon dioxide site is presumed to be about 3.2 A from the Zn(II) site, which means that there is a ligand between them. Evidence for a zinc-hydroxide mechanism has come from studies of the competition of water and cyanide and the role of cadmium as an inhibitor. At low pH, that is, below the pK of HCN, the Zn OH" model would suggest the reaction ... 

Before our work [39], only one catalytic mechanism for zinc dependent HDACs has been proposed in the literature, which was originated from the crystallographic study of HDLP [47], a histone-deacetylase-like protein that is widely used as a model for class-I HDACs. In the enzyme active site, the catalytic metal zinc is penta-coordinated by two asp residues, one histidine residues as well as the inhibitor [47], Based on their crystal structures, Finnin et al. [47] postulated a catalytic mechanism for HDACs in which the first reaction step is analogous to the hydroxide mechanism for zinc proteases zinc-bound water is a nucleophile and Zn2+ is five-fold coordinated during the reaction process. However, recent experimental studies by Kapustin et al. suggested that the transition state of HDACs may not be analogous to zinc-proteases [48], which cast some doubts on this mechanism. 

Finally we should briefly mention the purple acid phosphatases, which, unlike the alkaline phosphatases, are able to hydrolyse phosphate esters at acid pH values. Their purple colour is associated with a Tyr to Fe(III) charge transfer band. The mammalian purple acid phosphatase is a dinuclear Fe(II)-Fe(III) enzyme, whereas the dinuclear site in kidney bean purple acid phosphatase (Figure 12.13) has a Zn(II), Fe(III) centre with bridging hydroxide and Asp ligands. It is postulated that the iron centre has a terminal hydroxide ligand, whereas the zinc has an aqua ligand. We do not discuss the mechanism here, but it must be different from the alkaline phosphatase because the reaction proceeds with inversion of configuration at phosphorus. 

In the 2nd period ranging from the 1930s to the 1950s, basic research on flotation was conducted widely in order to understand the principles of the flotation process. Taggart and co-workers (1930, 1945) proposed a chemical reaction hypothesis, based on which the flotation of sulphide minerals was explained by the solubility product of the metal-collector salts involved. It was plausible at that time that the floatability of copper, lead, and zinc sulphide minerals using xanthate as a collector decreased in the order of increase of the solubility product of their metal xanthate (Karkovsky, 1957). Sutherland and Wark (1955) paid attention to the fact that this model was not always consistent with the established values of the solubility products of the species involved. They believed that the interaction of thio-collectors with sulphides should be considered as adsorption and proposed a mechanism of competitive adsorption between xanthate and hydroxide ions, which explained the Barsky empirical relationship between the upper pH limit of flotation and collector concentration. Gaudin (1957) concurred with Wark s explanation of this phenomenon. Du Rietz... 

In another set of experiments in alkaline solution it seems that with three hydroxides zinc reacts rather rapidly with porphyrins, but with four hydroxides it doesn t react nearly as rapidly. And if you replace the hydroxide by cyanides, you can stop the reaction altogether. I think this goes back to the remarks I made this morning, that each system may have its own particular mechanism. 

In a 250 ml, four-necked flask fitted with mechanical stirrer, reflux condenser, thermometer, and with provision for adding solids, a mixture of 90 ml of 95% ethanol, 15 ml of 12N sodium hydroxide solution, and 7 gm (0.047 mole) of p-nitrostyrene is heated to reflux temperature with vigorous stirring while 25 gm (0.39 gm-atom) of zinc dust is added in small portions so that the reaction is kept under control. After the addition has been completed, reflux is continued for hr. Then the hot reaction mixture is filtered in such a manner that the filtrate drops directly into a large volume of cold water. The resultant gummy precipitate is filtered off, dried, and sublimed at 120°C (2 mm Hg). The sublimed solid is then recrystallized repeatedly from 95 % ethanol to afford 0.95 gm (17%) of product, m.p. 138°-138.5°C. 

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