Metal ions hydrolysis

RNA hydrolysis, 45 285-287, 297-299 metalloenzymes, 45 251-252 bleomycin, 45 252-260, 299 nucleic acid hydrolysis metal ions and, 45 283-285 by oligonucleotide modified with metal complexes, 45 297-299 of phosphodiesters, 45 251, 287-297 by ribozymes, 45 285-287 cleavage by iron bleomycin, 43 140 polymerase, arsonomethyl phosphonate analogue, 44 201-202 substructures, 43 133-134 transfer... 

Hydrolysis of carboxylic and phosphoric esters is also a slow process at neutral pH, and is catalyzed by acids and bases by mechanisms similar to those involved in amide and peptide hydrolysis. Metal ions are also good catalysts of both carboxylic and phosphoric ester hydrolysis, typically with rate increases much higher than those observed for hydrolysis of amides or peptides (Table... 

K2 is called the hydrolysis constant for sodium ethanoate. Hydrolysis occurs when salts involving weak acids or bases are dissolved in water. It is often also found with metal ions in solution. The ion [M(H20) ] dissociates to the hydroxy species [M(H20) , (OH)]f 1. ... 

Perhaps the most extensively studied catalytic reaction in acpreous solutions is the metal-ion catalysed hydrolysis of carboxylate esters, phosphate esters , phosphate diesters, amides and nittiles". Inspired by hydrolytic metalloenzymes, a multitude of different metal-ion complexes have been prepared and analysed with respect to their hydrolytic activity. Unfortunately, the exact mechanism by which these complexes operate is not completely clarified. The most important role of the catalyst is coordination of a hydroxide ion that is acting as a nucleophile. The extent of activation of tire substrate througji coordination to the Lewis-acidic metal centre is still unclear and probably varies from one substrate to another. For monodentate substrates this interaction is not very efficient. Only a few quantitative studies have been published. Chan et al. reported an equilibrium constant for coordination of the amide carbonyl group of... 

Only for hydrolysis reactions also monodentate substrates are encountered, but for these systems the extent of activation of these compounds by the metal ion is still under debate. 

Inspired by the many hydrolytically-active metallo enzymes encountered in nature, extensive studies have been performed on so-called metallo micelles. These investigations usually focus on mixed micelles of a common surfactant together with a special chelating surfactant that exhibits a high affinity for transition-metal ions. These aggregates can have remarkable catalytic effects on the hydrolysis of activated carboxylic acid esters, phosphate esters and amides. In these reactions the exact role of the metal ion is not clear and may vary from one system to another. However, there are strong indications that the major function of the metal ion is the coordination of hydroxide anion in the Stem region of the micelle where it is in the proximity of the micelle-bound substrate. The first report of catalysis of a hydrolysis reaction by me tall omi cell es stems from 1978. In the years that... 

In a back titration, a slight excess of the metal salt solution must sometimes be added to yield the color of the metal-indicator complex. Where metal ions are easily hydrolyzed, the complexing agent is best added at a suitable, low pH and only when the metal is fully complexed is the pH adjusted upward to the value required for the back titration. In back titrations, solutions of the following metal ions are commonly employed Cu(II), Mg, Mn(II), Pb(II), Th(IV), and Zn. These solutions are usually prepared in the approximate strength desired from their nitrate salts (or the solution of the metal or its oxide or carbonate in nitric acid), and a minimum amount of acid is added to repress hydrolysis of the metal ion. The solutions are then standardized against an EDTA solution (or other chelon solution) of known strength. 

Metals and Metallic Ions. Under appropriate conditions, ozone oxidizes most metals with the exception of gold and the platinum group. When oxidized by ozone, heavy metal ions, such as Fe and Mn , result in the precipitation of insoluble hydroxides or oxides upon hydrolysis (48—50). Excess ozone oxidizes ferric hydroxide in alkaline media to ferrate, and Mn02 to MnO. ... 

Whereas decomposition is slow in pure solutions, it is accelerated enormously by alkali and traces of many metal ions. Indeed, hydrolysis to H2O2, followed by its disproportionation, is the main path for decomposition of inorganic peroxides. 

The ease of hydrolysis of metal alkoxides makes metal analysis a comparatively simple task. In many cases, the metal may be estimated by hydrolysis of a sample in a cmcible, and ignition to the metal oxide. Alternatively, the metal ion may be brought into solution by hydrolysis of a sample with dilute acid, followed by a standard analytical procedure for a solution of that particular metal. If the alcohol Hberated during the hydrolysis is likely to cause interference, it may be distilled from the solution by boiling. 

Silica Polymei Metal Ion Interactions in Solution. The reaction of metal ions with polymeric sihcate species in solution may be viewed as an ion-exchange process. Consequently, it might be expected that sihcate species acting as ligands would exhibit a range of reactivities toward cations in solution (59). Sihca gel forms complexes with multivalent metal ions in a manner that indicates a correlation between the ligand properties of the surface Si-OH groups and metal ion hydrolysis (60,61). For Cu +, Fe +, Cd +, and Pb +,... 

The increased acidity of the larger polymers most likely leads to this reduction in metal ion activity through easier development of active bonding sites in siUcate polymers. Thus, it could be expected that interaction constants between metal ions and polymer sdanol sites vary as a function of time and the sihcate polymer size. The interaction of cations with a siUcate anion leads to a reduction in pH. This produces larger siUcate anions, which in turn increases the complexation of metal ions. Therefore, the metal ion distribution in an amorphous metal sihcate particle is expected to be nonhomogeneous. It is not known whether this occurs, but it is clear that metal ions and siUcates react in a complex process that is comparable to metal ion hydrolysis. The products of the reactions of soluble siUcates with metal salts in concentrated solutions at ambient temperature are considered to be complex mixtures of metal ions and/or metal hydroxides, coagulated coUoidal size siUca species, and siUca gels. 

Many metal ions react with water to produce hydrolysis products that are multiply charged inorganic polymers. These may react specifically with negative sites on the colloidal particles to form relatively strong chemical bonds, or they may be adsorbed at the interface. In either case, the charge on the particle is reduced. 

Various chemical species influence the rates of hydrolysis of penicillins, e.g. metal ions (Cu >Zn >Ni Co ) (80JCS(P2)1725), carbohydrates (78MI51101), certain amine-containing catechol derivatives (69JPS1102) and /3-cyclodextrin (71JA767). Some of these even show some of the characteristics of enzyme-catalyzed hydrolyses. 

As metal ion concentration increases in the crevice, a net positive charge accumulates in the crevice electrolyte. This attracts negatively charged ions dissolved in the water. Chloride, sulfate, and other anions spontaneously concentrate in the crevice (Figs. 2.4 and 2.5). Hydrolysis produces acids in the crevice, accelerating attack (Reactions 2.5 and 2.6). Studies have shown that the crevice pH can decrease to 2 or less in salt solutions having a neutral pH. 

AgN03, EtOH, Pyr, 90°, 1.5 h H2S, 47% yield. An 5-triphenylmethyl thioether can be selectively cleaved in the presence of an 5-diphenylmethyl thioether by acidic hydrolysis or by heavy-metal ions. As a result of the structure of the substrate, the relative yields of cleavage by AgN03 and Hg(OAc)2 can be reversed. 

The rates of hydrolysis of the ester group in compounds A and B have been compared. The effect of an added metal ion (Np+) on the rate of hydrolysis has been studied, and the observed rate constants for attack by OH are tabulated. Suggest the most favorable transition-state stmcture for the addition step of the hydrolysis reaction for each substrate under each set of conditions. Discuss the relationship between the stmctures of these transition states and the relative rates of attack by hydroxide ion. 

Despite the statement above concerning the acid lability of cyclic formals, Gold and Sghibartz have shown that the acid catalyzed hydrolysis of these compounds is markedly depressed by some metal ions . Although the smaller cyclic formals did not exhibit a substantial rate reduction even in the presence of small cations like lithium, in certain larger systems the rate reduction was more than an order of magnitude. 

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