Metal ions hydrolytic cleavage

By analogy with the behavior of low molecular weight amino acid esters, the metal-promoted hydrolytic cleavage of PVA-QA may be described as a two-step process (Equation 1). The equilibrium formation of the intermediate polymeric metal complex, PVA-QA-M is followed by hydroxide ion cleavage of the ester bond and liberation of the free metal chelate. At constant pH and [M]>>[Ester], and provided steady-state conditions apply to the intermediate, the kinetics of formation of the product are first-order and the observed rate constant is given by Equation 5. jf the intermediate PVA-QA-M is not fully formed under the reaction conditions, both Equations 5 and 6 require that a plot of 1/kobs versus 1/[M] be linear with an intercept of l/ka. 

Although, as stated above, we wiU mostly focus on hydrolytic systems it is worth discussing oxidation catalysts briefly [8]. Probably the best known of these systems is exemphfied by the antitumor antibiotics belonging to the family of bleomycins (Fig. 6.1) [9]. These molecules may be included in the hst of peptide-based catalysts because of the presence of a small peptide which is involved both in the coordination to the metal ion (essential co-factor for the catalyst) and as a tether for a bisthiazole moiety that ensures interaction with DNA. It has recently been reported that bleomycins will also cleave RNA [10]. With these antibiotics DNA cleavage is known to be selective, preferentially occurring at 5 -GpC-3 and 5 -GpT-3 sequences, and results from metal-dependent oxidation [11]. Thus it is not a cleavage that occurs at the level of a P-O bond as expected for a non-hydrolytic mechanism. 

RNA is as suitable (if not more so) than DNA as a cleavage target [37]. In contrast to DNA, RNA is substantially less prone to oxidative cleavage [38] as a consequence of the higher stability of the glycosidic bond in ribonucleotides compared to that in deoxyribonucleotides. On the basis of the properties described in the introductory sections RNA is by contrast, much less stable to hydrolytic cleavage. For this reason the hydrolysis of the phosphate bond in this system can be successfully catalyzed not only by metal ions but also by ammonium ions. 

As we have already seen zinc-finger peptides are well-studied polypeptide motifs that have found many applications in synthetic systems, mostly because of their abihty to bind metal ions and interact with oligonucleotides. In this context the report by lima and Crooke [44] of the hydrolytic cleavage by a zinc-finger peptide devoid of any metal ion is a surprising. The system they studied, a 30-amino acid sequence, is based on a catalytic mechanism very similar to that discussed above... 

Metal ions can promote hydrolytic cleavage in the following ways ... 

The focus of this account is on low molecular weight metal complexes that mimic the cooperation of two metal ions in the hydrolytic cleavage of phosphate ester bonds. 

A quite new type of antibiotic and one of the few naturally-occurring boron compounds is boromycin (86). Hydrolytic cleavage of D-valine with the M(7) hydroxides gave caesium and rubidium salts of this antibiotic, and crystal structure analysis established the formula as (XIIT). The rubidium ion is irregularly coordinated by eight oxygen atoms. Experiments with models showed that the cation site would be the natural place for the—NH3+ end of the D-valine residue, and the whole structure raises the possibility that transport of larger alkali metals is related to the N-ends of peptides and proteins. 

In a number of nonenzymatic reactions catalyzed by pyridoxal, a metal ion complex is formed—a combination of a multivalent metal ion such as cupric oi aluminum ion with the Schiff base formed from the combination of an amino acid and pyridoxal (I). The electrostatic effect of the metal ion, as well as the electron sink of the pyridinium ion, facilitates the removal of an a -hydrogen atom to form the tautomeric Schiff base, II. Schiff base II is capable of a number of reactions characteristic of pyridoxal systems. Since the former asymmetric center of the amino acid has lost its asymmetry, donation of a proton to that center followed by hydrolytic cleavage of the system will result in racemic amino acid. On the other hand, donation of a proton to the benzylic carbon atom followed by hydrolytic cleavage of the system will result in a transamination reaction—that is, the amino acid will be converted to a keto acid and pyridoxal will be converted to pyridoxamine. Decarboxylation of the original amino acid can occur instead of the initial loss of a proton. In either case, a pair of electrons must be absorbed by the pyridoxal system, and in each case, the electrostatic effect of the metal ion facilitates this electron movement, as well as the subsequent hydrolytic cleavage (40, 43). 

By far the greatest attention has been to the complexing of the transition metal ions, often with surprising results. Thus, Cu11 can be shown to bind to N-7 of the adenine moiety of ATP and yet it can considerably enhance the hydrolytic cleavage of the phosphate bonds under the same conditions. Earlier suggestions were that chelate formation occurred via the N-7 and phosphate O... 

Collman and Buckingham (1963) have reported preliminary results of studies on the hydrolytic cleavage of amino-terminal peptide bonds by m-hydroxyaquotriethylenetetraaminecobalt(III) ions. The amino-terminal residues of di- and tripeptides are selectively hydrolyzed by one equivalent of metal chelate and are converted to an inert metal complex. The reaction proceeds as shown on p. 63. 

The modulation of the coordination to the transition metal has not necessarily positive implications on the reactivity. For instance, we observed [50] that the copper(II) complex (8) of tetramethyl-l,2-diaminoethane catalyzes the hydrolysis of the phosphoric acid triester PNPDPP via an electrophilic mechanism which involves the pseudointramolecular attack of deprotonated water, as illustrated in (9). The electrophilic mechanism contribution to the hydrolytic process totally disappears in micellar aggregates made of the amphiphilic complex (10). Clearly, micellization does not allow the P O group of the substrate to interact with the metal ion. This could be a result of steric constraint of the substrate when bound to the micelle and/or the formation of binuclear dihydroxy complexes, like (11), in the aggregate. So, in spite of the quite large rate accelerations observed [51] in the cleavage of PNPDPP in metallomicelles made of the amphiphilic complex (10), the second-order rate constant [allowing for the difference in pXa of the H2O molecules bound to copper(II) in micelles and monomers] is higher for (8) than for (10) (k > 250). 

Early studies in the area of metal-promoted peptide cleavage employed metal complexes in combination with oxidoreductive additives to achieve either oxidative (68-82) or hydrolytic (83) cleavage of the target proteins. For the oxidative cleavage, various redox active metal ions, such as Cu(I)-Cu(II), Cr(III), Ce(V), Fe(II)-Fe(III), Ni(II), and V(V) were employed. 

Last, it must be mentioned that metal coordination to the purine N7 position can also indirectly promote strand cleavage, although not through direct hydrolytic reaction on the sugar-phosphate backbone. Metal ions such as Pd and Cu, through coordination at N7, promote depurination. The depurinated site then becomes easily susceptible to hydrolysis upon treatment with mild base. 

In the transamination reaction (shown in reaction scheme LVIII), a pyridoxal amino acid Schiff base chelate is first formed, and a shift of the hydrogen atom in the a-carbon takes place to give a tautomeric Schiff base, which finally undergoes hydrolytic cleavage. The result is a transamination reaction in which the amino acid is converted to a keto acid and the pyridoxal to pyridoxamine. In this type of reaction, the metal ion serves to maintain the planarity of the Schiff base chelate and exerts an electron-withdrawing action in the same direction as the heterocyclic ring (149). 

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