Metaphosphate mechanism, hydrolysis

All arguments for the metaphosphate mechanism appear persuasive only when considered together since individual arguments could also apply to other hydrolysis mechanisms of phosphoric esters. 

Converging lines of evidence have led to a general acceptance of the monomeric metaphosphate mechanism for the hydrolysis of phosphate monoester monoanions. The pH rate profile for aryl and alkyl phosphate monoester hydrolyses commonly exhibits a rate maximum near pH 4. where the concentration of the monoanion is at a maximum. The proposed mechanism is based on these principal points of evidence (a) a general observation of P-O bond cleavage (b) the entropies of activation for a series of monoester monoanions are all close to zero, which is consistent with a unimolecular rather than a bi-molecular solvolysis where entropies of activation are usually more negative by 20 eu7 (c) the molar product composition (methyl phosphate inorganic phosphate) arising from the solvolysis of the monoester monoanion in a mixed methanol-water solvent usually approximates the molar ratio of methanol ... 

Although the metaphosphate mechanism for hydrolysis is well documented, such a pathway remains to be demonstrated in a biological system. Our present knowledge of many enzymic reactions allows, at best, the formulation of a preliminary mechanism, i.e. the chemical identity of substrates and enzymic intermediates and the minimal kinetic scheme. For example, much recent attention has been focused on the remarkable stability of the covalent phos-phoryl-enzyme (an O-phosphoryl serine) derived from E. coii alkaline phosphatase28 and inorganic phosphate, and on a systematic kinetic study of the enzyme s substrate specificity (O-, N- and S-monoesters) -9. Dephosphorylation of the enzyme, however, does not appear to be via a metaphosphate mechanism30. 

Phosphate esters have a variety of mechanistic paths for hydrolysis. Both C-O and P-0 cleavage are possible depending on the situation. A phosphate monoanion is a reasonable leaving group for nucleophilic substitution at carbon and so 8 2 or SnI reactions of neutral phosphate esters are well known. PO cleavage can occur by associative (by way of a pentacoordinate intermediate), dissociative (by way of a metaphosphate species), or concerted (avoiding both of these intermediates) mechanisms. 

There has been controversy over the mechanism of hydrolysis of ethylene methyl phosphate and investigations into the nature of metaphosphate and its role in the hydrolysis of phosphorus esters have continued. Elegant experiments relating to each of these problems have been reported. 

On the other hand, it is found that only partial racemization occurs on alkaline hydrolysis of optical active 198 in aqueous methanol136) and no racemization takes place in the hydrolysis of 199 in dioxane/water137). Moreover, the latter reaction is only ca. 80 times faster at 29 °C than that of the analogous morpholide 200, for which a metaphosphorimidate mechanism is precluded a priori by the absence of an NH function and whose hydrolysis is likewise stereospecific,37). Clearly a free metaphosphorimidothioate of type 191 cannot be involved in this case. The experimental findings are compatible, however, with the hypothesis that the nucleophile water attacks a metaphosphorothioimidate/phenolate associate 201. The question of how free metaphosphates occur in solution is of a general nature it has also been considered in the previous Section. 

Mechanisms may be readily envisaged where this reaction may lead to phos-phoryl transfer through nucleophilic attack with ring opening on phosphorus followed by hydrolysis of the acyclic acyl phosphate. In biological systems this may function as an alternative to the metaphosphate reaction. 

As was the case with other phosphotransfer reactions, the hydrolysis reaction catalyzed by PTPs lies along a mechanistic continuum between the limiting cases of associative and dissociative mechanisms. In the case of Yersina PTP, a crystal structure was solved with nitrate, a mimic of the transition state similar to the use of AIF3 with protein kinase A. In this case, analysis of the structure is proposed to be more consistent with a -1 charged metaphosphate, dissociative mechanism (11). 

Linear free-energy relationships (LFER) with monoanionic phosphorylated pyr-idines indicate a loose transition state in which metaphosphate is not an intermediate.16 The hydrolysis of the monoanion of 2,4-dinitrophenyl phosphate is thought to be concerted,39 but the possibility of a metaphosphate intermediate has not been ruled out with esters having less activated leaving groups. A stereochemical study of the hydrolysis of phenyl phosphate monoanion indicates that the reaction proceeds with inversion.21 This result implies either a concerted mechanism, or a discrete metaphosphate intermediate in a pre-associative mechanism. 

Fig. 8 Potential mechanisms for hydrolysis of phosphomonoester monoanions. In mechanism (a), proton transfer from the phosphoryl group to the ester oxygen (probably via the intermediacy of a water molecule) yields an anionic zwitterion intermediate. This may react in either concerted fashion (upper pathway) or via a discrete metaphosphate intermediate in a preassociative mechanism (bottom pathway). Mechanism (b) denotes proton transfer concerted with P-O(R) bond fission. As with (a), such a mechanism could either occur with concerted phosphoryl transfer to the nucleophile (upper pathway) or via a discrete metaphosphate intermediate in a preassociative mechanism (bottom pathway).

Phosphoric Acid and its Derivatives.— Amongst hydrolysis studies reported during the year are those of di(2,4-dichlorophenyl) phosphate tri(4-iodobenzyl) phosphate in 0.5—4.5M-HC1 in 50% dioxan and l-nitro-2-naphthyl and 4-nitro-l-naphthyl phosphates in 0.01—6M-HC1. The hydrolysis of 2,4-dinitrophenyl dibenzyl phosphate exhibits a 0 kinetic isotope effect that is suggestive of the intermediacy of a monomeric metaphosphate species, whereas an 5 N2-like mechanism is apparent... 

Hydrolysis of Phosphates. The conclusion that phosphoryl transfer between amines is concerted does not exclude the formation of a metaphosphate intermediate in reactions with weaker nucleophiles, including water. Pyridine is a much stronger nucleophile than water and it is possible that phosphoryl transfer between pyridines is concerted because the metaphosphate ion does not have a significant lifetime in the presence of pyridine the metaphosphate ion might exist for a short time in water. However, it is much more difficult to determine whether or not the mechanism is concerted for a reaction with water than for a reaction with pyridine. 

The second mechanism for a hydrolysis reaction at phosphorus involves the initial dissociative loss of the leaving group to generate the metaphosphate anion... 

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