Phosphate monoester monoanions, hydrolysis

Phosphate monoesters [R —OPO(OH)2] resist alkaline hydrolysis by converting to unreactive dianions [R —OP03 ]. Phosphate monoester monoanions [R —0P0(0H)0 ] are, however, susceptible to facile hydrolysis under slightly acidic conditions. 

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 ... 

In contrast, the acid-catalyzed hydrolysis of alkyl selenates is A-2158. The actual species which undergoes decomposition to alcohol and sulfur trioxide is probably the zwitterion as in the case of phosphate monoester monoanions. Evidence for sulfur trioxide as the reactive initial product of the A-1 solvolysis is obtained from the product compositions arising with mixed alcohol-water solvents. The product distribution is identical to that found for sulfur trioxide solvolysis, with the latter exhibiting a three-fold selectivity for methanol. Although the above entropies of activation and solvent deuterium isotope effects do not distinguish between the conventional A-l mechanism and one involving rate-limiting proton transfer, a simple calculation, based on the pKa of the sulfate moiety and the fact that its deprotonation is diffusion controlled. 

Grzyska PK, Czyryca PG, Purcell J, Hengge AC. Transition state 42. differences in hydrolysis reactions of alkyl versus aryl phosphate monoester monoanions. J. Am. Chem. Soc. 2003 125 13106-13111. 

Taken together, the data indicate direct phosphoryl transfer to a metal-bound water molecule without a phosphoenzyme intermediate. A Bronsted analysis found a value of (j of -0.3 for V/K,137 similar to the value for the uncatalyzed hydrolysis of phosphate monoester monoanions, which could be indicative of charge neutralization on the leaving group in the transition state via protonation. 

From a detailed comparison of the rates of aqueous hydrolysis of a very wide range of phosphate monoester monoanions it was found that there exists... 

A. S. Kearney and V. J. Stella, Hydrolysis of pharmaceutically relevant phosphate monoester monoanions Correlation to an established structure-reactivity relationship, J. Pharm. Sci. 82,69-72(1993). 

The reactive species under acidic conditions is the neutral ester. This reaction is believed to proceed by transfer of the proton from the sulfuryl group to the leaving group, as in reactions of phosphate monoester monoanions. A reduced value for d a solvent deuterium isotope effect of 2.43 " are consistent with proton transfer to the leaving group in the transition state. The intermediacy of free SO3 in the acid hydrolysis is sometimes assumed, but has not been proven. 

The spontaneous hydrolysis of 2-pyridyl phosphate (PP) is a good model for the special mechanism (Scheme 15) for the hydrolysis of phosphate monoester monoanions (M ), which are believed to react via an initial proton transfer equilibrium to form the highly reactive species the proton transfer occurring from a hydroxyl group attached... 

The pronounced proclivity of phosphoric monoester monoanions to eliminate POf is not always recognizable from the characteristic pH profile of Fig. 1. The hydrolysis rate maximum at pH w 4 may be masked by a faster reaction of the neutral phosphoric ester, as in the case of a-D-glucose 1-phosphate63) or on hydrolysis of monobenzyl phosphate 64). In the latter case, the known ability of benzyl esters to undergo SN1 and SN2 reactions permits fast hydrolysis of the neutral ester with C/O bond breakage. The fact that the monoanion 107 of the monobenzyl ester is hydrolyzed some 40 times faster than the monoanion 108 of the dibenzyl ester at the same pH again evidences the special hydrolysis pathway of 107, rationalized by means of the metaphosphate hypothesis. 

The pH-rate profile for unbuffered hydrolysis of glyceraldehyde-3-phosphate (6-3-P) has been attributed to hydrolysis of the monoanion of the phosphate monoester at pH < 4, spontaneous formation of glyceraldehyde from the phosphate dianion at pH 7-8, and, at higher pH, hydroxide-catalysed methylglyoxal formation. Reaction of the dianion is not subject to a solvent isotope effect and is believed to occur by the irreversible ElcB mechanism whereby an enediolate intermediate, formed on rate-determining C(2) deprotonation, subsequently expels phosphate trianion by C—0 bond breaking. The diethylacetal and 2-methyl-G-3-P do not hydrolyse under the same conditions.5... 

In 1955 Westheimer, Bunton, Vernon and their coworkers suggested that the hydrolysis of the monoanions of phosphate monoesters proceeds through the formation of an intermediate monomeric metaphosphate monoanion, which reacts rapidly with water to give inorganic phosphate (equation 1)(7,2). 

The stereochemical consequences of the methanolyses of the monoanion of phenyl [ 0, 0, 0]phosphate, the dianion of 4-nitrophenyl [" 0, 0, 0] phosphate, and [ 0, 0, 0]phosphocreatine have been determined by Knowles and co-workers (35). The monoanion of phenyl phosphate behaves as a typical phosphate monoester in that its rate of hydrolysis is maximal at pH 4, where an intramolecular proton transfer is possible. The dianion of 4-nitrophenyl phosphate is highly reactive since protonation of the leaving group is not necessary. Finally, A-phosphoguanidines have been reported to be the most reactive phosphoryl compound (the chiral phosphocreatine can be enzymically synthesized from [ y- 0, 0, 0]ATP). Thus, the solvolyses of all three of these compounds are believed to involve the participation of metaphosphate anion. The meth-anolysis of each of these compounds proceeds with quantitative inversion of configuration. 

All phosphatases catalyze the same net reaction, the hydrolysis of a phosphate monoester to inorganic phosphate and the alcohol or phenol from the ester group. As already mentioned, despite the thermodynamic favorability of this reaction, the kinetic barrier is formidable. A number of the enzymes that catalyze this reaction have been characterized. Phosphatases vary in their preference for the charge state of the substrate (either the monoanion or the dianion), in the presence or absence of a metal center, and in the utilization of a phosphoenzyme intermediate versus direct attack by water. Even among metallophosphatases, there are variations in the means by which the dinuclear metal center participates in binding and catalysis. 

The dissociative mechanism (1) for the hydrolysis of phosphate monoesters was originally invoked to explain the enhanced reactivity of the monoanion-monoacid form of methyl phosphate (Bunton et al., 1958 Butcher and Westheimer, 1955). The facts that were cited in support of the applicability... 

Reactions of less activated (aliphatic) phosphates were also investigated by the Wolfenden group.In case of these phosphates, the solvent used was cyclohexane, which is stiU aprotic (as DMSO) but also nonpolar, and the phosphates were synthesized as their tetrabutylammonium salts (as for reactions of aryl monoesters). The previous results strongly suggested an enthalpic contribution for the reactions of pNPP", whereas results for the hydrolysis of neopentyl phosphate monoester dianion (NP ) in wet cyclohexane su ested a significant entropic contribution to the overall 2.5 X 10 rate acceleration. In this solvent, the monoanionic and dianionic species of NP reacted with the same rate this result was attributed to the removal of water molecules proposed to assist hydrolysis of monoester monoanions in aqueous solution. Unsurprisingly, hydrolysis of phosphate diesters was also accelerated by cyclohexane, with a rate acceleration of 10 -fbId for dineopentyl phosphate at 25 °C, and by acetone, with a rate acceleration of 5 X 10 -fbId. The reduced amount of catalysis in case of alkyl diesters, relative to alkyl monoester dianions, parallels what was observed with the corresponding aryl species (see above). 

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