The substrate-assisted general base catalysis model for phosphate monoester hydrolysis: Evaluation using reactivity comparisons

Reactions of phosphate monoesters are ubiquitous in biological chemistry. H ence, this class of reactions has been subjected to extensive mechanistic a nalysis by physical organic chemists seeking to understand the nonenzymatic reactions and to apply this understanding to the corresponding enzymatic r eactions. Substrate-assisted general base catalysis of phosphoryl transfer, in which a proton from the nucleophile is transferred to a nonbridging pho sphoryl oxygen of the substrate prior to attack, has recently been proposed as a mechanism for both nonenzymatic and enzymatic reactions of phosphate monoester dianions and related compounds, in opposition to the previously a ccepted mechanism of direct nucleophilic reaction. We have evaluated this n ew mechanism for the hydrolysis of a phosphate monoester dianion in solutio n by considering the reactivity of the monoester monoanion that is a reacti on intermediate in the proposed proton transfer. The monoanion of the monoe ster 2,4-dinitrophenyl phosphate (DNPP-) and its diester analogue, methyl 2 ,4-dinitrophenyl phosphate monoanion (MDNPP-), have similar rate constants for reaction with several nucleophiles (k(rel) = k(DNPP)/k(MDNPP) approxima te to 10). In contrast, the substrate-assisted catalysis proposal requires that the rate constant for reaction of hydroxide ion with DNPP- be similar to 10(9)-fold larger than the experimentally determined rate constant for t he corresponding reaction of hydroxide ion with MDNPP-. These and additiona l observations render substrate-assisted general base catalysis an unlikely alternative to the classical mechanism for nonenzymatic phosphoryl transfe r.