Hydrolysis of 4-nitrophenyl acetate by a (N2S(thiolate))zinc hydroxide complex:: A model of the catalytically active intermediate for the zinc form of peptide deformylase

A novel zinc(II) hydroxide complex with a rare alkylthiolate donor in the coordination sphere is formed in aqueous solution from the dissolution of the zinc alkyl precursor complex (PATH)ZnCH3 (PATH = 2-methyl-1-[methyl(2-pyridin-2-ylethyl)amino]propane-2-thiolate) in H2O and protonolysis of the Zn-C bond to give (PATH)ZnOH (1). The (PATH)ZnOH complex has been shown to promote the hydrolysis of 4-nitrophenyl acetate (4-NA) by a detailed kinetic study and is the first functional model for the zinc form of the enzyme peptide deformylase. From a fit of the sigmoidal pH-rate profile a kinetic pK(a) of 8.05(5) and a pH-independent second-order rate constant (k"(max)) of 0.089(3) M-1 s(-1) have been obtained. The kinetic pK(a) is similar to the pK(a) of 7.7(1) determined by a potentiometric study (25 degreesC, I = 0.1 (NaNO3)). Observation of both rate enhancement and turnover shows that 1 acts as a catalyst for the hydrolysis of 4-NA, although the turnovers are modest. Activation parameters have been obtained from a temperature-dependence study of the rate constants (E-a = 54.8 kJ mol(-1), DeltaH(double dagger) = 52.4 kJ mol(-1), and DeltaS(double dagger) = -90.0 J mol(-1) K-1), and support a reaction mechanism which depends on nucleophilic attack of 1 in the rate-determining step. This is the first kinetic and thermodynamic study of a 4-coordinate zinc hydroxide complex containing a thiolate donor. In addition it is only the second time that a complete set of activation parameters have been obtained for the zinc-promoted hydrolysis of a carboxylic ester. This study puts the basicity and nucleophilicity of a (N2S)ZnOH complex in context with those of other LnZnOH complexes and enzymes.