| Abstract
| - The complex (PATH)ZnOH (1), which is formed in aqueous solution from (PATH)ZnCH3, is a rare example of a thiolate-containing 4-coordinate zinc hydroxide complex, and represents a reactive model for the zinc form of the enzyme peptide deformylase (PDF). This complex promotes the hydrolysis of 4-nitrophenyl acetate, and the pH-independent second-order rate constant, Ea, ΔH⧧, and ΔS⧧ have been determined for this reaction, along with the pKa of the ZnOH unit.
- A novel zinc(II) hydroxide complex with a rare alkylthiolate donor in the coordination sphere is formed in aqueoussolution 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 detailedkinetic study and is the first functional model for the zinc form of the enzyme peptide deformylase. From a fit ofthe sigmoidal pH−rate profile a kinetic pKa of 8.05(5) and a pH-independent second-order rate constant (k‘ ‘max) of0.089(3) M-1 s-1 have been obtained. The kinetic pKa is similar to the pKa of 7.7(1) determined by a potentiometricstudy (25 °C, I = 0.1 (NaNO3)). Observation of both rate enhancement and turnover shows that 1 acts as acatalyst for the hydrolysis of 4-NA, although the turnovers are modest. Activation parameters have been obtainedfrom a temperature-dependence study of the rate constants (Ea = 54.8 kJ mol-1, ΔH⧧ = 52.4 kJ mol-1, and ΔS⧧= −90.0 J mol-1 K-1), and support a reaction mechanism which depends on nucleophilic attack of 1 in therate-determining step. This is the first kinetic and thermodynamic study of a 4-coordinate zinc hydroxide complexcontaining a thiolate donor. In addition it is only the second time that a complete set of activation parameters havebeen obtained for the zinc-promoted hydrolysis of a carboxylic ester. This study puts the basicity and nucleophilicityof a (N2S)ZnOH complex in context with those of other LnZnOH complexes and enzymes.
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