Tetranuclear iron(III) complexes with a carboxylate-rich ligand as synthetic mimics of phosphoesterases in aqueous media

Abstract

The characterization and catalytic activities of two tetranuclear Fe(III) complexes of a carboxylate-rich ligand, N,N’-Bis[2-carboxybenzomethyl]-N,N’ -Bis[carboxymethyl]-1,3-diaminopropan-2-ol (H5ccdp) ligand towards the hydrolysis of p-nitrophenyl phosphate (PNPP) and bis(p-nitrophenyl) phosphate (BNPP) substrates in aqueous systems are described. Kinetic investigations were carried out using UV–vis spectrophotometric techniques at 25 °C and 37 °C and varied pH conditions (7 – 9.5). The kinetic studies showed that the turnover rate (kcat) values for the hydrolysis of PNPP by [Fe4(ccdp)2(μ-O)(μ-OH)(d-GluA)2]3- were marginally higher than [Fe4(ccdp)2(OH)2(o-phth)2]4- at 9.24 × 10-6 s−1 (pH 8 and 37 °C) and 4.27 × 10-7 s−1 (pH 8 and 25 °C), respectively. However, similar comparisons among the BNPP hydrolysis showed the turnover rate values for [Fe4(ccdp)2(OH)2(o-phth)2]4- was marginally better than [Fe4(ccdp)2(μ -O)(μ –OH)(d-GluA)2]3- with values of 1.85 × 10-6 s−1 (pH 8 and 37 °C) and 6.23 × 10-7 (pH 8 and 37 °C). ESI-MS techniques detected catalyst-substrate adduct species for both metal complexes with PNPP. Additionally, both complexes also showed evidence for complex-substrate-nucleophile adduct formation with both BNPP and PNPP substrates via ESI-MS. Based on the kinetic activity and structural information obtained in this study, a reaction mechanism for the hydrolysis of phosphorester has been proposed.