Unsymmetrical FeIIICoII and GaIIICoII Complexes as Chemical Hydrolases: Biomimetic Models for Purple Acid Phosphatases (PAPs)

The design and development of suitable biomimetic catalytic systems capable of mimicking the functional properties of enzymes continues to be a challenge for bioinorganic chemists. In this study, we report on the synthesis, X-ray structures, and physicochemical characterization of the novel isostructural [(FeCoII)-Co-III(BPBPMP)(mu-OAc)(2)]ClO4 (1) and [(GaCoII)-Co-III(BPBPMP)(mu-OAc)(2)]ClO4 (2) complexes with the unsymmetrical dinucleating ligand H2BPBPMP {2-bis[{(2-pyridyl-methyl)-aminomethyl)-6-{(2-hydroxy-benzyl)-(2-pyridyl-methyl)}-aminomethyl]-4-methylphenol). The previously reported complex [(FeZnII)-Zn-III(BPBPMP)(mu-OAc)(2)]ClO4 (3) was investigated here by electron paramagnetic resonance for comparison with such studies on 1 and 2. A magneto-structural correlation between the exchange parameter J (cm(-1)) and the average bond lengh d (angstrom) of the [Fe-III-O-M-II] structural unit for 1 and for related isostructural (FeMII)-M-III complexes using the correlation J = -10(7) exp(-6.8d) reveals that this parameter is the major factor that determines the degree of antiferromagnetic coupling in the series [(BPBPMP)Fe-III(mu-OAc)(2)M-II](+) (M-II = Mn, Fe, Co, Ni) of complexes. Potentiometric and spectrophotometric titrations along with electronic absorption studies show that, in aqueous solution, complexes 1 and 2 generate the [(HO)M-III(mu-OH)Co-II(H2O)] complex as the catalytically active species in diester hydrolysis reactions, Kinetic studies on the hydrolysis of the model substrate bis(2,4-dinitrophenyl)phosphate by 1 and 2 show Michaelis-Menten behavior, with 2 being 35% more active than 1. In combination with k(H)/k(D) isotope effects, the kinetic studies suggest a mechanism in which a terminal M-III-bound hydroxide is the hydrolysis-initiating nucleophilic catalyst. In addition, the complexes show maximum catalytic activity in DNA hydrolysis near physiological pH. The modest reactivity difference between 1 and 2 is consistent with the slightly increased nucleophilic character of the Ga-III-OH terminal group in comparison to Fe-III-OH in the dinuclear (MCoII)-Co-III species.