Ga-III Complexes as Models for the M-III Site of Purple Acid Phosphatase: Ligand Effects on the Hydrolytic Reactivity Toward Bis(2,4-dinitrophenyl) phosphate

Abstract

The effects of a series of Galli complexes with tripodal ligands on the hydrolysis rate of the activated phosphate diester bis(2,4-dinitrophenyl)phosphate (BDNPP) have been investigated. In particular, the influence of the nature of the ligand donor sites on the reactivity of Ga-III which represents a mimic of the Fe-III ion in purple acid phosphatase has been evaluated. It has been shown that replacing neutral nitrogen donor atoms and carboxylate groups by phenolate groups enhanced the reactivity of the Ga complexes. Bell-shaped pH-rate profiles and the measured solvent deuterium isotope effects are indicative of a mechanism that involves nucleophilic attack on the coordinated substrate by Ga-OH. The trend in reactivity found for the different Ga complexes reveals that of the two effects of the metal, Lewis acid activation of the substrate and nucleophile activation, the latter one is more important in determining the intrinsic reactivity of the metal catalyst. The relevance of the present findings for the modulation of the activity of the M-III ion in purple acid phosphatase whose active site contains a phenolate (tyrosine side chain) is discussed.