Phosphoester hydrolysis using structural phosphatase models of tren based zinc(II) complexes and X-ray crystal structures of [Zn(tren)(H2O)](ClO4)2 and [Zn(tren)(BNPP)]ClO4

Abstract

New tris(2-aminoethyl)amine (tren) L1 based ligands, namely N,N′,N″-tris(2-benzylaminoethyl)amine L2 and N,N′,N″-tris(im-benzyl-L-histidylethylaminoethyl) amine L3 have been synthesized and characterized. Complexation studies on Zn2+ complexes 1, 2 and 3 derived from L1, L2, and L3 showed that the presence of benzyl and benzyl-histidyl moieties attached to the tripodal ligand L1 side arms decrease the pKa of the Zn-bound water molecule: 10.72 for 1, 9.61 for 2 and 7.43 for 3, respectively. The zinc complex of 3 was a much more active catalyst for the hydrolysis of bis(p-nitrophenyl)phosphate (BNPP-) and tris(p-nitrophenyl)phosphate (TNPP) compared with 1 and 2. In the case of 1 and 2, the pH-dependence of their observed pseudo-first-order rate constants kobsd showed sigmoidal pH-rate profile, while 3 gave bell-shape curve with a maximum rate constant of around 1.0 × 10-5 s-1 at pH 8.5. The pH dependence of kobsd indicated that the Zn-bound hydroxo species is responsible for catalytic activity. The crystal structures of [L1Zn-H2O]2+ (1) and [L1Zn-BNPP]+ (4) have been determined and showed trigonal-bipyramidal configurations around the central Zn. The zinc complexes of 1 and 4 served as structural models for the binding mode of coordinated water as well as substrates in the active site of zinc enzyme. © 2001 Elsevier Science B.V.