Rapid Phosphodiester Hydrolysis by an Ammonium-Functionalized Copper(II) Complex. A Model for the Cooperativity of Metal Ions and NH-Acidic Groups in Phosphoryl Transfer Enzymes

The copper(II) complexes [( L n )Cu(NO 3 ) 4 ·2(H 2 O)] ( n = 1: 1, n = 2: 2 ) of the ammonium-functionalized ligands [6,6‘-(Me 2 HNCH 2 C⋮C) 2 bpy] 2+ ( L 1 ) and [6,6‘-(Me 3 NCH 2 C⋮C) 2 bpy] 2+ ( L 2 ) were prepared. Hydrolysis of the activated phosphodiester bis( p -nitrophenyl) phosphate (BNPP) by these complexes in ethanol−water 19:1 at 20 °C was investigated. The rate constants for cleavage of the bound phosphodiester at pH 6.6 are k cat = 4.4(±0.4) × 10 - 3 s - 1 for ( L 1 )Cu and k cat = 4(±1) × 10 - 6 s - 1 for ( L 2 )Cu. ( L 1 )Cu accelerates hydrolysis of BNPP 4 × 10 7 -fold and is 1000 times more reactive than ( L 2 )Cu. This suggests that the high reactivity of ( L 1 )Cu is related to the interaction of the acidic −NMe 2 H + group with the phosphodiester substrate. Bifunctional binding of a phosphate ester by metal coordination and hydrogen bonding with one ammonium group is observed in the crystallographically characterized complex [( L 1 ) 2 Cu 2 (1,3-μ-O 3 POPh) 2 (OH 2 ) 2 ](NO 3 ) 4 ( 3 ). A plausible mechanism of BNPP cleavage by ( L 1 )Cu includes metal-hydroxide attack to the phosphodiester which is doubly activated by coordinative and hydrogen bonding. The copper(II) complex of L 1 represents a simple model for the efficient cooperativity of metal ions and NH-acidic amino acid side chains (Lys-ammonium, Arg-guanidinium, His-imidazolium) in enzymes that catalyze the cleavage of phosphate di- and monoesters.