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(NO3)(4) . 2(H2O)] (n = 1: 1, n = 2: 2) of the ammonium-functionalized ligands [6,6'-(Me(2)HNCH(2)C=C)(2)b py](2+) (L(1)) and [6,6'-(Me(3)NCH(2)C=C)(2)bpy](2+) (L(2)) were prepa red. Hydrolysis of the activated phosphodiester bis(p-nitrophenyl) pho sphate (BNPP) by these complexes in ethanol-water 19:1 at 20 degrees C was investigated. The rate constants for cleavage of the bound phosph odiester at pH 6.6 are k(cat) = 4.4(+/-0.4) x 10(-3) s(-1) for (L(1))C u and k(cat) = 4(+/-1) x 10(-6) s(-1) for (L(2))Cu. (L(1))Cu accelerat es hydrolysis of BNPP 4 x 10(7)-fold and is 1000 times more reactive t han (L(2))Cu. This suggests that the high reactivity of (L(1))Cu is re lated to the interaction of the acidic -NMe(2)H(+) group with the phos phodiester substrate. Bifunctional binding of a phosphate ester by met al coordination and hydrogen bonding with one ammonium group is observ ed in the crystallographically characterized complex (L(1))(2)Cu-2(1,3 -mu-O(3)POPh)(2)(OH2)(2)](NO3)(4) (3). A plausible mechanism of BNPP c leavage by (L(1))Cu includes metal-hydroxide attack to the phosphodies ter which is doubly activated by coordinative and hydrogen bonding. Th e copper(II) complex of L(1) represents a simple model for the efficie nt cooperativity of metal ions and NH-acidic amino acid side chains (L ys-ammonium, Arg-guanidinium, His-imidazolium) in enzymes that catalyz e the cleavage of phosphate di- and monoesters.