EFFICIENT CATALYTIC PHOSPHATE DIESTER CLEAVAGE BY THE SYNERGETIC ACTION OF 2 CU(II) CENTERS IN A DINUCLEAR CIS-DIAQUA CU(II) CALIX[4]ARENE ENZYME MODEL

A calix[4]arene derivative 2-[Cu(II)](2) functionalized with two cis-d iaqua Cu(II) centers at the distal positions of the upper rim was synt hesized and investigated as a model for dinuclear metalloenzymes that catalyze chemical transformations of phosphate esters. The flexible di nuclear calix[4]arene efficiently catalyzes the transesterification of the RNA model 2-hydroxypropyl-p-nitrophenyl phosphate (HPNP) and the hydrolysis of the DNA model ethyl-p-nitrophenyl phosphate (EPNP) with turnover conversion, thereby exhibiting rate enhancement factors of 1. 0 x 10(4) and 2.7 x 10(4), respectively. The mononuclear reference com plex, 3-Cu(II), lacking the macrocyclic backbone, has a much lower act ivity, showing that the high catalytic activity of the dinuclear calix [4]arene complex is due to synergetic action of the two Cu(II) centers . Saturation kinetics and pH variation studies point to the formation of a Michaelis-Menten complex in which the phosphate group is doubly L ewis acid activated by coordination to the two Cu(II) centers. In this complex, a Cu(II) bound hydroxide ion, which is present already at pH 6.5, can act as a base in the intramolecular transesterification of H PNP or as a nucleophile in the hydrolysis of EPNP. The remarkably low pK(a) of the Cu(II) bound water molecules in the hydrophobic calix[4]a rene 2-[Cu(II)](2) mimics the low pK(a) of metal bound water molecules in hydrophobic enzyme active sites, which makes the enzyme (model) ac tive under slightly acidic to neutral conditions. The high catalytic e fficiency of this enzyme model is attributed to a dynamic binding of t he substrate and (pre)-transition state, possible by rapid low energy conformational changes of the flexible calix[4]arene backbone.