2-hydroxypropyl 4-nitrophenyl phosphate HPNP transesterification

The 2-hydroxypropyl 4-nitrophenyl phosphate (HPNP) transesterification reactivity (Fig. 40, bottom) of binuclear zinc complexes of the dianionic ICIMP and trianionic BCIMP ligands (Fig. 62) have been compared.245 Notably, both of these ligands contain imidazole and carboxylate donors akin to protein-derived residues that are found as ligands to the zinc centers in phosphate-ester-hydrolyzing enzymes such as phosphotriesterase. The initial rate for HPNP transesterification promoted by [(ICIMP)Zn2(Ph2Ac)]C104 in 50% acetonitrile-water solution (tris buffer) was determined to be five times faster than the reaction promoted by [(BCIMP)Zn2(Ph2Ac)]. 

In spite of the above mentioned Co(EII) compounds, kinetically labile metal complexes may provide fast product/substrate exchange and some of these systems show real catalytic activity. In native dinuclear phosphatases Mg(II), Mn(II), Fe(II/III), or Zn(II) ions are present in the active centers. Although the aqua complexes of the weakest Lewis acids, Mg(H) and Mn(II), show measurable acceleration of e.g. the transesterification of 2-hydroxypropyl p-nitrophenyl phosphate HPNP, [Mn(II)] = 0.004 M, kobs/ uncat = 73 at pH 7 and 310 K, [38] or the hydrolysis of S -uridyluridine (UpU) [39], only a few structural [40] but no functional phosphatase-mimicking dinuclear complexes have been reported with these metal ions. 

Fig. 40 (top) Cationic portion of bifunctional zinc complexes and relative second-order rate constant values and (bottom) intramolecular transesterification reaction of 2-hydroxypropyl 4-nitrophenyl phosphate (HPNP). 

The bimetallic complex 2—Zu2 is a highly efficient turnover catalyst of the transesterification of the RNA model compound 2-hydroxypropyl p-nitrophenyl phosphate (HPNP) (Eq. 26.1). A 23,000-fold rate enhancement is observed in the presence of 2—Zn2 (0.48 mM catalyst concentration, 50 % MeCN-20 mM aqueous buffer, pH 7,25 °C) [16]. The two metal ions in 2—Zn2 efficiently cooperate both in... 

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