TRANSESTERIFICATION OF A PHOSPHATE DIESTER BY DIVALENT AND TRIVALENT METAL-IONS

The transesterification of the 2-(4-nitrophenyl phosphate) ester of propylene glycol (1) was promoted by La3+, Nd3+, Eu3+, Gd3+, Tb3+, Yb3+, Lu3+, Pb2+, Zn2+, Cu2+, Ni2+, Co2+, Mn2+, Mg2+, and Ca2+ at 37-degrees-C in 0.01 M Hepes buffer (N-(2-hydroxy-ethyl)piperazine-N'-ethanesulfonic acid), pH 6.85. The ability of metal ions to promote transesterification of 1, as measured by apparent-second-order rate constants, followed the order Tb3+ > Gd3+ > Yb3+ > Eu3+ > Nd3+ > Pb2+ > Lu3+ > La3+ >> Cu2+ > Zn2+ >> Co2+ > Mn2+ > Ni2+ >> Mg2+ > Ca2+. The transesterification of 1 by La3+, Pb2+, Zn2+, Cu2+, Ni2+, Co2+, or Mn2+ exhibited saturation kinetics, consistent with the formation of a reactive metal ion complex with 1. Metal ion association constants (K) for formation of the metal-1 complex and a first-order catalytic rate constant (k(cat)) for decomposition of the metal-1 complex were determined for metal ions that showed saturation kinetics. K varied by only a factor of 5, whereas k(cat) varied by a factor of 29. Transesterification of 1 bound to Pb2+, La3+, Cu2+, Zn2+, Co2+, Mn2+, Ni2+ occurred 2.1 x 10(4), 1.4 x 10(4), 4.6 x 10(3), 3.6 x 10(3), 9.2 x 10(2), 7.7 x 10(2), or 7.7 x 10(2) times more rapidly, respectively, than the transesterification of free 1. Properties of metal ions that may affect their ability to promote transesterification of phosphate esters are discussed; these include the Lewis acidity of the metal ion, the formation of metal hydroxide complexes, metal ion radius, and stabilization of a hypervalent phosphorus (V) intermediate. These metal ion model studies are put in context with metal ion requirements in self-cleaving RNAs.