ARTIFICIAL METALLOPHOSPHOESTERASES BUILT ON POLY(ETHYLENIMINE)

Various derivatives of poly(ethylenimine) (PEI)-containing macrocyclic metal centers were prepared by condensation of PEI with several dicarbonyl compounds in the presence of a series of metal ions involved as templates. In view of the biological functions of transphosphorylation reactions, the PEI-based macrocyclic complexes were tested for their activity as artificial metallophosphoesterases. Hydrolysis of bis(p-nitrophenyl) phosphate (BNPP) and p-nitrophenyl phosphate (NPP) was catalyzed by the PEI-based macrocyclic complexes. Kinetic data for the hydrolysis of BNPP or NPP revealed complex formation between the substrate and the catalysts, followed by efficient dephoephorylation of the complexed substrate. Depending upon the structure of the catalyst and the reaction conditions, the degree of acceleration observed for BNPP and NPP hydrolysis was up to 6 orders of magnitude. The most effective catalyis was achieved by the macrocyclic complex prepared by condensation of PEI with glyoxal in the presence of Co(II) ion. The PEI-based macrocycles containing divalent metal ions such as Fe(III), Ni(II), Cu(II), or Zn(II) ion were also quite effective catalysts for the hydrolysis of BNPP or NPP. The catalytic activity of the PEI-based macrocyclic complexes containing divalent metal ions was much greater than that of other synthetic catalysts containing divalent metal ions ever reported. Possible origins of the effective catalysis are discussed, in view of the mechanisms proposed for analogous enzymatic and nonenzymatic hydrolysis of phosphate esters. Characteristics of actions of metalloenzymes such as essential involvement of metal ions, complex formation with the substrates, and efficient catalysis in the conversion of bound substrates were reproduced by the artificial metallophosphoesterases. The metal centers of the artificial metallophosphoesterases are involved both in substrate binding and in catalytic transformation. Catalytic repertoires played by the metal ions are discussed.