Ligand effects on the phosphoesterase activity of Co(II) Schiff base complexes built on PAMAM dendrimers

Abstract Two types of cobalt complexes were created on polyamidoamine dendrimer generation four (PAMAM G4) as potential hydrolytic catalysts. The first types were simple aqueous mixtures of Co(II)–G4 or Co(III)–G4, and the second type were Co(II)–Schiff-base complexes made from template reactions of the terminal amines on G4 with different dialdehyde compounds. The simple Co(III)–G4 mixtures gave only ligand exchange reactions with inorganic phosphate (or with the phosphate esters) based on changes in the visible spectra. The Co(II) Schiff base complexes catalyzed the hydrolysis of bis-nitrophenylphosphate (BNPP) and nitrophenylphosphate (NPP). The Schiff base complex derived from glyoxal (gly), Co(II)[PAMAM G4–gly] ( A ), was very effective for the hydrolysis of the phosphate monoester, NPP [ k bi (= k cat / K m )=0.15±0.02 M −1 s −1 at 50°C, pH 7.3]. The other complex derived from 2,6-pyridine dialdehyde (PDA), Co(II)[PAMAM G4–PDA] ( B ), followed Michaelis–Menten kinetics with both substrates ( K m =0.13 and 0.15 mM; k cat =5.8×10 −6 and 6.2×10 −6 s −1 for BNPP and NPP, respectively). Reactions with A did not show any saturation effects. The pH–rate profiles for A and B peak over a narrow pH range implicating a metal-hydroxide form of the catalyst as the active species. The rate enhancements here are compared to an analogous set of catalysts, Co(II)–PEI–glyoxal and Co(II)–PEI–2,6-diacetylpyridine, where PEI is poly(ethylenimine) (N. Kim, J. Suh, J. Org. Chem. 59 (1994) 1561). The ability of the PAMAM platform to yield more homogeneous Co(II) sites than the Co(II)–PEI complexes is discussed.