Phosphodiesterolytic activity of lanthanide (III) complexes with α-amino acids

Kinetics of the hydrolysis of BNPP (bis(4-nitrophenyl)phosphate) mediated by lanthanide - samarium (III) and ytterbium (III) - alone and in the presence of various alfa amino acids has been systematically studied at 37.0 °C and I = 0.15 M in NaClO₄, in the pH interval of 7-9. The rate of BNPP cleavage is sensitive to metal ion concentration, pH, and ligand to metal molar ratio. Hydrolysis follows Michaelis-Menten-type saturation kinetics. For both metals, high pH values markedly increase the observed activity. Besides, potentiometric titrations of all these systems under identical conditions allowed us to identify the active coordination compounds towards hydrolysis. The results show that complexes with phosphodiesterolytic activity are monomeric cationic species such as Ln(aa)₃(OH)]²⁺ or Ln(aa)₂(OH)₂]⁺. Since phosphodiesterolytic activity is evident above pH 7 and it is increased with increasing pH, hydrolytic reactions of the metals are competitive processes that could lead to their precipitation as Ln(OH)₃(s). In this sense, ligand excess (for example, ligand to metal molar ratio equal to 30) was employed. Furthermore, due to its more extended hydrolysis, ytterbium shows, in general, less activity than samarium under the studied conditions. In general, a good phosphodiesterolytic activity is observed for these complexes under similar conditions to the physiological ones. Amino acids could be easily derivatized without changing their coordinating ability, leading to lanthanide complexes possibly capable of efficiently hydrolyzing the phosphodiester linkages of nucleic acids.