Potentiometric titrations of the mixtures of lanthanide(III) perchlorates and bis-Tris propane (BTP) reveal formation of dinuclear hydroxo complexes M2(BTP)2(OH)n(6-n), where M = La(III), Pr(III), Nd(III), Eu(III), Gd(III), and Dy(III) and n = 2, 4, 5, or 6, in the pH range 7-9. ESI-MS data confirm the presence of dinuclear species. Kinetics of the hydrolysis of bis(4-nitrophenyl) phosphate (BNPP), mono-4-nitrophenyl phosphate (NPP), and 4-nitrophenyl acetate (NPA) in the lanthanide(III)-BTP systems has been studied at 25 degrees C in the pH range 7-9. The second-order rate constants for the hydrolysis of BNPP by individual lanthanide hydroxo complexes have been estimated by using the multiple regression on observed rate constants obtained at variable pH. For a given metal, the rate constants increase with increasing in the number n of coordinated hydroxide ions. In a series of complexes with a given n, the second-order rate constants decrease in the order La > Pr > Nd > Eu > Gd > Dy. Hydrolysis of NPP follows Michaelis-Menten-type "saturation" kinetics. This difference in kinetic behavior can be attributed to stronger binding of NPP dianion than BNPP monoanion to the lanthanide(III) species. Activities of lanthanide complexes in the hydrolysis of NPA, which is 10(6) times more reactive than BNPP in alkaline or aqueous hydrolysis, are similar to those in BNPP hydrolysis indicating unique capability of lanthanide(III) cations to stabilize the transition state of phosphate diester hydrolysis. Results of this study are analyzed together with literature data for other metal cations in terms of the Brønsted correlation and transition state-catalyst complexation strength.
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