NMR AND LUMINESCENCE STUDIES ON THE FORMATION OF TERNARY ADDUCTS BETWEEN HSA AND LN(III)-MALONATE COMPLEXES (LN = EU, GD, TB)

At physiological pH and in the presence of an excess of malonate ligan d (MAL), the lanthanide ions (Ln = Eu(III) Gd(III) and Tb(III)) are un der the form of [Ln(MAL)(2)(H2O)(4)](-). Upon addition of human serum albumin (HSA), formation of two different ternary adducts of stoichiom etry HSA-Ln(MAL)(x)(H2O)(q) (x = 2, q = 2; x = 2, q = 4) is detected. On the basis of the reasonable assumption that the binding strength fo r the two sites on the protein are inversely proportional to the hydra tion state of the metal ion, stability constants of 4.0 . 10(3) M-1 an d 3.5 . 10(2) M-1 have been evaluated for the system with q = 2 and q = 4, respectively. Whereas for the stronger binding site it is suggest ed that the protein provides two or three donor atoms to the coordinat ion cage of the Ln(m) ion, in the case of the weaker binding site it i s likely that it corresponds to a simple electrostatic interaction bet ween the negatively charged [Ln(MAL)(2)(H2O)(4)](-) and positively cha rged groups on the surface of the protein. (C) 1998 Elsevier Science B .V. All rights reserved.