The binding of Eu^3^+ to human serum apotransferrin has been studied by monitoring the change of difference UV spectra at 245nm. Conditional equilibrium constants for the complexation of Eu^3^+ by human serum apotransferrin in 0.1mol.dm^-^3 hepes, pH7.4, at 25℃ have been measured. The results are logK~C=8.42±0.12, logK~N=6.03±0. 42 for complexation of Eu^3^+. The molar absorptivity per binding site for Eu^3^+ is (2.2±0.1)×10^4cm^-^1.mol^-^1.dm^. Titration of both C and N terminal monoferric transferrins with Eu^3^+ indicate that Eu^3^+ binding is stronger at the C therminal binding site than the N terminal binding site. Linear free energy relationships for Eu^3^+ and R~E^3^+ (R~E=Nd, Sm, Gd and Tb) have been established. There is a size restriction for the binding of lanthanide ions on C terminal binding site of apotransferrin.

Key words: EUROPIUM ION