Interaction between the buffer Tris and the eu(III) ion: Luminescence and potentiometric investigation

The interaction between the buffer 2‐amino‐2‐(hydroxymethyl)propane‐1,3‐diol ( Tris ) and the Eu(III) ion has been studied by luminescence spectroscopy in D 2 O. Emission and excitation spectra ( 5 D 0 ← 7 F 0 transition) indicate an interaction with both [ Tris H] + and neutral Tris species. The former is weak and probably of the outer‐sphere type. The latter is of inner‐sphere type and corresponds to the formation of the [Eu( Tris )] 3+ species (estimated log K 1 = 2.3 ± 0.3). Buffer Tris is also demonstrated to prevent the formation of an Eu‐hydroxo species in the pD range of 7–8. Potentiometric measurements in H 2 O allowed a more precise calculation of the stability constant: log K 1 = 2.44 ± 0.07. Comparison with the data for aliphatic amines and other metal ions lead to the conclusion that the Eu/ Tris interaction is mainly achieved through the amino group. 1 H‐NMR spectra in presence of Tb(III) ions confirmed both this assumption and the presence of a weak interaction with Tris H + . Quantitative determinations of association constants between lanthanide ions and macromolecules of biological interest performed in presence of Tris should, therefore, be corrected for the Eu/ Tris interaction.