Binding of luminescent metal probes to proteins: the case of terbium

Abstract

Some lanthanides show room temperature phosphorescence related to f–f transitions. The excitation to the emitting state can be accomplished by either direct or indirect absorption of light by the rare earth ions. However, the low oscillatory strength of f–f transitions makes the direct excitation not very useful to probe dilute protein-metal complexes. Indirect (sensitized) emission occurs than a ligand or a nearest chromophore is able to transfer non radiatively the energy absorbed on the lanthanide. This process is particularly effective with protein Tb 3+ complexes. The green Tb 3+ phosphorescence can be increased more than 10 4 times upon binding of the metal to binding sites of proteins. Terbium may exchange with ligands 69 bonds, which preferentially involve charged or uncharged oxygen atoms. Up to date Tb 3+ has been shown to substitute more or less conservatively in proteins calcium, iron or manganese. The former substitution is particularly relevant since calcium is spectroscopically silent. The substitution of iron and manganese by Tb 3+ may give very useful information on the environment of the metal site. The Tb 3+ substitution for Ca 2+ in hemocyanin, a copper protein where Ca 2+ plays an important regulatory role, and in concanavalin, a plant agglutinin containing both Mn 2+, and for Fe 2+ or Fe 3+ in ferritin, a metal storage protein, will be discussed in detail.