Application of circularly polarized luminescence spectroscopy to Tb(III) and Eu(III) complexes of transferrins

Circularly polarized luminescence (CPL) and total luminescence of Tb(III) and Eu(III) as substitutional replacements for iron in a series of Fe binding transferrins are reported. The measurement of the total emission intensity is a direct measure of binding of the lanthanide ions and illustrates that approximately 2 equiv of Tb(III) and Eu(III) is bound to these proteins. In agreement with previous work, circularly polarized luminescence from Tb(III) bound to the transferrins is quite large. Additional measurements show that the net CPL displays no dependence on concentration of the metal ion and shows little variation between the two binding sites. CPL from lactoferrin is very similar to serum transferrin, however, ovotransferrin shows differences in line shape and magnitude. These results are discussed in the context of the previously published crystal structure of the binding sites of the three proteins.