The extent of exposed hydrophobic patch on apoovotransferrin (apoOTf) and various species of europium(III) ovotransferrin (Eu_N-OTf and Eu₂-OTf) were investigated using 2-p-toluidinylnaphthalene-6-sulfonate (TNS) as a fluorescence probe. The result showed that the extent of exposed hydrophobic patch on the protein was in turn: apoOTf＞Eu_N-OTf＞Eu₂-OTf. And apoovotransferrin (apoOTf) undergoes a large conformational change when Eu^3＋ binds to the N-terminal site, but a small conformational change when the ion binds to the C-terminal site. In addition, the guanidine hydrochloride denaturation experiments on three kinds of protein showed that the binding of Eu³⁺ to apoOTf led to a stable structure of OTf. Stability is in turn apoOTf＜Eu_N-OTf＜Eu₂-OTf. Ion strength effects demonstrate that internal hydrophobic interactions were enhanced in accordance with Eu₂-OTf＞Eu_N-OTf＞apoOTf, thus stabilizing the protein conformation accordingly. Such information may afford our understanding of the biological effect on rare earth.

Key words: apoovotransferrin, Eu^3＋, conformational change