The interactions between tannic acid (TA) and apoovotransferrin (apoOTF) were studied by fluorescence spectroscopy with the spectroscopic probes of Tb^3＋ and intrinsic fluorescence of the protein in 0.01 mol•L^－1 Hepes, at pH 7.4 and room temperature. The results showed that the complex with the molar ratio 1∶1 was formed between TA and apoOTF, Tb_N^3＋-apoOTF, Tb_N^3＋-apoOTF-Tb_C^3＋, respectively. The binding constants of TA to apoOTF, Tb_N^3＋-apoOTF and Tb_N^3＋-apoOTF-Tb_C^3＋ was 7.15×10⁵, 4.16×10⁶ and 3.77×10⁶ mol^－1•L, respectively. What’s more, the tannic acid could bind to terbium ions, whose binding molar ratio was about 1∶2, and the binding ability of Tb^3＋ to apoOTF was weaker than that of Tb^3＋ to the tannic acid. Moreover, binding constant for attachment of TA-Tb₂^3＋ to apoOTF was 1.86×10⁵ mol^－1•L, and the number of binding sites of the protein for TA-Tb₂^3＋ was about 1.

Key words: apoovotransferrin, tannic acid, terbium ion probe, fluorescence spectroscopy