In 0.01 mol/L N-(2-hydroxyethyl)-piperazine-N'-2-ethanesulfonic acid (Hepes), pH 7.4, the binding of N,N'-bis(2-hydroxybenzyl) ethylenedinitrilo-N,N'-diacetic acid (HBED) with Tb(III) was monitored by fluorescence spectra. As the exchange interaction between HBED and Tb(III), the Tb(III) emission intensity at 545 nm is greatly enhanced. The conditional equilibrium constant for Tb-HBED is lgK = 14.30 ± 0. 49. HBED emission intensity at 311 nm or Tb(III) at 545 nm decreases as the temperature rises. For the complex of Tb(III) ions with apotransferrin, Tb_N-apoTf-Tb_C, the protein emission intensity at 338 nm decreases as the temperature rises, while the Tb(III) emission intensity at 549 nm increases gradually with increased temperature. The experimental phenomenon may be resulted from energy transfer between Tb(III), the acceptor, and Trp residue in helix 5, the donor. According to Forster type dipole-dipole radiationless energy transfer, the average efficiency of energy transfer from Trp to Tb(III) increases 29% over 55 ℃ temperature range, this represents a 4.2% decreases in donor-acceptor distance (R). The denaturation of the protein begins at 60 ℃after Tb(III) ions bind to apotransferrin. For the complex of Tb(III) ions with apoovotransferrin, Tb_N-apoOTf-Tb_C, there is a decrease in protein emission intensity at 336 nm as temperature rises, but there is no significent increase in Tb(III) emission intensity at 549 nm. Tb(III) binding to apoovotransferrin has no effect on the denaturation point of the protein. It means that there is different conformational behavior for Tb_N-apoTf-Tb_C and Tb_N-apoOTf-Tb_C.

Key words: TERBIUM, CONFORMATION, FLUOROSPECTROPHOTOMETRY, ETHANEDIAMINE, ACETIC ACID P, PROTEIN