BINDING EFFECT OF TB3+ AND CA2+ IONS ON T RICHOSANTHIN

Tb3+ has been used as a fluorescent probe to study the structure of tr ichosanthin (TCS), The nonradioactive energy transfer from fluorescent active tryptophan(Trp) residue to the bound Tb3+ ions take place and the fluorescence of Trp is decreased when Tb3+ ions are bound to TCS t o replace the Ca2+ in it, According to the results of Trp fluorescence and tl ie information given from CD and UV absorption spectra, we hav e found that the affinity of Tb3+ to TCS is lower than that of Ca2+ io ns to TCS, Significant change of TCS structure was not observed with t he Tb3+ ions binding, The microenvironment surrounded Trp and Tyr may be changed slightly with the Tb3+ binding, The conformation of Ca2+-fr ee TCS was significantly changed compared with that oi native TCS with the Trp residue moving into the interior of TCS molecules. The Ca2+-b inding site in TCS can be maintained partly when the Ca2+ is removed f rom TCS molecules.