Tb(III) and Eu(III) as fluorescent probes to investigate the metal-binding sites of trichosanthin.

Tb(III) and Eu(III) ions were used as fluorescent probes in the study of the trichosanthin (TCS). The fluorescence of Tb(III) was increased considerably when bound to TCS to replace the Ca(II) ions. The nonradiative energy transfer from fluorescent tryptophan (Trp) residues to the bound Tb(III) or Eu(III) took place. From a Foster d-d nonradiative energy transfer mechanism, it was obtained that the average distance between the bound Tb(III) and the Trp residue is 1.27nm. The results indicate that the major groups in TCS bound to Ca(II) ions should be the carboxylic side groups of the glutamic acid and/or aspartic acid. The fluorescent quenching of Ca(II)-free TCS by adding Tb(III) or Eu(III) into TCS from which the Ca(II) ions had been removed was discussed as well.