TECHNETIUM METALLOTHIONEIN - SPECTROSCOPIC AND EXAFS STUDY OF (TCO3-TC-99 BINDING TO ZN-7-METALLOTHIONEIN())

Vibrational, absorption, circular dichroism, and Tc K-edge extended X- ray absorption fine structure (EXAFS) measurements have been made on t hree samples of metallothionein (MT) containing 1.6, 3.9, and 6.2 mol equiv of Tc-99, prepared by the metal displacement (transchelation) re action of TcO(glucoheptonate)(2)(-) with Zn-7-MT. For all three sample s the TcO3+ ion is bound to the protein in a square pyramidal coordina tion with equatorial ligands consisting of similar to 2 cysteine thiol ates and, surprisingly, similar to 2 O/N donor ligands. As the Tc cont ent increases, there is very little change in the Tc coordination, exc ept for a decrease in the number of cysteine ligands per TcO3+ for Tc- 6.2-MT. The unusual ligand set for this metal ion when it is bound to MT, the absence of bridging cysteine ligands, and the significant numb er of cysteines not involved in binding TcO3+, indicate that transchel ation does not involve a simple substitution of TcO3+ for Zn(II).