Technetium coordination ability of cysteine-containing peptides: X-ray absorption spectroscopy of a 99Tc labelled endothelin derivative

The coordination mode of a 99Tc-labelled endothelin derivative, Asp-Gly-Gly-Cys-Gly-Cys-Phe-(D-Trp)-Leu-Asp-Ile-Ile-Trp, in solution was determined by X-ray absorption spectroscopy (XAS). XAS analysis revealed that coordination of the [TcO]3+ core is restricted to the sequence -Cys-Gly-Cys-. The preferred coordination by the cysteinyl thiol group prevents involvement of any donor atom other than sulphur, thus forming purely S-coordinate 1:2 complexes. Two occurring 99Tc complex species with identical coordination spheres were identified and seem likely to represent parallel and antiparallel peptide chain orientation isomers.