EXAFS spectroscopy of technetium and rhenium complexes relevant to nuclear medicine

Technetium and rhenium in the form of special complexes are important metals for the use in nuclear medicine. In order to design such complexes, detailed knowledge about the complexation behaviour of ligands and complex structures is required. EXAFS spectroscopy using synchrotron radiation provides the possibility to investigate complex structures in solid and soluted state. In the present study, EXAFS spectroscopy was applied to study the co-ordination spheres of technetium-99 and rhenium-187 complexes of different kinds of complexes. Rhenium complexes of model peptides were investigated and the extent to which the co-ordination mode depends on the peptide sequences was shown. EXAFS measurements at different pH values revealed changes in the metal co-ordination spheres. In case of the dipeptide Cys-Gly, a 1:2 complex with a Re(V) oxo core and a formal S2N2 co-ordination sphere is formed. By increasing the pH, a Re(V) dioxo core is generated. The tripeptide Gly-Gly-Cys was shown to exhibit a formal SN3 co-ordination with the Re(V) oxo metal centre. Using multiple scattering analysis, the involvement of the peptide backbone in the metal co-ordination was found. Under alkine conditions, the co-ordination of a hydroxyl group at the metal core takes place. EXAFS spectroscopy was further applied to study the technetium binding sites in large, biologically relevant peptides. The technetium complex of an endothelin derivative was shown to be a 1:2 Tc(V) oxo complex with a formal S4 co-ordination as realised by the peptide cysteine residues. A LHRH derivative bearing a cysteine residue at the side chain of a lysine residue was analysed to represent a 1:2 Tc(V) oxo complex with a formal S2N2 co-ordination sphere. Thereby, the metal co-ordination is ensured by the free cysteine residue of the peptides. The investigation of mixed ligand complexes of technetium and rhenium using the "3+1"-concept showed characteristic EXAFS features of this class of complexes. The co-ordination spheres as known from XRD analyses could be confirmed for solution. Reaction pathways leading to different products during the formation of "3+1" complexes were monitored and helped to understand reaction mechanisms. Furthermore, stability studies of the mixed-ligand complexes under biological conditions were performed and showed the possibility to record EXAFS data in situ. As a further complex category, mixed ligand Tc and Re carbonyl compounds were studied. The metal co-ordination spheres gave typical EXAFS patterns with the carbonyl co-ordination detectable by single and multiple scattering effects with excellent correlation between EXAFS and known XRD data found. By measurements in solution, significant changes in the metal environment of the metal carbonyl precursor molecules were observed. In conclusion, EXAFS spectroscopy is an excellent tool for structural analysis of technetium and rhenium complexes relevant to nuclear medicine.