Solution structures of rhenium (V) oxo peptide complexes of glycylglycylcysteine and cysteinylglycine as studied by capillary electrophoresis and X-ray absorption spectroscopy

Rhenium (V) oxo complexes of the peptides glycylglycylcysteine (GlyGlyCys) and cysteinylglycine (CysGly) have been synthesized by ligand exchange reactions starting from rhenium (V) oxo gluconate. For the first time, the complexes were characterized in solution by extended X-Ray absorption fine structure spectroscopy (EXAFS), pH dependent capillary electrophoresis (CE) with simultaneous UV absorption spectroscopy and mass spectroscopy, performed by the coupling of CE with an electrospray mass selective detector (CE-ES-MSD), EXAFS data analysis provided information on the complex structures by single and multiple scattering analysis, The investigations confirm a rhenium oxidation state of +5 with typical rhenium (V) oxo patterns. The peptides differ in their mode to coordinate the rhenium (V) oxo metal core. GlyClyCys forms a 1:1 rhenium (V) oxo complex with a formal SN3 coordination. whereby the peptide backbone is involved in the coordination and showed multiple scattering signals for the peptide carbonyl oxygen atoms. The complex is neutral at low pH and anionic at neutral pH. CysGly forms a 1:2 complex with a formal S2N2 coordination, wherein the terminal amine nitrogen atom is involved in the rhenium coordination. A cationic complex species is detected at low pH, and increasing pH values lead to the formation of an anionic species. The complex compositions were confirmed by mass spectroscopic measurements. The results are considered to be helpful in a better understanding of the Tc and Re coordination abilities with peptides. This knowledge is of great relevance for modern nuclear medicine in its commitment to utilize radio-labeled target-specific peptide radiopharmaceuticals. (C) 1998 Elsevier Science Inc. All rights reserved.