A critical quantum chemical and experimental study of the potentiality of direct labeling of the CN group with [(99m)Tc(CO)(3)](+) or [(186/188)Re(CO)(3)](+) in CN containing biomolecules.

Introduction: It was determined recently that [(99m)Tc(OH(2))(2)(X(-))(CO(3))(3)] could strongly bind to the CN group, allowing direct labeling of CN in vitamin B(12) despite the presence of a benzimidazole group. The aim of this paper was to perform a critical study of this potentiality, coupling quantum chemical calculations to experimental evidence.

Methods: Computational methods: Within the density functional theory calculations, the 6-31+G basis set (C, H, O, N atoms) and the LANL2DZ basis set (Tc,Re) were used. Stability calculations of the [RCNM(CO)(3)](+)) (M=Tc,Re) complexes were performed with the Gaussian 03 suite of programs, while for the evaluation of relative stability substitution reactions were used. Radiochemistry: Vitamin B(12), 4-hydroxy-benzylcyanide and 4-methoxy-benzonitrile were labeled at 100 degrees C during 30 min. High-performance liquid chromatography analysis was performed using radioactive and UV detection.

Results: Computational methods: The influence of different ligands on the stability yielded a sequence: imidazole>tBuCN>NH(3) approximately CH(3)CN>HCN (mimicking the best CoCN)>H(2)O. The transmetalation reaction indicates that all ligands prefer Re to Tc. The preference for the nitrogen atom of imidazole to the cyanide nitrogen atom for complex formation with [Tc(CO)(3)(H(2)O)(3)](+) is interpreted in terms of the hard and soft acid and base properties principle. Radiochemistry: 4-Hydroxy-benzylcyanide and 4-methoxy-benzonitrile did not show any labeling. An excess of acetonitrile did not inhibit the labeling of vitamin B(12) as expected if the CN group should be involved, indicating that the labeling occurs on a stronger complexing group present like benzimidazole.

Conclusion: Both theory and experiments prove that [CN-Tc(CO)(3)(H(2)O)((2-x))L(x)](+) complexes are weak and that in vitamin B(12) most probably the benzimidazole group is involved.