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

Abstract 

Introduction

It was determined recently that [^99mTc(OH₂)₂(X^-)(CO₃)₃] could strongly bind to the CN group, allowing direct labeling of CN in vitamin B₁₂ 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 B₁₂, 4-hydroxy-benzylcyanide and 4-methoxy-benzonitrile were labeled at 100°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>NH₃∼CH₃CN>HCN (mimicking the best CoCN)>H₂O. 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(H₂O)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 B₁₂ 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(H₂O)_(2−x)L_x]⁺ complexes are weak and that in vitamin B₁₂ most probably the benzimidazole group is involved.

Keywords: [^99mTc(CO)₃]⁺, Direct labeling R-CN, Quantum chemistry, Biomolecules