Labeling of proteins by a triosmium carbonyl cluster via a Bolton-Hunter-like procedure

The labeling of proteins employing succinimido 4-pentynoate or pyrylium derivatives of mono- and dinuclear organometallic complexes is now a well-established procedure. We have covalently linked a 5d trimetallic cluster fragment, namely "Os₃(CO)₁₀", to a model amine (i.e. benzylamine), to model amino acids (i.e. β-alanine and Nα-protected lysines), and finally to bovine serum albumin (BSA). The complete reaction sequence, starting from Os₃(CO)₁₀(μ₃-η²-succinimido 4-pentynoate) as the Bolton-Hunter-like reagent, was verified at the molecular level by spectroscopic techniques. The average coupling ratio (CR) between the Os₃ marker and BSA is fairly high, i.e. around 20. This result highlights the potential use of the Os₃-labeling procedure for improving the X-ray structural determination and transmission electron microscopy (TEM) definition of proteins.