Questioning the Affinity of Electrophilic Astatine for Sulfur-containing Compounds: Unexpected Bindings Revealed

The affinity of AtO⁺ for around 20 model ligands (L), carrying functionalized oxygen, sulfur, and nitrogen atoms, has been assessed through a combined experimental and theoretical methodology. Significant equilibrium constants (K_L ∼ 10⁴) have been measured for sulfur-containing compounds, in agreement with the previously highlighted, relatively stable radiolabeling of SH-containing proteins with ²¹¹At. Conversely, no interaction occurs in the aqueous phase for their oxygenated counterparts, but higher affinities (K_L > 10⁶) have been determined for nitrogen-based ligands, including aromatic nitrogen heterocycles. The quantum mechanical calculations definitively ruled out any rationale based on either the metallic character of astatine or its guessed softness; the favored interactions all involve specifically the oxygen atom of AtO⁺, leading to the formation of covalent O-S or O-C single bonds.