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 (KL ∼ 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 (KL > 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.