<div>There have been significant advances in the biological use of hypervalent selenium and tellurium compounds as cysteine protease inhibitors over the recent past. However, the full understanding of their reaction mechanisms in aqueous medium and the mechanism of cysteine proteases inhibition is still elusive. Kinetic studies suggest an irreversible inhibition mechanism, which was explained by forming a covalent bond between the enzyme sulfhydryl group and the chalcogen atom at its hypervalent state (+4). However, it is still unclear the active form of the inhibitor present in the aqueous biological media. To uncover this question, we performed a theoretical investigation using density functional theory (DFT). This study investigated chloride ligand exchange reactions by oxygen and sulfur nucleophiles on hypervalent selenium and tellurium compounds. All tetra- and tri-coordinate chalcogen compounds and distinct protonation states of the nucleophiles were considered, totaling 34 unique species, 7</div><div>nucleophiles and 155 free energies rections. We discovered that chloride is easily replaced by a nonprotonated nucleophile (SH<sup>–</sup> or OH<sup>– </sup>) in R<sub>2</sub>SeCl<sub>2</sub> . We also found that</div><div>tri-coordinate species are more stable than their tetra-coordinate counterparts, with selenoxide (R<sub>2</sub>SeO) protonation being strongly exergonic in acid pH. These results suggest that the protonated selenoxide (R<sub>2</sub>SeOH<sup>+</sup>) is the most probable active chemical species in biological media. The computed energetic profiles paint a possible picture for the selenurane activity, with successive exergonic steps leading to a covalent inhibition of thiol dependent enzymes, like cysteine proteases. A second less exergonic pathway has also been uncovered, with a direct reaction to chalcogenonium cation (R<sub>2</sub>SeCl<sup>+</sup>) as the inhibition step. The trends observed for the telluranes were similar, albeit with</div><div>more exergonic reactions and a stronger trend to form bonds with oxygen species then selenuranes.</div><div><br></div>
Equilibrium Between Tri- and Tetra-Coordinate Chalcogenuranes Is Critical for Cysteine Protease Inhibition
