Still, we anticipated that some thiol-exchange of A to D is unavoidable, particularly with monothiols due to their high cellular concentrations (Fig. ), so it is important to assess the fate of D. We estimated that rates of cargo release directly from D (or B) by 6-exo-trig "on-reductant" cyclisation of its thiol should be insignificant, particularly compared to those for 5-exo-trig cyclisation of the more acidic and more nucleophilic selenolate in fully reduced C. Thus, we expected that A will significantly release cargo only if fully reduced to C. TrxR completes this reduction in a preorganised unimolecular step (B→C), whereas monothiols would require collision and reaction with a second equivalent of RSH before fast intramolecular reclosure returns A. Thus we also expected a very strong kinetic advantage for the rate of TrxR-induced rather than monothiol reduction to C, which could combine to give overall monothiol resistance.