Having tested the major biological monothiol, monoselenol, and selenolthiol reductants, we finally examined the effect of vicinal dithiols on probe activation. We began by screening the probes' doseresponse to dithiothreitol (DTT), a nonphysiological vicinal dithiol that is ca. 50 mV more reducing than Trx1, but which reproduces some aspects of its behaviour and so can be helpful to study dithiol reactivity. Excess DTT triggered all A-and G-type selenenylsulfide probes. Although selenenylsulfides are more thermodynamically resistant to reduction than disulfides, their reaction with DTT was more rapid and more sensitive than that of the corresponding S1-S3 series (Fig. ). This is a reminder of the importance of kinetics to probe release, as the greater electrophilicity of Se in selenenylsulfides than S in disulfides determined the assay outcomes.