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A simple molecular probe displays highly selective turn-on response toward NO by the unprecedented NO-induced formation of a 1,2,3,4-oxatriazole ring exhibiting no interference from various endogenous biomolecules including DHA, AA, etc. Kinetics of the reactions between NO and the probe provide a mechanistic insight into the formation of 1,2,3,4-oxatriazole which showed that, though initially 1,2,3,4-oxatriazole is formed and extractable in solid form, it exists in equilibrium with the ring opened azide form which ultimately hydrolyzed and converted to carboxylic acid and nitrate. The reaction displays second-order dependence on [NO] and first-order on [Probe]. The probe is water-soluble, cell permeable, and noncytotoxic and appropriates for live cell imaging. This constitutes the first report where there is a direct evidence of NO-induced ring closing reaction of an acyl hydrazide moiety leading to the formation of 1,2,3,4-oxatriazole.

A smart molecule, QT490, containing thiosemicarbazide moiety acts as a highly selective turn-on in vitro NO sensor through the unprecedented NO-induced transformation of thiosemicarbazide moiety to 1,3,4-oxadiazole heterocycle with the concomitant release of HSNO, thereby eliminating any interference from various endogenous biomolecules including dehydroascorbic acid, ascorbic acid, etc. The kinetic studies of the reactions between QT490and NO provide a mechanistic insight into formation of HSNO/RSNO from the reaction between H2S/RSH and NO in the biological system. This novel probe is non-cytotoxic, cell permeable, water-soluble, and appropriate for intracellular cytoplasmic NO sensing with the possibilities of in vivo applications.