Fluorescence turn-on probe for biothiols: intramolecular hydrogen bonding effect on the Michael reaction

Weakly fluorescent coumarinyl enones are rapidly transformed into strongly fluorescent molecules through the Michael addition reaction of a thiol group, where an intramolecular hydrogen bond plays a critical role in the reaction rate. The molecular probe (3) with an ortho hydroxyl group to a carbonyl group exhibits a rapid response toward GSH owing to the stabilization of the possible oxyanion intermediate by a preferable intramolecular hydrogen bond. Probe 1 with an o-hydroxyl group also showed a moderately enhanced reaction rate with GSH and soluble in HEPES buffer to exhibit a highly selective and sensitive fluorescence turn-on response toward biothiols.