Charge-driven tripod somersault on DNA for ratiometric fluorescence imaging of small molecules in the nucleus

Although fluorescence tracing of small bioactive molecules in living cells has been extensively studied, it is still a challenging task to detect their variations in the nucleus mainly due to the impermeable nuclear membrane and nucleic acid interference. Herein, we take advantage of the nucleic acid enriched environment in the nucleus to establish a strategy, named "charge-driven tripod somersault on DNA", for ratiometric fluorescence imaging of small bioactive molecules in the nucleus. Taking SO₂ derivatives as a typical target analyte, a tripodal probe has been constructed by conjugating two DNA binding groups containing a SO₂ derivative reaction site. Mechanism studies demonstrate that upon encountering and reacting with SO₃ ^2-/HSO₃ ^-, a charge variation occurs at the responsive arm of the tripodal probe, triggering a tripod somersault on DNA, resulting in the conformational rearrangement of the DNA binding modes with DNA-modulated fluorescence change, which allows the second emission feature to emerge. In this strategy, probe-DNA binding is not influenced by RNA or non-specific protein association, thus making it ideal for tracing nucleus-localized analytes. The application of this strategy has realized both in vitro and in vivo ratiometric fluorescence imaging of the variations of endogenous SO₂ derivatives in the nucleus for the first time, with high specificity and selectivity. Also, in theory, this strategy opens up a new avenue for the design of fluorescence probes for the nucleus-localized biological analytes.