Bioorthogonal Cyclopropenones for Investigating RNA Structure.

RNA sequences encode structures that impact protein production and other cellular processes. Misfolded RNAs can also potentiate disease, but a complete picture is lacking. To establish more comprehensive and accurate RNA structure-function relationships, new methods are needed to interrogate RNA in native environments. Existing tools rely primarily on electrophiles that are constitutively "on" or triggered by UV light, often resulting in high background. Here we describe an alternative, chemically triggered approach to cross-link RNAs using bioorthogonal cyclopropenones (CpOs). These reagents selectively react with phosphines to provide ketenes─electrophiles that can trap neighboring nucleophiles to forge covalent cross-links. As a proof-of-concept, we conjugated a CpO motif to thiazole orange (TO-1). TO-1-CpO bound selectively to a model RNA aptamer (Mango) with nanomolar affinity, as confirmed by fluorescence turn-on. After phosphine administration, covalent cross-links were formed between the CpO and RNA. Cross-linking was both time and dose dependent. We further applied the chemically triggered tools to model RNAs under biologically relevant conditions. Collectively, this work expands the toolkit of probes for studying RNA and its native conformations.