Photolabile nucleotides that disrupt nucleic acid structure are useful mechanistic probes and can be used as tools for regulating biochemical processes. Previous probes can be limited by the need to incorporate multiple modified nucleotides into oligonucleotides and in kinetic studies by the ratelimiting step in the conversion to the native nucleotide. Photolysis of aryl sulfide 1 produces high yields of 5-methyluridine and product formation is complete in less than a microsecond. Aryl sulfide

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... prevents RNA hairpin formation and complete folding of the preQ1 class I riboswitch. Proper folding is achieved in each instance upon photolysis at 350 nm. Aryl sulfide 1 is a novel tool for modulating RNA structure and formation of 5-methyluridine within a radical cage suggests that it will be useful in kinetic studies.