LAUSR

In this study, we investigated the photoreaction of BrU in a pyrene‐labeled DNA duplex, RNA duplex, and DNA/RNA hybrids. We found that the photoreactivity of BrU changed dramatically from hydrogen abstraction to cross‐linking by changing the conformation of the duplex from the B‐form to the A‐form. Among three A‐form structures, the largest amount of cross‐linked products was observed when BrU was incorporated into the RNA strand and the pyrene was conjugated to the 5′ end of the DNA. These results indicate that the contact manner of pyrene was different between A‐ and B‐form duplexes. This is a rare example of the use of the reactivity of bromouracil to analyze the contact between a small molecule with a weak binding affinity and a nucleic acid.