Methods for chemoselective modification of a target molecule comprising an amine N-oxide in a biological sample are provided. Aspects of the methods include selectively reacting the amine N-oxide group of the target molecule with a boron agent, where the reacting reduces the amine N-oxide to an amine to produce a modified target molecule. Modification of the target molecule using the subject methods may produce an activated target molecule, e.g., a detectable or bioactive. In some cases, chemoselective modification leads to cleavage of the modified target molecule to produce a first target fragment and a second target fragment. Also provided are compositions useful in practicing various embodiments of the subject methods.
