[reaction: see text]. Biological methylation plays a vital role in regulatory mechanisms of gene transcription. Methylation of both promoter sequences within the genome, as well as protein substrates, has a profound impact upon gene transcription. Yet, few tools exist by which to identify sites of biological methylation in complex biological mixtures. We have generated a novel adenosine-derived N-mustard that serves as an efficient synthetic cofactor and allows for subsequent "click" chemistry involving the modified nucleic acid substrate.