2-Aminopurine

2-Aminopurine, a purine analog of guanine and adenine, is a fluorescent molecular marker used in nucleic acid research. It most commonly pairs with thymine as an adenine-analogue. It uses a different ketone oxygen on thymine for H-bonding and forms a stronger bond.
It can also pair with cytosine at low pH in a protonated way like a functional analogue of guanine. For this reason it is sometimes used in the laboratory for mutagenesis. A mixture of structures are present according to NMR. An atomic-resolution structures of the 2AP·C pair has been produced.