The use of diaminopurine to investigate structural properties of nucleic acids and molecular recognition between ligands and DNA.

2,6-Diaminopurine (DAP) is an analogue of adenine which can be converted to nucleotides that serve as substrates for incorporation into nucleic acids by polymerases in place of (d)AMP. It pairs with thymidine (or uracil), engaging in three hydrogen bonds of the Watson-Crick type. The result of DAP incorporation is to add considerable stability to the double helix and to impart other structural features, such as an altered groove width and disruption of the normal spine of hydration. DNA containing DAP may or may not be recognized by restriction endonucleases; RNA containing DAP may not engage in normal splicing. The DAP.T pair affects the local flexibility of DNA and impedes the interaction with helix bending proteins. By providing a non-canonical hydrogen bond donor in the minor groove and/or blocking access to the floor of that groove it strongly affects interactions with small molecules such as antibiotics and anticancer drugs. Examples which illustrate altered recognition of nucleotide sequences in DAP-containing DNA are presented: changed sites of cutting by bleomycin, photocleavage by uranyl nitrate and footprinting with mithramycin. Using DNA in which both A-->DAP and G-->Inosine substitutions have been made it is possible to assess precisely the role of the purine 2-amino group in ligand-DNA recognition.