Multiple structures for the 2-aminopurine-cytosine mispair

The base pair formed between 2-aminopurine (2AP) and cytosine (C) is an int ermediate in I transition mutations generated by 2AP. To date, several stru ctures have been proposed for the 2AP-C mispair, including those involving a rare tautomer, a protonated base pair, and a neutral wobble structure. In this paper, we describe a series of UV, fluorescence, and NMR studies whic h demonstrate that an equilibrium exists between the neutral wobble and the protonated Watson Crick structures. The apparent pK value for the transiti on between the structures is 5.9-6.0. Formation of a Watson-Crick base pair is accomplished predominantly by protonation of the 2AP residue as indicat ed by UV spectral changes, fluorescence quenching, and changes in proton ch emical shifts. Rapid transfer of the shared proton between the 2AP and cyto sine residues is indicated by the rapid exchange of the cytosine amino prot ons of the protonated Watson-Crick configuration. The relative contribution of the neutral wobble and protonated Watson-Crick configurations to 2AP-in duced transition mutations is discussed.