Solution structures of oligonucleotides containing either a guanine or a cytosine in front of a gap of one nucleotide

We report NMR and molecular modelling studies on two DNA duplexes containin g a gap of one nucleotides. The difference between the two oligonucleotides lies in the central base face to the gap, a guanine or a cytosine. For the gapG, we observed in solution a B-form conformation where the guanine stac ks in the helix. For the gapC, we reveal the existence of two species, one majority where the cytosine is inside the helix and a second for which the cytosine is extrahelical.