500 MHz 1H NMR studies using 2D-NOESY indicate that the oligonucleotide d(CTCTCT) at low pH forms a parallel double helix with cytosine·cytosine base pairs and thymine·thymine bulges. This unusual structure may explain the hypersensitivity of S1 nuclease at low pH towards supercoiled plasmids containing d(CT) n inserts.
