The oligonucleotide d(A5C5) in solution forms a parallel self-duplex at neutral and low pH values. H2O NMR spectra at pH 5.1 indicate the presence of five imino resonances at lower temperatures; and the structure is stable up to 60°C. These signals ran arise only from the hemiprotonated C+·C pairs [Westhof, E. and Sundaralingham, M. (1980) Biochemistry 77, 1852–1856; Westhof, E. and Sundaralingham, M. (1980) J. Mol. Biol. 142, 331–361] and constitute the first direct observation of C+·C hemiprotonated pairs in solution. The cross peaks from Hl's and more than five distinct AH8's in 500 MHz 1H 2D-NOESY spectra indicate that there are two conformationally different and energetically similar A-tracts. There is good qualitative agreement between NOESY data and two theoretically derived structures in which A-tracts are reverse Watson-Crick and reverse Hoogsteen base-paired, respectively.
