NMR study of self‐paired parallel duplex of d(AAAAACCCCC) in solution

Abstract
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.