title: Solution structures of the i-motif tetramers of d(TCC), d(5methylCCT) and d(T5methylCC): novel NOE connections between amino protons and sugar protons.
abstract: BACKGROUND
  At slightly acid or even neutral pH, oligodeoxynucleotides that include a stretch of cytidines form a tetramer structure in which two parallel-stranded duplexes have their hemi-protonated C.C+ base pairs face-to-face and fully intercalated, in a so-called i-motif, first observed serendipitously in [d(TC5)]4.
  
  
  RESULTS
  A high-definition structure of [d(TCC)]4 was computed on the basis of inter-residue distances corresponding to 21 NOESY cross-peaks measured at short mixing times. A similarly defined structure of [d(5mCCT)]4 was also obtained. A small number of very characteristic (amino proton)-(sugar proton) cross-peaks entails the intercalation topology. The structure is generally similar to that of [d(TC5)]4. The sequence d(T5mCC) forms two tetramers in comparable proportions. The intercalation topologies are read off the two patterns of (amino proton)-(sugar proton) cross-peaks: one is the same as in the d(TCC) tetramer, the other has the intercalated strands shifted by one base, which avoids the steric hindrance between the methyl groups of the 5mC pairs of the two duplexes.
  
  
  CONCLUSIONS
  The structures obtained in this work and the procedures introduced to characterize them and to solve the problems linked to the symmetry of the structure provide tools for further exploring the conditions required for formation of the i-motif.
authors.affiliation: Groupe de Biophysique de l'Ecole Polytechnique, Palaiseau, France.
authors.forename: J L
authors.initials: JL
authors.lastname: Leroy
authors.forename: M
authors.initials: M
authors.lastname: Guéron
mesh_terms: D001483:Base Sequence; D006860:Hydrogen Bonding; D009682:Magnetic Resonance Spectroscopy / Q000379:methods; D008969:Molecular Sequence Data; D009690:Nucleic Acid Conformation; D009838:Oligodeoxyribonucleotides / Q000737:chemistry*; D013816:Thermodynamics
chemical_list: D009838:Oligodeoxyribonucleotides