[5mCCTCTCTCC]4: an i-motif tetramer with intercalated T*T pairs.

The i-motif is a four-stranded structure built by intercalation in a head-to-tail orientation of two parallel duplexes associated by hemiprotonated C(*)C(+) pairs. T*T pairs are nearly isomorphic of C*C(+) pairs; however the structural investigations of i-motif tetramers containing thymidines suggest that the i-motif cannot accommodate T*T pairs in a face-to-face orientation. The tetramer of 5mCCTCTCTCC make an exception. It includes two symmetry related open/closed T3/T7 groups, but the central thymidines form two long-lived T5*T5 pairs that are intercalated in a face-to-face orientation. This observation provides indications of the origin of the conflict that usually hinders T*T intercalation into i-motif structures and more generally of the constraints influencing i-motif formation.