[5mCCTCTCTCC]_4: An i-Motif Tetramer with Intercalated T·T Pairs

摘要

At slightly acid pH, C rich sequences form tetrameric structures in which two parallel duplexes held together by hemiprotonated C·C~+ pairs are intercalated in each other. This structure, called an i-motif, is exceptional in that it involves systematic base-pair intercalation. Thymidines can form symmetrical T·T base pairs that are nearly isomorphic of C·C~+ pairs; however the structural investigations of the [5mCCTCC]_4~2 and [5mCCTC_nTCC]_4 (n = 1, 2, 3) tetramers and of the [5mCCTCACTCC]_2 dimer show that incorporation of two T·T base pairs in a face-to-face orientation in the i-motif is hindered. All these structures exhibit a common structural feature including the T·T pair of one duplex, the unstacked thymidines of the other, and the adjacent C·C~+ pairs of each thymidine. This assembly has been designated as an "open/paired T/T motif"(Figure 1).