Structural analysis of telomere G-quartet and molecular design of new nucleic acid aptamer

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The P-chiral stereodefined phosphorothioate groups have been introduced into all the four internucleotide positions of d(T_<PS1>C_<PS2>C_<PS3>C_<PS4>C) (PSn : phosphorothioate group), and among possible 16 diastereomers of PS-d(TC_4), 10 stereomers have been synthesized to investigate the effects of the sense of the P-chirality upon the structure and stability of the i-motif structure. The temperature dependence of circular dichroism spectra showed melting temperature (Tm) of the [all R_p]PS-d(TC_4) i-motif was 31 ℃ identical to that of the parent oligomer, PO-d(TC_4), while that of the [all S_p]PS-d(TC_4) i-motif was largely decreased by 11℃. Single substitution of R_p with S_p caused a decrease of T_m by 3〜4℃ at positions of PS1, 2, and 3, and by 1℃ at that of PS4, showing the additive property of the T_m suppression. The comparison of the NOESY spectra between [all R_p]PS-, [all S_p]PS-, and PO-d(TC_4) showed that intra-residual H6-H3' and H2"-H4' NOE cross-peaks of the [all S_p] isomer are weaker than those of the [all R_p] isomer and PO-d(TC_4), indicating the change in the C3'-endo conformation and glycosidic bond angle. The structural alternation for the i-motif formed by [all S_p]PS-d(TC_4) is also suggested by the chemical shift differences of C2/C3/C4 H2"and H4' protons from those of [all R_p]PS-d(TC_4) and PO-d(TC_4). These results suggest that the S_p configuration at phosphorus of the phosphorothioate linkage change the sugar-phosphorothioate conformation and intermolecular interaction in the narrow groove, leading to the destabilization of the i-motif structure.