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

在d(T_&lt；PS1&gt；C_&lt；PS2&gt；C_&lt；PS3&gt；C_&lt；PS4&gt；C)的4个核苷酸间基团(PSn：硫代硫酸基)上引入了P-手性立体定义的硫代硫酸酯基团，并在PS-d(TC_4)的16个可能的非对映异构体中合成了10个立体异构体，考察了P-手性对I基序结构和稳定性的影响。圆二色谱的温度依赖关系表明，[ALL R_p]PS-d(TC_4)I基序的熔融温度(Tm)与母体齐聚物PO-d(TC_4)的熔融温度(Tm)相同，而[ALL S_p]PS-d(TC_4)I基序的熔融温度(Tm)显著降低了11℃。用S_p单独替代R_p，在P_s_1、P_2、P_3处T_m降低3~4℃，P_s_4处T_m降低1℃，表现出抑制T_m的相加性。[ALL R_p]PS-、[ALL S_p]PS-和PO-d(TC_4)的NOESY谱比较表明，[ALL S_p]异构体的H6-H3‘和H2“-H4’NOE交叉峰比[ALL R_p]异构体和PO-d(TC_4)弱，表明C3‘-内切构象和糖苷键角度发生了变化。C2/C3/C4H2“和H4‘质子与[ALL R_p]PS-d(TC_4)和PO-d(TC_4)的化学位移差异也表明了[ALL S_p]PS-d(TC_4)形成的I基序的结构变化。这些结果表明，硫代磷键上的S p构型改变了糖-硫代硫酸盐的构象和分子间的相互作用，导致了I基序结构的失稳。

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.