Hardin C C, Watson T, Corregan M, Bailey C
Department of Biochemistry, North Carolina State University, Raleigh 27695.
Biochemistry. 1992 Jan 28;31(3):833-41. doi: 10.1021/bi00118a028.
The DNA oligonucleotide d(CGCG3GCG) can form either a Watson-Crick (WC) hairpin or a parallel-stranded quadruplex structure containing six G-quartet base pair assemblies. The exchange between these forms and single strands can be monitored using circular dichroism (CD). NMR results verified the assignment of specific CD bands to quadruplex and hairpin species, respectively. Cations stabilize the quadruplex in the order K+ greater than Ca2+ greater than Na+ greater than Mg2+ greater than Li+ and K+ greater than Rb+ greater than Cs+, indicating that K+ has an optimum ionic radius for complex formation and that ionic charge affects the extent of ion-induced stabilization. The quadruplex is stable in the presence of 40 mM K+ at micromolar DNA concentration and can be kinetically trapped as a metastable form when prepared at millimolar DNA concentration and then diluted into buffer containing 40 mM Na+. The concentration of K+ required to reverse the equilibrium from the hairpin to the quadruplex decreases sharply with increased DNA concentration. The quadruplex has an unusual pKa of ca. 6.8, indicating that C.C+ base pairs are probably forming. This system provides insights into some of the detailed structural characteristics of a ["G4-DNA".ion] complex and an experimental model for the recently proposed "sodium-potassium conformational switch" [Sen, D., & Gilbert, W. (1988) Nature 334, 364-366; Sen, D., & Gilbert, W. (1990) Nature 344, 410-414]. These results may help to explain the lack of cytidine residues in G-rich telomeric DNAs and suggest that methylation of GC-rich duplex DNAs in "GpC islands" may induce quadruplex formation within heterochromatin domains, resulting in reversible chromosomal condensation.
DNA寡核苷酸d(CGCG3GCG) 可以形成沃森-克里克（WC）发夹结构或包含六个G-四联体碱基对组装体的平行链四链体结构。这些形式与单链之间的交换可以使用圆二色性（CD）进行监测。核磁共振（NMR）结果分别验证了特定CD谱带对应于四链体和发夹物种的归属。阳离子按K+＞Ca2+＞Na+＞Mg2+＞Li+以及K+＞Rb+＞Cs+的顺序稳定四链体，这表明K+具有形成复合物的最佳离子半径，并且离子电荷会影响离子诱导的稳定程度。在微摩尔DNA浓度下，四链体在40 mM K+存在时是稳定的，当在毫摩尔DNA浓度下制备然后稀释到含有40 mM Na+的缓冲液中时，可以动力学捕获为亚稳态形式。使平衡从发夹结构向四链体逆转所需的K+浓度随着DNA浓度的增加而急剧下降。四链体具有约6.8的异常pKa，表明可能正在形成C.C+碱基对。该系统为["G4-DNA.离子]复合物的一些详细结构特征提供了见解，并为最近提出的“钠-钾构象开关” [森，D.，&吉尔伯特，W.（1988年）《自然》334，364 - 366；森，D.，&吉尔伯特，W.（1990年）《自然》344，410 - 414]提供了一个实验模型。这些结果可能有助于解释富含G的端粒DNA中缺乏胞嘧啶残基的现象，并表明“GpC岛”中富含GC的双链DNA的甲基化可能会在异染色质结构域内诱导四链体形成，从而导致可逆的染色体凝聚。
