James Graham Brown Cancer Center, University of Louisville, Louisville, KY 40202, United States.
Biophys Chem. 2011 Nov;159(1):205-9. doi: 10.1016/j.bpc.2011.06.012. Epub 2011 Jun 28.
Formation of DNA quadruplexes requires monovalent cation binding. To characterize the cation binding stoichiometry and linkage between binding and folding, we carried out KCl titrations of Tel22 (d[A(GGGTTA)(3)]), a model of the human telomere sequence, using a fluorescent indicator to determine K(+) and circular dichroism to assess the extent of folding. At K(+)=5 mM (sufficient for >95% folding), the apparent binding stoichiometry is 3K(+)/Tel22; at K(+)=20 mM, it increased to 8-10K(+)/Tel22. Thermodynamic analysis shows that at K(+)=5 mM, K(+) binding contributes approximately -4.9 kcal/mol for folding Tel22. The overall folding free energy is -2.4 kcal/mol, indicating that there are energetically unfavorable contributions to folding. Thus, quadruplex folding is driven almost entirely by the energy of cation binding with little or no contribution from other weak molecular interactions.
形成 DNA 四链体需要单价阳离子结合。为了描述阳离子结合的化学计量关系以及结合与折叠之间的联系，我们使用荧光指示剂来确定 [K(+)]（游离），并使用圆二色性评估折叠程度，对 Tel22（d[A(GGGTTA)(3)]）（人端粒序列的模型）进行 KCl 滴定。在 [K(+)]（游离）=5 mM（足以实现>95%的折叠）时，表观结合化学计量比为 3K(+)/Tel22；在 [K(+)]（游离）=20 mM 时，增加到 8-10K(+)/Tel22。热力学分析表明，在 [K(+)]（游离）=5 mM 时，K(+)结合对折叠 Tel22 的贡献约为-4.9 kcal/mol。总折叠自由能为-2.4 kcal/mol，表明折叠存在不利的能量贡献。因此，四链体折叠几乎完全由阳离子结合的能量驱动，而其他弱分子相互作用的贡献很小或没有。
