1H nuclear magnetic resonance study on equilibrium between two four-stranded solution conformations of short d(CnT)
1H nuclear magnetic resonance study on equilibrium between two four-stranded solution conformations of short d(CnT)
复制标题
DOI：
10.1021/bi980492g
复制
发表时间：
1998-09-15
期刊：
影响因子：
2.9
通讯作者：
Makino, K
中科院分区：
文献类型：
--
作者：
Kanaori, K;Maeda, A;Makino, K
d（C4 T）的NMR分析显示溶液中两种不同的四聚体（各自完全对称）之间的缓慢交换。对于一个四聚体，在C1和T5之间观察到i基序结构（H1 '-H1'和H6-H1 '/H1'-H6）的NOE交叉峰模式特征，表明该四聚体采取完全插入的构象，其中T5残基堆叠在C1上。另一个双链体的C1（+）对（S型）。另一个是四聚体，其中一个双链体移动了一个核苷酸单位（R型），导致3'端胸苷残基不堆叠，而堆叠的C数量相等。C++与S-形式配对。对于d（C3 T）观察到相同的光谱特征，但对于d（TC 3）和d（TC 4）都没有观察到，这表明胸苷残基的位置在四分体异构化中的关键作用。从NMR变性曲线，发现S-形式比R-形式更稳定，以及平衡常数的对数（K = [四聚体]/[单聚体]（4））和温度的倒数（1/T）对于两种形式都被证实，表明符合两态转变模型。由四条单链形成S型的焓和熵值均比R型的焓和熵值更负，焓项有助于S型在低温下的稳定。发现自由能值的差[Δ G度（S-形式）-Δ G度（R-形式）]为-2.1和-2.7 kT。mol（-1），说明S-型的稳定性较高。随着温度的升高，这两种拓扑结构在溶液中平衡存在，并通过解离成单链进行缓慢交换。这种拓扑异构化的生物作用也建议。
NMR analysis of d(C4T) showed the slow exchange between two distinct tetramers (each fully symmetric) in solution. For one tetramer, NOE cross-peak patterns characteristic of an i-motif structure (H1'-H1' and H6-H1'/H1'-H6) were observed between C1 and T5, indicating that this tetramer takes a completely intercalated conformation where the T5 residue is stacked on the C1 . C1(+) pair of the other duplex (S-form). The other was found to be a tetramer in which one of the duplexes is shifted by one nucleotide unit (R-form), resulting in nonstacking 3' end thymidine residues and an equal number of stacked C . C+ pairs to that of the S-form. The same spectral features were observed for d(C3T) but neither for d(TC3) nor d(TC4), indicative of the critical role of the position of the thymidine residue in the tetrad isomerization, From NMR denaturation profiles, the S-forms were found to be more stable than the R-forms, and the linear relationship between the logarithm of the equilibrium constant (K = [tetramer]/[single](4)) and the inverse of temperature (1/T) was confirmed for both forms, indicating conformity to the two-state transition model. Both enthalpy and entropy values of the formation of the S-form from four single strands were more negative than those of the R-form, The enthalpy term should contribute to the stabilization of the S-forms at low temperatures. The difference of the free energy values [Delta G degrees(S-form) - Delta G degrees(R-form)] was found to be -2.1 and -2.7 kT . mol(-1) at 20 degrees C for d(C4T) and d(C3T), respectively, explaining the higher stability of the S-forms, With increasing temperature, these two topologies were found to comparably exist at equilibrium in solution with slow exchange via dissociation to the single strands. A biological role of this topological isomerization is also suggested.