Dattagupta N, Hogan M, Crothers D M
Proc Natl Acad Sci U S A. 1978 Sep;75(9):4286-90. doi: 10.1073/pnas.75.9.4286.
We report equilibrium, relaxation kinetic, and transient electric dichroism studies on the complex of the diamino steroid irehdiamine A with DNA. The results are consistent with a beta-kinked structure for the complex at saturation, with a kink in the DNA structure induced by a bound steroid every second base pair. The results that favor this hypothesis include an apparent length decrease of rod-like bacterial DNA molecules when only a small amount of drug is bound, followed by an apparent length increase at saturation. The limiting dichroism amplitude implies a substantial increase in the tilt of the bases relative to the orientation axis; at saturation the base UV transition moments are tilted about 31 degrees from the plane perpendicular to the orientation axis. Because of the direction of polarization of the 260-nm transition moments, the results indicate that the tilt of the bases must be predominantly in the short rather than the long axis of the base pair. The large hyperchroism of the complex is consistent with loss of base stacking, as required by a kinked structure. The kinetic results imply a bimolecular reaction mechanism, with a temperature-dependent association rate constant of roughly 10(8) M(-1) sec(-1), and a dissociation rate constant of about 5 x 10(3) sec(-1), nearly independent of temperature. The association activation energy and apparent reaction enthalpy vary from 12 to 22 kcal mol(-1); heat is absorbed on complex formation as expected for loss of base-stacking interactions. An anomalous result of the experiments is the larger apparent length increase (13%) exhibited by two eukaryotic DNAs, compared to 6% for three prokaryotic DNAs. Differences were also observed in the kinetic properties of the complexes.
我们报告了关于二氨基甾体irehdiamine A与DNA复合物的平衡、弛豫动力学及瞬态电二色性研究。结果表明，该复合物在饱和时具有β-扭结结构，甾体结合会使DNA结构每隔一个碱基对产生一个扭结。支持这一假说的结果包括：当仅结合少量药物时，杆状细菌DNA分子的表观长度减小，而在饱和时表观长度增加。极限二色性振幅表明碱基相对于取向轴的倾斜度大幅增加；在饱和时，碱基的紫外跃迁矩相对于垂直于取向轴的平面倾斜约31度。由于260纳米跃迁矩的极化方向，结果表明碱基的倾斜主要发生在碱基对的短轴而非长轴方向。复合物的大增色效应与扭结结构所需的碱基堆积丧失一致。动力学结果暗示了一种双分子反应机制，缔合速率常数与温度有关，约为10(8) M(-1) sec(-1)，解离速率常数约为5 x 10(3) sec(-1)，几乎与温度无关。缔合活化能和表观反应焓在12至22千卡/摩尔之间变化；正如碱基堆积相互作用丧失所预期的那样，复合物形成时吸收热量。实验的一个异常结果是，两种真核生物DNA的表观长度增加幅度（13%）比三种原核生物DNA的（6%）更大。在复合物的动力学性质方面也观察到了差异。
