Chaires J B
J Biol Chem. 1986 Jul 5;261(19):8899-907.
Absorbance and fluorescence methods were used to measure the binding of the anticancer drug daunomycin to poly (dGdC) under ionic conditions that initially favor the left-handed Z conformation of the polymer. Drug binding was cooperative under these conditions and may be fully accounted for by an allosteric model in which the drug binds preferentially (but not exclusively) to the right-handed B conformation and shifts the polymer from the Z to an intercalated right-handed conformation. Quantitative analysis of binding isotherms in terms of the allosteric model allowed for estimation of the equilibrium constants for the conversion of a base pair at a B-Z interface from the Z to the B conformation and for the formation of a base pair in the B conformation within a stretch of helix in the Z conformation. The free energy of the Z to B conversion of a base pair was calculated from this data and ranges from +0.03 to +0.3 kcal/mol over the NaCl range of 2.4-3.5 M. The free energy for the formation of a B-Z junction was nearly constant at +4.0 kcal/mol over the same range of NaCl concentrations. The salt dependence of the free energy of the Z to B transition indicates preferential Na+ binding to the Z form and that there is a net release of Na+ upon conversion of a base pair from the Z to the B conformation. The energetically unfavorable Z to B transition was found by this analysis to be driven by coupling to the energetically favorable interaction of daunomycin with B form DNA. In 3.5 M NaCl, for example, the free energy change for the overall reaction (Z DNA base pairs) + (daunomycin) in equilibrium with (right-handed complex) is -7.0 kcal/mol, nearly all of which is contributed by the binding of drug to B DNA. Analysis using the allosteric model also shows that the number of base pairs converted from the Z to the B conformation per bound drug molecule is salt dependent and provides evidence that drug molecules partition into regions of the polymer in the right-handed conformation.
在最初有利于聚合物左手Z构象的离子条件下，采用吸光度和荧光法测量抗癌药物柔红霉素与聚（dGdC）的结合。在这些条件下，药物结合具有协同性，并且可以通过变构模型完全解释，在该模型中，药物优先（但不唯一）结合到右手B构象，并使聚合物从Z构象转变为插入的右手构象。根据变构模型对结合等温线进行定量分析，可以估算B-Z界面处碱基对从Z构象转变为B构象以及在Z构象的螺旋段内形成B构象碱基对的平衡常数。根据该数据计算出碱基对从Z转变为B的自由能，在2.4-3.5 M的NaCl浓度范围内，其范围为+0.03至+0.3 kcal/mol。在相同的NaCl浓度范围内，B-Z连接形成的自由能几乎恒定为+4.0 kcal/mol。Z到B转变的自由能对盐的依赖性表明Na+优先结合到Z形式，并且当碱基对从Z构象转变为B构象时会有Na+的净释放。通过该分析发现，能量上不利的Z到B转变是由柔红霉素与B型DNA的能量上有利的相互作用耦合驱动的。例如，在3.5 M NaCl中，（Z DNA碱基对）+（柔红霉素）与（右手复合物）平衡的总反应的自由能变化为-7.0 kcal/mol，几乎全部由药物与B DNA的结合贡献。使用变构模型的分析还表明，每个结合的药物分子从Z构象转变为B构象的碱基对数量取决于盐，并提供了药物分子分配到右手构象的聚合物区域的证据。
