Scott E V, Jones R L, Banville D L, Zon G, Marzilli L G, Wilson W D
Department of Chemistry, Georgia State University, Atlanta 30303.
Biochemistry. 1988 Feb 9;27(3):915-23. doi: 10.1021/bi00403a012.
Imino proton and 31P NMR studies were conducted on the binding of actinomycin D (ActD) to self-complementary oligodeoxyribonucleotides with adjacent 5'-GC-3' sites. ActD showed very high specificity for binding to GC sites regardless of oligomer length and surrounding sequence. For a first class of duplexes with a central GCGC sequence, a mixture of 1:1 complexes was observed due to the two different orientations of the ActD phenoxazone ring system. Analysis of 1H chemical shifts suggested that the favored 1:1 complex had the benzenoid side of the phenoxazone ring over the G base in the central base pair of the GCGC sequence. This is the first case in which an unsymmetrical intercalator has been shown to bind to DNA in both possible orientations. A unique 2:1 complex, with significantly different 1H and 31P chemical shifts relative to those of the 1:1 complexes, was formed with these same oligomers, again with the benzenoid side of the ActD molecule over the G base of the central GC base pair. There is considerable anticooperativity to binding of the second ActD in a GCGC sequence. In titrations of oligomers with the GCGC sequence, only the two 1:1 complexes are found up to ratios of one ActD per oligomer. Increasing the ActD concentration, however, resulted in stoichiometric formation of the unique 2:1 adduct. Spectrophotometric binding studies indicated that the apparent binding equilibrium constant for a GC site adjacent to a bound site is reduced by approximately a factor of 20 relative to the ActD binding constant to an isolated GC site.
对放线菌素D（ActD）与具有相邻5'-GC-3'位点的自互补寡脱氧核糖核苷酸的结合进行了亚氨基质子和³¹P核磁共振研究。无论寡聚物长度和周围序列如何，ActD对与GC位点的结合都表现出非常高的特异性。对于第一类具有中心GCGC序列的双链体，由于ActD吩恶嗪环系统的两种不同取向，观察到1:1复合物的混合物。¹H化学位移分析表明，有利的1:1复合物在GCGC序列中心碱基对的G碱基上方具有吩恶嗪环的苯型侧。这是首次证明不对称嵌入剂能够以两种可能的取向与DNA结合。与这些相同的寡聚物形成了一种独特的2:1复合物，其¹H和³¹P化学位移与1:1复合物的化学位移有显著差异，同样ActD分子的苯型侧位于中心GC碱基对的G碱基上方。在GCGC序列中第二个ActD的结合存在相当大的反协同作用。在用GCGC序列的寡聚物进行滴定实验时，在每个寡聚物一个ActD的比例之前，只发现了两种1:1复合物。然而，增加ActD的浓度会导致独特的2:1加合物的化学计量形成。分光光度法结合研究表明，与结合位点相邻的GC位点的表观结合平衡常数相对于ActD与孤立GC位点的结合常数降低了约20倍。
