Howard F B, Limn W, Miles H T
Biochemistry. 1985 Sep 10;24(19):5033-9. doi: 10.1021/bi00340a012.
The ribopolynucleotide poly(2-amino-8-methyladenylic acid), (r2NH2(8)MeA)n, has been synthesized, and its physical and chemical properties have been examined. The study reveals competing effects on these properties of the 2-NH2 and 8-Me substituents. In marked contrast to the analogous (r8MeA)n, the new polymer readily interacts to form double helixes with complementary pyrimidine polynucleotides. Triple helixes are not formed. The 8-Me group is strongly destabilizing for helix formation (delta Tm approximately 65 degrees C), presumably by favoring a syn conformation, which blocks heteroduplex formation with ribohomopolymers. The 2-NH2 substituent stabilizes helixes in the ribo series by about 30 degrees C in Tm by forming a third interbase hydrogen bond. We suggest that the free energy from the 2-NH2 interaction drives the syn-anti equilibrium to the purine polymer to the anti form present in the double helix. CD spectra of the homopolymers (r2NH2A)n and (r2NH2(8)MeA)n are completely different, reflecting major differences of conformation. The double helixes formed by these polymers with (rT)n and (rBrU)n, on the other hand, have closely similar CD spectra, supporting our proposal of a major change in conformation of (2NH2(8)MeA)n on going from single strand to double helix.
已合成了核糖多聚核苷酸聚（2-氨基-8-甲基腺苷酸），即(r2NH2(8)MeA)n，并对其物理和化学性质进行了研究。该研究揭示了2-NH2和8-Me取代基对这些性质的竞争效应。与类似的(r8MeA)n形成鲜明对比的是，这种新聚合物很容易与互补的嘧啶多核苷酸相互作用形成双螺旋。未形成三螺旋。8-Me基团对螺旋形成具有很强的去稳定作用（ΔTm约为65℃），推测是因为它有利于顺式构象，从而阻碍了与核糖均聚物形成异源双链体。2-NH2取代基通过形成第三个碱基间氢键，使核糖系列中的螺旋在Tm时稳定约30℃。我们认为，2-NH2相互作用产生的自由能将嘌呤聚合物的顺式-反式平衡驱动到双螺旋中存在的反式构象。均聚物(r2NH2A)n和(r2NH2(8)MeA)n的圆二色光谱完全不同，反映了构象上的主要差异。另一方面，这些聚合物与(rT)n和(rBrU)n形成的双螺旋具有非常相似的圆二色光谱，这支持了我们关于(2NH2(8)MeA)n从单链转变为双螺旋时构象发生重大变化的提议。
