Muraoka M, Miles H T, Howard F B
Biochemistry. 1980 May 27;19(11):2429-39. doi: 10.1021/bi00552a022.
Random copolymers of adenylic acid and 2-aminoadenylic acid form both double and triple helices with poly(uridylic acid) [poly(U)]. Transition temperatures of two-stranded helices increase with 2NH2A content, exhibiting a slight positive departure from linearity and indicating that the contribution to helix stability arising from introduction of the 2-amino group does not significantly depend upon base sequence. We have shown previously that poly(2NH2A) . poly(U) does not undergo a disproportionation reaction (2 leads to 3 transition). Extrapolation from melting curves of 1:1 complexes between A,2NH2A copolymers and poly(U) indicates a Tm for the 2 leads to 3 transition of poly(2NH2A) . poly(U) which is too high to be observable under normal conditions. Addition of an organic solvent (50% ethylene glycol), however, lowers Tm by promoting unstacking of single-stranded poly(2NH2A) sufficiently to permit observation of the disproportionation of poly(2NH2A) . poly(U) for the first time. Transition breadths of 1:1 complexes of A,2NH2A copolymers with poly(U) are greater than those of either of the homopolymer complexes in the middle range of composition (67 and 48% A) but not at 25% A. These results are consistent with previous calculations on the effect of heterogeneity in base-pair stability on DNA transition breadths. In the poly(A), poly(U) system, Et4N+ counterion reduces the Tm of the double and triple helices by 26 and 41 degrees C, respectively. The larger depression in the latter case arises from the higher charge density of the triple helix and less effective counterion screening by Et4N+. In the poly(2NH2A), poly(U) system Tm,2 leads to 1 is reduced by 24 degrees C, but extrapolation of the copolymer results indicates a reduction of approximately 100 degrees C for Tm,3 leads to 2, accounting for previous failure to observe a triple helix in this system. CD spectra of A,2NH2A copolymers suggest that much of the spectral region can be regarded as a contribution of the CD spectra of the parent polymers poly(A) and poly(2NH2A) but that the region from 255 to 275 nm requires that contributions made by longer range interactions be taken into account.
腺苷酸与2 - 氨基腺苷酸的无规共聚物能与聚（尿苷酸）[聚（U）]形成双链和三链螺旋结构。双链螺旋的转变温度随2NH₂A含量的增加而升高，呈现出轻微的正偏离线性关系，这表明引入2 - 氨基对螺旋稳定性的贡献并不显著依赖于碱基序列。我们之前已经表明聚（2NH₂A）·聚（U）不会发生歧化反应（2转变为3）。从A、2NH₂A共聚物与聚（U）之间1:1复合物的熔解曲线外推可知，聚（2NH₂A）·聚（U）从2转变为3的熔点太高，在正常条件下无法观察到。然而，添加有机溶剂（50%乙二醇）会通过充分促进单链聚（2NH₂A）的解堆积来降低熔点，从而首次使得聚（2NH₂A）·聚（U）的歧化反应得以观察到。在组成的中间范围（67%和48%的A），A、2NH₂A共聚物与聚（U）的1:1复合物的转变宽度大于两种均聚物复合物的转变宽度，但在A含量为25%时并非如此。这些结果与先前关于碱基对稳定性异质性对DNA转变宽度影响的计算结果一致。在聚（A） - 聚（U）体系中，四乙基铵离子（Et₄N⁺）分别使双链和三链螺旋的熔点降低26℃和41℃。后一种情况下熔点降低幅度更大是因为三链螺旋的电荷密度更高，而Et₄N⁺对其反离子屏蔽效果较差。在聚（2NH₂A） - 聚（U）体系中转变为1的熔点降低了24℃，但共聚物结果的外推表明转变为2的熔点降低约100℃，这解释了之前在该体系中未能观察到三链螺旋的原因。A、2NH₂A共聚物的圆二色光谱表明，大部分光谱区域可视为母体聚合物聚（A）和聚（2NH₂A）圆二色光谱的贡献，但255至275nm区域需要考虑长程相互作用的贡献。
