Hattori M, Frazier J, Miles H T
Biochemistry. 1975 Nov 18;14(23):5033-45. doi: 10.1021/bi00694a002.
Poly(8-aminoguanylic acid) has in neutral solution a novel ordered structure of high stability. The 8-amino group permits formation of three hydrogen bonds between two residues along the "top", or long axis, of the purines. The usual hydrogen bonding protons and Watson-Crick pairing sites are not involved in the association. The bonding scheme has a twofold rotation axis and is hemiprotonated at N(7). Poly(8NH2G) is converted by alkaline titration (pK = 9.7) to a quite different ordered structure, which is the favored form over the range approximately pH 10-11. The bonding scheme appears to be composed of a planar, tetrameric array of guanine residues, in which the 8-amino group does not participate in interbase hydrogen bonding. Poly (8NH2G) does not interact with poly(C) in neutral solution because of the high stability of the hemiprotonated G-G self-structure. Titration to the alkaline plateau, however, permits ready formation of a two-stranded Watson-Crick helix. In contrast to the monomer 8NH2GMP, poly(8NH2G) does not form a triple helix with poly(C) under any conditions. The properties of the ordered structures are interpreted in terms of a strong tendency of the 8-amino group to form a third interbase hydrogen bond, when this possibility is not prevented by high pH.
聚（8-氨基鸟苷酸）在中性溶液中具有一种新型的高稳定性有序结构。8-氨基基团允许沿着嘌呤的“顶部”或长轴在两个残基之间形成三个氢键。通常的氢键质子和沃森-克里克配对位点不参与这种缔合。这种键合模式具有一个二重旋转轴，并且在N(7)处半质子化。通过碱滴定（pK = 9.7），聚（8NH₂G）转变为一种截然不同的有序结构，在大约pH 10 - 11的范围内，这种结构是更有利的形式。这种键合模式似乎由鸟嘌呤残基的平面四聚体阵列组成，其中8-氨基基团不参与碱基间的氢键形成。由于半质子化的G-G自结构的高稳定性，聚（8NH₂G）在中性溶液中不与聚（C）相互作用。然而，滴定至碱性平台允许容易地形成双链沃森-克里克螺旋。与单体8NH₂GMP不同，聚（8NH₂G）在任何条件下都不与聚（C）形成三链螺旋。根据8-氨基基团在不被高pH阻止时形成第三个碱基间氢键的强烈倾向来解释这些有序结构的性质。
