Kang C H, Berger I, Lockshin C, Ratliff R, Moyzis R, Rich A
Department of Biology, Massachusetts Institute of Technology, Cambridge 02139.
Proc Natl Acad Sci U S A. 1994 Nov 22;91(24):11636-40. doi: 10.1073/pnas.91.24.11636.
The crystal structure of d(C3T), solved at 1.4 A resolution, reveals that the molecule forms a four-stranded intercalated complex. It consists of two parallel-stranded duplexes, each of which is held together by cytosine-protonated cytosine base pairs. The two duplexes are intercalated with each other and have opposite strand orientation. The molecule has a flat, lath-like appearance, and the covalently bonded cytosines have a slow right-handed twist of 17.1 degrees. However, there is considerable asymmetry. On one of the flat sides, the phosphate groups are rotated away from the center of the molecule. They are held in this orientation by bridging water molecules that bind the NH of cytosine and a phosphate group of an opposite chain. There is also considerable microheterogeneity in the structure. The cytosine hemiprotonation occurs even at pH 7 where stable crystals form.
以1.4埃分辨率解析的d(C3T)晶体结构显示，该分子形成了一个四链插入复合物。它由两个平行链的双链体组成，每个双链体通过胞嘧啶-质子化胞嘧啶碱基对维系在一起。这两个双链体相互插入且链的方向相反。该分子具有扁平的板条状外观，共价键连接的胞嘧啶有17.1度的缓慢右旋。然而，存在相当大的不对称性。在其中一个扁平面上，磷酸基团从分子中心旋转开。它们通过连接胞嘧啶的NH和相反链的一个磷酸基团的桥连水分子保持在这个方向。该结构中也存在相当大的微观异质性。即使在pH 7形成稳定晶体的情况下，胞嘧啶半质子化也会发生。
