Kiran M R, Bansal M
Molecular Biophysics Unit, Indian Institute of Science, Bangalore, India.
Indian J Biochem Biophys. 1995 Dec;32(6):391-403.
DNA triple helices containing two purine strands and one pyrimidine strand (C.GG and T.AA) have been studied, using model building followed by energy minimisation, for different orientations of the third strand resulting from variation in the hydrogen bonding between the Watson-Crick duplex and the third strand and the glycosidic torsion angle in the third strand. Our results show that in the C.GG case the structure with a parallel orientation of the third strand, resulting from Hoogsteen hydrogen bonds between the third strand and the Watson-Crick duplex, is energetically the most favourable while in the T.AA case the antiparallel orientation of the third strand, resulting from reverse Hoogsteen hydrogen bonds, is energetically the most favourable. These studies when extended to the mixed sequence triplexes, in which the second strand is a mixture of G and A, correspondingly the third strand is a mixture of G and A/T, show that though the parallel orientation is still energetically more favourable, the antiparallel orientation becomes energetically comparable with an increasing number of thymines in the third strand. Structurally, for the mixed triplexes containing G and T in the third strand, it is seen that the basepair non-isomorphism between the C.GG and the T.AT triplets can be overcome with some changes in the base pair parameters without much distortion of either the backbone or the hydrogen bonds.
我们利用模型构建并随后进行能量最小化的方法，研究了包含两条嘌呤链和一条嘧啶链（C.GG和T.AA）的DNA三链螺旋结构，针对由于沃森-克里克双链体与第三条链之间氢键的变化以及第三条链中糖苷扭转角的变化而导致的第三条链的不同取向进行了研究。我们的结果表明，在C.GG的情况下，第三条链与沃森-克里克双链体之间通过 hoogsteen 氢键形成平行取向的结构在能量上最为有利；而在T.AA的情况下，第三条链通过反向 hoogsteen 氢键形成反平行取向的结构在能量上最为有利。当这些研究扩展到混合序列三链体时，其中第二条链是G和A的混合物，相应地第三条链是G和A/T的混合物，结果表明虽然平行取向在能量上仍然更有利，但随着第三条链中胸腺嘧啶数量的增加，反平行取向在能量上变得与之相当。在结构上，对于第三条链中含有G和T的混合三链体，可以看到通过碱基对参数的一些变化，可以克服C.GG和T.AT三联体之间的碱基对非同构现象，而不会对主链或氢键造成太大的扭曲。
