Solution structure of a hexitol nucleic acid duplex with four consecutive T center dot T base pairs

The structure of the hexitol nucleic acid (HNA) h(GCGCTTTTGCGC) was determi ned by NMR spectroscopy. This unnatural nucleic acid was developed as a mim ic for A-RNA. In solution, the studied sequence is forming a symmetric doub le-stranded structure with four central consecutive T.T wobble pairs flanke d by G.C Watson-Crick base pairs. The stem regions adopt an A-type helical structure. Discrete changes in backbone angles are altering the course of t he helix axis in the internal loop region. Two H-bonds are formed in each w obble pair, and base stacking is preserved in the duplex, explaining the st ability of the duplex. This structure elucidation provides information abou t the influence of a (T)(4) fragment on local helix geometries as well as o n the nature of the T.T mismatch base pairing in a TTTT tract.