PARALLEL NUCLEIC-ACID HELICES WITH HOOGSTEEN BASE-PAIRING - SYMMETRY AND STRUCTURE

Molecular structures for parallel DNA and RNA double helices with Hoog steen pairing are proposed for the first time. The DNA helices have su gars in the C2'-endo region and the phosphodiester conformations are ( trans, gauche(-)), and the RNA helices have sugars in the C3'-endo reg ion and the phosphodiester conformations are (gauche(-), gauche(-)). A pseudorotational symmetry relates the two parallel strands of DNA hel ices and a screw symmetry relates the two strands of RNA helices, whic h have an associated tilt of the bases. The conformational space of pa rallel helices with Hoogsteen base pairing, unlike the Watson-Crick du plex, is highly restricted due to the unique positioning of the symmet ry axis in the former case. The features of the parallel double helix with Hoogsteen pairing are compared with the Watson-Crick duplex and t he corresponding triple helix. (C) 1994 John Wiley & Sons, Inc.