A MODEL FOR THE [C-G-CENTER-DOT-C](N) TRIPLE-HELIX DERIVED FROM OBSERVATION OF THE C+-G-CENTER-DOT-C BASE TRIPLET IN A CRYSTAL-STRUCTURE()

A molecular modelling study on the [C+-G . C](11) triple helix is repo rted, We have observed the C+-G . C base triplet in the crystal struct ure of an oligonucleotide-drug complex, between the minor-groove drug netropsin and the decanucleotide d(CGCAATTGCG)(2). The complex was cry stallised at pH 7.0, but the crystal structure, at a resolution of 2.4 Angstrom, shows that a terminal cytosine has become protonated and pa rticipates in a parallel C+-G . C base triplet, The structure of this triplet and its associated sugar-phosphate backbones have been energy- refined and then used to generate a triple helix, This has characteris tics of the B-type family of DNA structures for two strands, with the third, the C+ strand, having backbone conformations closer to the A fa mily. (C) 1997 Federation of European Biochemical Societies.