Triple-strand formation in the homopurine:homopyrimidine DNA oligonucleotides d(G-A)4 and d(T-C)4.

Interest in triple and quadruple DNA helices built from homopurine and homopyrimidine strands has recently intensified principally because such structures may occur in vivo but also because of the potential use of triplexes both in forming highly sequence-specific complexes for use in chromosome mapping and in repressing transcription. From fibre diffraction data, models for triplex structures with poly(U).poly(A).poly(U) and poly(dT).poly(dA).poly(dT) have been proposed, in which the purine and one pyrimidine strand are Watson-Crick paired in an A' helix, and the other pyrimidine strand is Hoogsteen base-paired parallel to the purine strand along the major groove. A similar base-pairing scheme involving G and C would require protonation of C for Hoogsteen base-pair formation, and models for such triplexes have been proposed by analogy to the single-sequence fibre diffraction data. To date, however, there have been no single crystal or NMR structural data on DNA triplexes, and no direct observation of the protonated C in such a context. We present here the first NMR evidence for triplex formation in DNA from the homopurine d(G-A) and homopyrimidine d(T-C) oligonucleotides, and report direct observation of imino protons from protonated cytosines in the triplex.