POLYPURINE POLYPYRIMIDINE HAIRPINS FORM A TRIPLE HELIX STRUCTURE AT LOW PH

1D and 2D NMR investigations of the 15 residue deoxynucleotide sequence d(TCTCTC-TTT-GAGAGA) show that above pH = 6.5 the molecule adopts a B-form hairpin conformation. As the pH is lowered below 6.5 molecules progressively associate in pairs to form a partially triple helical, partially single stranded structure in which the bases of the oligopyrimldine d(TC)3 tract from one molecule form Hoogsteen pairs with the d(G-A)3 tract of the other. Imino protons of protonated cytosines can be observed at very low field (≈15 ppm). The enthalpy of triplex formation was estimated by NMR techniques to be -16 kcal mol-1. Intense H6 to H3′ cross peaks from residues in all three strands suggest the presence of N-type sugars at some but not at all possible sites. Surprisingly strong cross peaks between H5′ or H5″ and non-exchangeable base protons are also observed. These suggest that certain of the O5′-C5′-C4′-C3′ phosphate backbone torsion angles (γ) are unusual. © 1990 Oxford University Press.