Hoogsteen hydrogen bonds using N-7 of the purine bases of the Watson–Crick base pairs (Figures 2.33
and 2.34).^36 The resulting base-triplets form the core of a triple helix. In theory, G can form a base-triple
with a CG pair and A with a TA pair (Figure 2.33b), but the only combinations that have isomorphous
location of their C-1atoms are the two triplets TxAT and CGxC(Figure 2.33a), where Cis the N-3
protonated form of cytosine. This means that the three strands of triple-helical DNA are normally two
50 Chapter 2
Figure 2.33 Base triples formed by (a) Hoogsteen bonding for TAxT and CGxCand (b) reversed Hoogsteen
binding for TAxA and CGxG. (c) Model of triple-helical DNA based on fibre diffraction of
poly(dA)2poly(dT) (kindly provided by the late Professor Claude Hélène). (d) A schematic representation
of H-DNA showing loci for attack by single-strand specific reagents