Figure 2

Examples of different structures formed by GAA•TTC repeats showing their constituent hydrogen bonding schemes. (A) A purine:purine:pyrimidine triplex [11, 12]. This triplex involves Hoogsteen hydrogen bonding between a purine already in a Watson-Crick base pair with a pyrimidine, and a purine from a different region of the repeat. (B) A GAA hairpin [61]. Various hydrogen bonding schemes involving G•G, G•A and A•A base pairs have been proposed [74] including the one shown. However, the precise molecular details of the GAA hairpin are unknown. (C) A parallel duplex in which, unlike the antiparallel configuration of normal Watson-Crick duplex, the polarity of the two base-paired strands is the same [75]. In this configuration the base pairs involve a reverse Watson-Crick orientation and two, rather than three, hydrogen bonds.