By using nuclear magnetic resonance relaxation dispersion spectroscopy in concert with steered molecular dynamics simulations, we have observed transient sequence-specific excursions away from Watson–Crick base-pairing at CA and TA steps inside canonical duplex DNA towards low-populated and short-lived A•T and G•C Hoogsteen base pairs. The observation of Hoogsteen base pairs in DNA duplexes specifically bound to transcription factors and in damaged DNA sites implies that the DNA double helix intrinsically codes for excited state Hoogsteen base pairs as a means of expanding its structural complexity beyond that which can be achieved based on Watson–Crick base-pairing.Geometrically, the Hoogsteen base pair is related to the Watson-Crick base pair by a 180-degree rotation about the glycosidic bond (N9–C1'). While the A•T Hoogsteen base pair is classic, the similar G•C+ Hoogsteen pair (with protonation of cytosine N3) is equally possible. The A•T and G•C Hoogsteen base pairs have two perfect H-bonds, so they are energetically stable. As for their existence in DNA duplex, the most direct evidence comes from the "trap" experiments (see Fig.3 of the paper). In the News & Views section, Honig and Rohs provide a nice recap of the main point and implications of this work.
As also observed in another recent publication, "Replication infidelity via a mismatch with Watson–Crick geometry", the base sequence has a subtle role in influencing the base-pairing schemes, three-dimensional structures and biological functions of DNA. However, we should not forget that only the Watson-Crick base pairs, and to a less extent, the G-U wobble pair, have the correct symmetry to ensure a "regular" double helical structure.