A Hoogsteen base pair is a variation of base-pairing in nucleic acids such as the A•T pair. In this manner, two nucleobases, one on each strand, can be held together by hydrogen bonds in the major groove. A Hoogsteen base pair applies the N7 position of the purine base (as a hydrogen bond acceptor) and C6 amino group (as a donor), which bind the Watson–Crick (N3–C4) face of the pyrimidine base. Ten years after James Watson and Francis Crick published their model of the DNA double helix Karst Hoogsteen reported a crystal structure of a complex in which analogues of A and T formed a base pair that had a different geometry from that described by Watson and Crick. Similarly, an alternative base-pairing geometry can occur for G•C pairs. Hoogsteen pointed out that if the alternative hydrogen-bonding patterns were present in DNA, then the double helix would have to assume a quite different shape. Hoogsteen base pairs are observed in alternative structures such as the four-stranded G-quadruplex structures that form in DNA and RNA. Here you can see a Hoogsteen pair U-U located in the crystal structure of a synthetic RNA dimer (PDB code: 1OSU)

#molecularart ... #Hoogsten ... #UU ... #pair ... #RNA ... #structure ... #dimer ... #noncanonical ... #basepairing ... #xray

Structure rendered with @proteinimaging and depicted with @corelphotopaint