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Watson-Crick versus Hoogsteen base pairing. (A) Hydrogen bonding in an A-T base pair. The structures of 2-aminopurine and purine are also shown. (B) Hydrogen bonding in a G-C base pair. The hydrogen bonding for a G · T Hoogsteen base pair is also shown, and the structures of 6O-methyl guanine and 8-oxoguanine are shown.
