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Inferred protonation states of C•A base pairs observed in the structures. In their canonical tautomeric state C and A do not pair (middle), because the two extracyclic amines clash. If either C or A tautomerizes (asterisk), a hydrogen-bonded base pair that mimicks a cognate shape can form (top). If A ionizes, a wobble base pair can form (bottom) (17, 18). Hydrogen bond donors and acceptors are colored blue and pink respectively.
