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An unexpectedly high affinity for guanine is shown by the rare RNA nucleobase 7,9-dimethylguanine (7,9-DimeG). At pH 7 it forms a partially protonated homo base pair (see sketch) with its conjugated acid (7,9-DimeGH+), the stability of which greatly exceeds that of the Watson-Crick base pair between guanine and cytosine.
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Metzger et al. (1996) studied this question.
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