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Sum electron density contoured at the 1.0 σ level (green meshwork) around the modified Y6·T19 and Y18·T7 along the normal to the base pairs, viewed (a) from the side and (b) from the top approximately the named to base pairs, revealing stacking interactions. (c) Watson–Crick base pairing of 7-deaza-dA·dT. Y6 and Y18 bases are shown in blue.
