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Figure 2.
Fig 2. Topotecan electron density. (A) Topotecan with
reversible hydrolysis of the E-ring from the closed lactone form
to the open carboxylate form is diagrammed. (B) A 3.0  ||Fo||-||Fc|| omit map
of electron density for Topotecan is illustrated. (C) A 3.0 ||Fo||-||Fc|| electron
density map calculated with the lactone form of Topotecan (100%
closed E-ring) is diagrammed. Negative electron density (red) is
seen in the vicinity of the lactone oxygen, and positive (blue)
electron density peaks are located nearby. (D) A 3.0 ||Fo||-||Fc|| electron
density calculated with the carboxylate form of Topotecan (100%
open E-ring) is diagrammed. Negative electron density (red)
surrounds the terminal hydroxyl and carboxylic acid moieties,
whereas a positive (blue) electron density peak is in the
location of what would be the lactone oxygen in the closed
E-ring conformation.
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