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Shown are the chemical structures of fentanyl (left), W-15 (center), and W-18 (right). The structural similarities between fentanyl and W-15/W-18 are shown in red. The presence of aryl sulfonamide group (in blue) in both W-15/W-18 makes the piperidine nitrogen not charged at physiological pH compared with the fentanyl piperidine nitrogen.
