| http://www.w3.org/ns/prov#value | - In general, a number of features have to be present for the opioid analgesic to be active, including the tertiary nitrogen with a small alkyl group (A); the presence of a piperidine ring (B); a quaternary carbon (D) and a benzene ring (C) - or species with the same shape -attached to it; and a C2 spacer (E), part of the piperidine ring, between the tertiary nitrogen (A) and the quaternary carbon (
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