Final answer:
The opioid with a pKa of 6.5, which is lower than physiological pH, is Meperidine. This property affects how the drug ionizes in the body, influencing its ability to cross biological membranes and become active pharmacologically.
Step-by-step explanation:
The pKa of a compound is a chemical property that influences its ionization within a given pH environment. In the context of opioids, pKa affects how much of the drug exists in an ionized versus non-ionized form and consequently, its ability to cross biological membranes and exert its pharmacological effects.
The only opioid listed with a pKa lower than physiological pH, making it predominantly non-ionized at physiological pH, is Meperidine (option c), with a pKa of 6.5. Fentanyl, Morphine, and Remifentanil all have pKa values higher than physiological pH.
Opioids such as Fentanyl, Morphine, Oxycodone, Vicodin, and methadone are powerful prescription pain relievers, but they also come with a high potential for abuse. The understanding of their chemical properties, like pKa, is essential for pharmacologists and healthcare providers when considering their pharmacokinetics and potential for abuse.