Table 2.

pKa values of common opioid ligands

Opioid ligand	pKa
Alfentanil	7.82a
Buprenorphine	8.31
Butorphanol	8.19
Codeine	8.21
Fentanyl	8.43
Heroin	7.95
Levorphanol	9.58
Meperidine	8.59
Methadone	8.94
Morphine	8.21
Nalbuphine	8.71
Naloxone	7.94
Oxycodone	8.53
Oxymorphone	8.17
Pentazocine	8.88
Remifentanil	7.51b
Sufentanil	7.85
Tramadol	9.41

All values (except ^a and ^b) are experimentally measured values from (126).

^aCalculated values from ChEMBL, no experimental values available.

^bCalculated values from DrugBank, no experimental values available.