Abstract
Dynorphin A (Dyn A) is a natural kappa opioid receptor (κ) agonist that contains a characteristic tetrapeptide sequence Tyr-Gly-Gly-Phe at the N-terminus termed the “message” sequence, followed by a unique C-terminal sequence termed the “address” [1]. The message sequence is proposed to confer opioid activity, whereas the address sequence is thought to impart high affinity for κ opioid receptors. However, the structural and conformational requirements for antagonist activity as opposed to agonist activity are not clear from the structure-activity relationship (SAR) studies of Dyn A.