Recent studies showed that dermorphin and enkephalin analogues containing two methyl groups at the 2', 6'-positions of the Tyr 1 aromatic ring and lacking an N-terminal amino group were moderately potent δ and μ opioid antagonists. These results indicate that a positively charged N-terminal amino group may be essential for signal transduction but not for receptor binding and suggested that its deletion in agonist opioid peptides containing ...
[Kawasaki, Andrew M.; Knapp, Richard J.; Kramer, Thomas H.; Wire, William S.; Vasquez, Olga S.; et al. Journal of Medicinal Chemistry, 1990 , vol. 33, # 7 p. 1874 - 1879]