Synthesis and biological activities of ykfa analogues: effects of position 4 substitutions and altered ring size on In vitro opioid activity

Substitution in position 4 of the potent opioid peptide YkFA with aliphatic hydrophobic residues resulted in compounds that retained low nanomolar activities at both μ and δ opioid receptors, while ring contraction by incorporation of diaminobutyric acid in position 2 resulted in a more pronounced decrease in potency at both receptors for the ψ[CH2NH] pseudopeptide as compared to the all amide parent.