cyclo[d-Asp(2),Dap(5)]Dyn A-(1-13)NH(2) (Dap, 2,3-diaminopropionic acid; Dyn A, dynorphin A), synthesized previously in our laboratory, showed sub-nanomolar affinity for kappa opioid receptors and potent agonist activity in the guinea pig ileum assay (Arttamangkul et al., J. Med. Chem. 1995, 38, 2410-2417). Various modifications were made in position 3 of cyclo[d-Asp(2),Dap(5)]Dyn A-(1-11)NH(2) that could influence the opioid receptor affinity, selectivity, and/or efficacy of this peptide. An optimized orthogonal synthetic strategy was developed for the synthesis of these cyclic peptides in which the final peptides could be cleaved from the solid support with trifluoroacetic acid. Substitutions of Gly(3) by Ala, d-Ala, Trp, and d-Trp in cyclo[d-Asp(2),Dap(5)]Dyn A-(1-11)NH(2) and its linear counterpart [d-Asp(2),Dap(5)]Dyn A-(1-11)NH(2) were generally well tolerated by both kappa and micro opioid receptors. Despite differences in the size and stereochemistry of the substitutions, most of the peptides (except for cyclo[d-Asp(2),Pro(3),Dap(5)]Dyn A-(1-11)NH(2) and [d-Asp(2),d-Ala(3), Dap(5)]Dyn A-(1-11)NH(2)) exhibited low nanomolar affinity for both kappa (K(i) = 0.21 to 2.2 nM) and micro (K(i) = 0.22 to 7.27 nM) opioid receptors. All of the 3-substituted cyclic and linear analogues synthesized showed reduced affinity for delta opioid receptors. Incorporation of d-Ala at position 3 of cyclo[d-Asp(2),Dap(5)]Dyn A-(1-11)NH(2) exhibited 2-fold higher kappa opioid receptor affinity and 16-fold higher selectivity for kappa over micro opioid receptors than the parent cyclic peptide. In contrast, substitution of Ala at position 3 resulted in an analogue with 2.4-fold lower affinity and very low preference for kappa over micro opioid receptors. The Trp and d-Trp cyclic and linear analogues exhibited similar nanomolar affinities for kappa opioid receptors. cyclo[d-Asp(2),Pro(3),Dap(5)]Dyn A-(1-11)NH(2) showed the largest decreases in affinity for all three opioid receptors compared to the parent cyclic peptide. Except for cyclo[d-Asp(2), Pro(3),Dap(5)]Dyn A-(1-11)NH(2), which was a partial agonist, all of the cyclic peptides exhibited full agonist activity in the adenylyl cyclase assay using cloned kappa opioid receptors.