(A) Illustration of the
compounds (i) heterocycle, (ii) AMW610,
(iii) AST3-88, and (iv) AMW6103. The stereochemistry of the thioether
ring is represented by an asterisk (*). Compounds AST3-88 and AMW610
contain l-Cys, and compound AMW6103 contains a d-Cys at this position. The side chain of His is depicted in green,
DPhe in orange, Arg in blue, the heterocycle moiety in pink, and the
disulfide bridge is circled in yellow. (B) Amino acid sequences of
key endogenous and synthetic melanocortin peptides.