Stabilization of $\beta$ -turn conformations in enkephalins. $\alpha$ -Aminoisobutyric acid analogs

Stereochemistry constraints were introduced into the enkephalin backbone by substituting $\alpha$-aminoisobutyryl (Aib) residues at positions 2 and 3, instead of Gly. 1H NMR studies of Tyr-Aib-Gly-Phe-Met-NH2 [69872-68-0], Tyr-Aib-Aib-Phe-Met-$NH_2$ [72564-51-3], and Tyr-Gly-Aib-Phe-Met-$NH_2$ [72564-52-4] demonstrate the occurrence of folded, intramolecularly H-bonded structures in org. solvents. Similar conformations are also favored in the corresponding tert-butyloxycarbonyl-protected tetrapeptides, which lack the Tyr residue. A $\beta$-turn centered at positions 2 and 3 is proposed for the $Aib_2-Gly_3$ analog. In the Gly2-Aib3 analog, the $\beta$-turn has $Aib_3-Phe_4$ as the corner residues. The $Aib_2-Aib_3$ analog adopts a consecutive $\beta$-turn or 310 helical conformation. High in vivo biol. activity is observed for the $Aib_2$ and $Aib_2-Aib_3$ analogs, while the $Aib_3$ peptide is less activ