Conformations and Mitochondrial Uncoupling Activity of Synthetic Emerimicin Fragments

Several amino terminal fragments of the emerimicins $(Ac-{Phe}^1-{Aib}^2-{Aib}^3-{Aib}4-{Va1}^5-{Gly}^6-{Leu}^7-{Aib}^8-{Aib}^9-{Hyp}^{10}-{Gln}^{11}-D-{Iva}^{12}-{Hyp}^{13}-Ala/{Aib}^{14}-{Phol}^{15})$ ranging in length from five to ten residues have been synthesized. Nuclear magnetic resonance studies have been carried out on the 1-5, 6-10, 1-6, 1-7, 1-8, 1-9, and 1-10 fragments. The number of solvent-shielded NH groups in $CD{Cl}_3$ solutions for 1-5, 1-6, 1-7, 1-8, 1-9, and 1-10 indicate that $3_{10}$-helical structures are favored in this solvent. In ${({CD}_3)}_2SO$, an additional NH group, assigned to Aib(3) NH is solvent exposed in the fragments longer than six residues, suggesting partial unfolding of the N-terminal \beta-turn or transition to an \alpha-helical conformation. The data for fragment 6-10 are consistent with a conformation having a single Leu-Aib \beta-turn. Infrared studies suggest an increase in the number of intramolecular hydrogen bonds with increasing peptide chain length. Appreciable mitochondrial uncoupling activity is observed for peptides with a chain length of at least seven residues. The order of efficiencies of the fragments is 1-7 < 1-8 - 1-10 < 1-9, with the decapeptide exhibiting anomalously low uncoupling activity