Labaditin, a cyclic peptide with rich biotechnological potential: preliminary toxicological studies and structural...

Cyclic peptides isolated from the plants of the Euphorbiaceae family have been largely studied due to their rigid conformation, which is considered significant for biologic activity. The peptide Labaditin (L(0)) and its open chain analogs (L(1)) were synthesized by the solid-phase peptide synthesis technique (Fmoc/tBu), and purified to elucidate its interaction with membrane models. A shift in λ(max) emission and Stern-Volmer constants values indicate that both tryptophans migrate to a more apolar environment, with L(1) decreasing less than L(0). A circular dichroism (CD) study revealed that L(0) was kept unstructured in aqueous media as much as in the presence of dipalmitoilphosphatidylcholine liposomes. The thermodynamic studies by differential calorimetry (DSC) show a ΔH increase (50 and 18 kcal/mol, for L(0) and L(1), respectively) with peptide concentrations, which is indicative of lipids associating with peptides, resulting in the inability of the lipids to participate in the main transition. Therefore, all CD, DSC, and fluorescence data suggest a greater L(0) membrane insertion. A probable mechanism for Labaditin interaction is based initially on the hydrophobic interaction of the peptide with the lipid membrane, conformational change, peptide adsorption on the lipid surface, and internalization process. Peptide's antibacterial effect was also evaluated and revealed that only L(0) showed reduction in viability in Gram-positive bacteria while no effects to the Gram-negative.