pH-Triggered nanostructural transformations in antimicrobial peptide/oleic acid self-assemblies

The delivery of poorly water-soluble antimicrobial peptides (AMPs) that are sensitive to degradation is a major challenge in the pharmaceutical field. In this study, we design and characterize a pH-sensitive nanocarrier with the potential for delivery of AMPs and their protection from degradation. These nanobiointerfaces are prepared through the self-assembly of oleic acid (OA) with the human cathelicidin LL-37 in excess water. Advanced experimental methods including synchrotron small angle X-ray scattering, cryogenic transmission electron microscopy, and dynamic light scattering were used to characterize the OA/LL-37 self-assemblies and their structural alterations in response to changes in pH and composition. Experimental findings reveal colloidal transformations from normal emulsions via micellar cubosomes and hexosomes to vesicles upon increasing the pH from 6.0 to 8.0 at a LL-37 content around 10 wt% relative to OA. Increasing the LL-37 content to 30 wt% in OA led to diminishing of micellar cubosomes and hexosomes in this narrow pH range, favoring the formation of micelles and vesicles of various shapes and sizes. Upon increasing the pH, with the strongest effect around pH 7.5, charge repulsions among the gradually deprotonating carboxylic groups of OA modified the geometric packing of the molecules, significantly affecting the nanostructure. These detailed insights into the formation of this unique family of nanobiointerfaces and their tunable structural features may contribute to the rational design of pH-responsive antimicrobial systems for the delivery of peptides, particularly poorly water-soluble AMPs.