Membrane Interaction of an Anti-Bacterial AApeptide Defined by EPR Spectroscopy

Abstract

Antibiotic resistance is one of the major threats to public health. AApeptides are a new class of synthetic anti-bacterial peptidomimetics that are not prone to antibiotic resistance, and are highly resistant to protease degradation. The broad-spectrum anti-bacterial activities of AApeptides are believed to be related to their unique structural features, which are capable of disrupting bacterial membranes selectively over human eukaryotic cells. How AApeptides selectively interact with bacterial membranes and alter lipid assembly and properties is unclear, but such information is essential in order to understand their antimicrobial activities. Here, by using electron paramagnetic resonance (EPR) techniques at 9 and 95 GHz, we have characterized the membrane interaction and destabilizing activities of an AApeptide, cyclic-γ-AApeptide, on liposomes mimicking bacterial and eukaryotic cell membranes. The analysis revealed specific interactions between cyclic-γ-AApeptides and negatively-charged lipid molecules. Subsequently, the AApeptide interacts strongly with the bacteria-mimic liposomes containing negatively-charged lipids, and thereby inhibits membrane fluidity. Furthermore, AApeptide binding induces significant lipid-lateral-ordering of the bacteria-mimic liposomes, detected by EPR at 95 GHz. In addition, AApeptide binding increases the membrane permeability of the bacteria-mimic liposomes. By contrast, minimal membrane fluidity and permeability changes were observed for liposomes mimicking eukaryotic cell membranes, consisting of neutral lipids and cholesterol, upon AApeptide binding. The results revealed that the intrinsic features of AApeptides are important for their ability to selectively disrupt bacterial membranes, the implications of which extend to developing new antibacterial biomaterials.