Towards Design of Novel Antimicrobial Agents: Role of the Conformational Rigidity

Abstract

Non-natural mimics of antimicrobial peptides (AMPs) are excellent candidates for anti-infectious agents due to their stability towards enzymatic degradation and broad adjustability of physicochemical properties. Conformationally flexible acyl-lysine oligomers (OAKs) and restrained arylamide foldamers have demonstrated capability to be fine-tuned to high antimicrobial activity and negligible toxicity towards human cells. In the present work we examine how structural rigidity affects interactions of the AMP analogs with model lipid monolayers at the air-liquid interface by constant-pressure insertion assays, epifluorescence microscopy (EFM), X-ray reflectivity (XR) and grazing incident-angle X-ray diffraction (GIXD) using synchrotron radiation. Simplified models of the outer Gram-negative and cytoplasmic Gram-positive membranes were represented Lipid A and DPPG monolayers, respectively, while mammalian plasma membrane was mimicked with zwitterionic DPPC/Cholesterol 6/4 monolayer mixture. Insertion assays show that both AMP analogs readily incorporate into the bacterial, but not mammalian, membrane mimics. Membrane-insertion of OAK and arylamide was accompanied by rapid deterioration of the structural order in lipids. Interestingly, flexible OAK was more efficient in disrupting Gram-negative rather than Gram-positive bacterial model membrane. Electron density profiles across the film, derived from XR data, demonstrate that after insertion the hydrophobic cores of OAK and arylamide were located within lipid acyl chains, inducing negative and positive local curvatures, respectively. Moreover, concentration of flexible OAK within Lipid A was higher than within DPPG, as opposed to restrained arylamide, as well as to natural AMPs we characterized previously, including LL-37, SMAP-29, and PG-1.