Dependence of Disruptive Ability of Amino Acid Fibrils upon Their Physico-Mechanical Characteristics: Investigations on a Model Phospholipid Bilayer

Neurodegenerative changes have been linked to the aggregation of proteins and deposition of amyloid fibrils. Single amino acids can also form toxic amyloid-like fibrils. In this study, we have investigated and compared the structural characteristics of fibrils of four levorotatory amino acids (tryptophan, Trp; phenylalanine, Phe; methionine, Met; and alanine, Ala), each differing in polarity and aromaticity. The effect of fibrils was studied on a model bilayer composed of soya phosphatidylcholine (SPC). Dynamic light scattering (DLS) was employed to monitor variations in the hydrodynamic diameter and surface charge of liposomes following their interactions with fibrils. Physico-mechanical properties of fibrils were studied using scanning electron microscopy (SEM) and atomic force microscopy (AFM). Isothermal titration calorimetry (ITC) was used for quantifying the energy changes in the bilayer during destabilization triggered by the specific amino acid fibril. Our results show high mechanical sturdiness of fibrils derived from aromatic amino acids, which could induce notable changes in the vesicle size. Among nonaromatic ones, mechanically weaker Met fibrils inflicted no changes in the size of vesicles, whereas Ala did not form fibrils. Energetics of fibril-bilayer interactions was found to be enthalpy-driven except that of Ala which exhibited entropy-driven binding. These changes appeared to correlate with the topography and stiffness of the fibrils. With their high stiffness and roughness, Trp fibrils inflicted a maximum impact on vesicles. The effects of Met and Ala remained minimal. Based on these findings, we conclude that liposomal bilayers can be used for understanding the effect of fibrillar aggregates a priori upon mammalian cells.