We used the Langmuir monolayers technique to study the surface properties of melittin toxin mixed with either liquid-condensed DSPC or liquid-expanded POPC phospholipids. Pure melittin peptide forms stable insoluble monolayers at the air-water interface without interacting with Thioflavin T (Th-T), a sensitive probe to detect protein amyloid formation. When melittin peptide is mixed with DSPC lipid at 50 % of peptide area proportion at the surface, we observed the formation of fibril-like structures detected by Brewster angle microscopy (BAM), but they were not observable with POPC. The nano-structures in the melittin-DSPC mixtures became Th-T positive labeling when the arrangement was observed with fluorescence microscopy. In this condition, Th-T undergoes an unexpected shift in the typical emission wavelength of this amyloid marker when a 2D fluorescence analysis is conducted. Even when reflectivity analysis of BAM imaging evidenced that these structures would correspond to the DSPC lipid component of the mixture, the interpretation of ATR-FTIR and Th-T data suggested that both components were involved in a new lipid-peptide rearrangement. These nano-fibril arrangements were also evidenced by scanning electron and atomic force microscopy when the films were transferred to a mica support. The fibril formation was not detected when melittin was mixed with the liquid-expanded POPC lipid. We postulated that DSPC lipids can dynamically trigger the process of amyloid-like nano-arrangement formation at the interface. This process is favored by the relative peptide content, the quality of the interfacial environment, and the physical state of the lipid at the surface.
Keywords: Interfacial amyloid facilitators; Lipid as inducer of amyloids; Melittin-amyloid formation; Melittin-lipid interaction.
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