An intermittent amyloid phase found in gemini (G5 and G6) surfactant induced β-sheet to α-helix transition in concanavalin A protein

Abstract

Surfactant-induced amyloid fibrillation has many implications in laboratory and industry. Previously, cetyltrimethylammonium bromide (CTAB; C19H42BrN) induced amyloid formation in concanavalin A (ConA) has been reported by authors (Khan, JM et al., 2016 A). In this work, cationic gemini surfactants pentanediyl‑1,5‑bis (dimethylcetylammonium bromide); 16-5-16 (G5), and hexanediyl‑1,6‑bis (dimethyl cetyl ammonium bromide); 16-6-16 (G6) were found to induce amyloid-like aggregation in ConA as studied with turbidity at 350 nm, Rayleigh scattering and Thioflavin T (ThT) binding assay and Far UV CD. The sizes of the aggregates were characterized with dynamic light scattering (DLS) and atomic force microscopy (AFM). Hydrodynamic radii (Rh) of the aggregates were found to be 74 nm and 122 nm with 50 μM of G5 and G6 respectively. Even at 7.5 μM, gemini surfactants seems disrupting hydrophobic pockets of ConA and result in aggregation. On the contrary, aggregation disappeared around 250 μM surfactant concentration with β-sheet → α-helix transition. The re-establishment of intra-molecular hydrogen bonding may have resulted in the formation of non-native α-helical structures. Alcohols, predominantly 2,2,2‑trifuloroethanol (TFE), have been reported to induce α-helices in β-sheet proteins by solvent engineering. There are very few reports of β-sheet to α-helix transition induced by non-alcoholic substances. Our lab is pioneer in reporting surfactant (CTAB) induced β-sheet to α-helix transition in ConA (Khan, JM et al., 2016 A). This is the first report of G5 and G6 gemini surfactant induced β-sheet to α-helix transition, as per our knowledge. ConA being apoptotic to cancerous cells and having potential therapeutic effect on hepatoma, the observations done in this work could be of immense interest in vesicular delivery of ConA.