Abstract
Although many reports have described protein-surfactant interactions but the mechanism of amyloid-like fibril formation by surfactants is not clear. In this study, we investigated the role of sodium dodecylbenzene sulfonate (SDBS) to induce aggregation in equine heart muscle myoglobin (E-Mb) at pH 4.5. The aggregation was analyzed by Rayleigh scattering (RLS), turbidity, Thioflavin-T binding (ThT), and Congo red (CR) binding, circular dichroism spectroscopy and transmission electron microscopy (TEM). Turbidity and RLS analysis demarcated the aggregate were formed at effective range of SDBS concentration at pH 4.5, this led to the observation that at low concentrations (0.05 mM-1.2 mM) SDBS cause aggregation of E-Mb monomers, in contrast at higher SDBS concentrations the aggregates were not formed. The following observations were further supported through ThT and CR binding results suggests that SDBS-induced E-Mb aggregates have amyloid-like structure. Far-UV CD results for aggregated E-Mb also displayed single negative peak between 222-235 nm, which is a characteristic of cross-beta-sheet. However, in the presence of high concentration of SDBS, the non-aggregated state of E-Mb obtained consisted of mixed alpha-helices and beta-sheets. SDBS-induced aggregates had amyloid-like morphology as confirmed by TEM. These results indicate that both electrostatic forces and hydrophobic interactions contribute towards SDBS-induced E-Mb aggregates formation with a characteristic amyloid-like structure. Also, these findings have significant implications for understanding mechanisms underlying amyloid fibril formation by anionic surfactants.