Micellization of surfactin and its effect on the aggregate conformation of amyloid beta(1-40)

The aggregation of amyloid beta-peptide (Abeta(1-40)) into fibrils is a key pathological process associated with Alzheimer's disease. This work has investigated the micellization process of biosurfactant surfactin and its effect on the aggregation behavior of Abeta(1-40). The results show that surfactin has strong self-assembly ability to form micelles and the micelles tend to form larger aggregates. Surfactin adopts a beta-turn conformation at low micelle concentration but a beta-sheet conformation at high micelle concentration. The effect of surfactin on the Abeta(1-40) aggregation behavior exhibits a strong concentration-dependent fashion. Below the critical micelle concentration of surfactin, the electrostatic binding of surfactin monomers on Abeta(1-40) causes Abeta(1-40) molecules to unfold. Assisted by the hydrophobic interaction among surfactin monomers on the Abeta(1-40) chain, the conformation of Abeta(1-40) transfers to the beta-sheet structure, which promotes the formation of fibrils. At low surfactin micelle concentration, besides the electrostatic force and hydrophobic interaction, hydrogen bonds formed between surfactin micelles and adjacent Abeta(1-40) peptide chains may promote the ordered organization of these Abeta(1-40) peptide chains, thus leading to the formation of beta-sheets and fibrils to a great extent. At high surfactin micelle concentration, the separating of Abeta(1-40) chains by the excessive surfactin micelles and the aggregation of the complexes of Abeta(1-40) with surfactin micelles inhibit the formation of beta-sheets and fibrils.