Solvent modulated structural transition of self-assemblies formed by bola-form hexapeptide amphiphiles

Abstract

Peptide molecules can self-assemble to form one-dimensional nanostructures, and the formation of these structures is closely related to the delicate balances of different non-covalent interactions. These non-covalent interactions can be finely tuned by changing the molecular structure and the external conditions. Among them, the introduction of organic solvents to peptide based systems is a simple and effective way for regulating the aggregate structure. In this study, we used three bola-form hexapeptide molecules Ac-KI4K-NH2, Ac-RI4R-NH2 and Ac-HI4H-NH2 with different hydrophilic amino acids as models to investigate the effect of methanol on the morphology of the self-assemblies. The self-assembly behavior of these peptides in methanol was thoroughly investigated by a combination of transmission electron microscopy (TEM), atomic force microscopy (AFM), circular dichroism spectroscopy (CD), and Fourier transform infrared spectroscopy (FTIR). The results demonstrated that both the solvent methanol and the hydrophilic amino acids in the peptides have a great impact on the morphologies of the self-assemblies. Ac-KI4K-NH2 self-assembled into thin nanofibers in methanol. In contrast, Ac-RI4R-NH2 formed a monolayer helical ribbons with an obvious dynamic process initiating from thin twist nanofibers, then thin helical ribbons, and finally evolved to wide helical ribbons. The dominant nanostructures for Ac-HI4H-NH2 were multi-layer flat ribbons but with a narrower width. These morphologies formed in methanol were significantly different from those formed in water, due to the weakened electrostatic, hydrogen bonding, and hydrophobic interactions between peptides or peptides and methanol. The introduction of methanol creates a completely hydrophobic environment and changes the non-covalent interactions within β-sheets, thus resulting a thin width. A gel phase with superior viscoelasticity was formed by Ac-KI4K-NH2 in methanol and it may have applications in various fields of chemistry, biomedicine, environment and electricity. These results can help us understand the roles of organic solvents in controlling the delicate balance of different non-covalent interactions as well as the final aggregate morphologies and may benefit further research in establishing the relationship among peptide solvent, peptide molecular structure, and the final aggregate morphology.