Tuning Chelation by the Surfactant-Like Peptide A₆H Using Predetermined pH Values

Abstract

We examine the self-assembly of a peptide A₆H comprising a hexa-alanine sequence A₆ with a histidine (H) “head group”, which chelates Zn²⁺ cations. We study the self-assembly of A₆H and binding of Zn²⁺ ions in ZnCl₂ solutions, under acidic and neutral conditions. A₆H self-assembles into nanotapes held together by a β-sheet structure in acidic aqueous solutions. By dissolving A₆H in acidic ZnCl₂ solutions, the carbonyl oxygen atoms in A₆H chelate the Zn²⁺ ions and allow for β-sheet formation at lower concentrations, consequently reducing the onset concentration for nanotape formation. A₆H mixed with water or ZnCl₂ solutions under neutral conditions produces short sheets or pseudocrystalline tapes, respectively. The imidazole ring of A₆H chelates Zn²⁺ ions in neutral solutions. The internal structure of nanosheets and pseudocrystalline sheets in neutral solutions is similar to the internal structure of A₆H nanotapes in acidic solutions. Our results show that it is possible to induce dramatic changes in the self-assembly and chelation sites of A₆H by changing the pH of the solution. However, it is likely that the amphiphilic nature of A₆H determines the internal structure of the self-assembled aggregates independent from changes in chelation