A 26-residue peptide containing six histidine (H) residues showed a pH-dependent interaction with a membrane. At a basic pH, where the H residues were deprotonated, the peptides formed a helical structure and adopted a transmembrane alignment in the lipid vesicles. At an acidic pH, where the H residues were protonated, they exhibited a mixture of beta and random forms and were aggregated or dissolved in the aqueous phase.