Self-assembling Peptides with Internal Ionizable Unnatural Amino Acids: A General Approach to pH-responsive Peptide Materials

Self-assembled peptides are an emerging family of biomaterials that show great promise for a range of biomedical and biotechnological applications. Introducing and tuning the pH-responsiveness of the assembly is highly desirable for improving their biological activities. Inspired by proteins with internal ionizable residues, we report a simple but effective approach to constructing pH-responsive peptide assembly containing unnatural ionic amino acids with an aliphatic tertiary amine side chain. Through a combined experimental and computational investigation, we demonstrate that these residues can be accommodated and stabilized within the internal hydrophobic compartment of the peptide assembly. The hydrophobic microenvironment shifts their pK_a significantly from a basic pH typically found for free amines to a more biologically relevant pH in the weakly acidic range. The pH-induced ionization and ionization-dependent self-assembly and disassembly are thoroughly investigated and correlated with the biological activity of the assembly. This new approach has unique advantages in tuning the pH-responsiveness of self-assembled peptides across a large pH range in a complex biological environment. We anticipate the ionizable amino acids developed here can be widely applicable to the synthesis and self-assembly of many amphiphilic peptides with endowed pH-responsive properties to enhance their biological activities toward applications ranging from targeted therapeutic delivery to proton transport.