Fine-Tuning and Enhancement of pH-Dependent Membrane Permeation of Cyclic Peptides by Utilizing Noncanonical Amino Acids with Extended Side Chains

The development of cyclic peptides that exhibit pH-sensitive
membrane
permeation is a promising strategy for tissue-selective drug delivery.
We investigated the pH-dependent interactions of designed cyclic peptides
bearing noncanonical amino acids of long acidic side chains with lipid
membranes, including surface binding, insertion, and translocation
across the membrane. As the length of the side chain of acidic amino
acid increased, the binding affinity of the peptides to phosphatidylcholine
bilayer surfaces decreased, while the pH for the 50% insertion of
the peptides into the bilayers increased. The pH for membrane permeation
of the peptides increased with the side chain length, resulting in
specific membrane permeation at pH ∼6.5. The longer side chain
of acidic amino acids improved the maximum rate of membrane permeation
at low pH, where both entropic and enthalpic contributions affected
the permeation. Our peptide also showed intracellular delivery of
cargo molecules into living cells in a pH-dependent manner.