Engineering pH-responsive switching of donor-π-acceptor chromophore alignments along a peptide nanotube scaffold

Abstract

A cyclic tri-β-peptide cyclo(β-Ala-β-Ala-β-Lys) having diethylaminonaphthalimide at the β-Lys side chain (CP3Npi) self-assembled into a peptide nanotube in a solution of HFIP and water. CD spectra of the CP3Npi nanotubes show a negative Cotton effect at 441 nm and a positive Cotton effect at 393 nm, indicating that D-π-A naphthalimide chromophores are aligned in a left-handed chiral way along the nanotube. The CP3Npi nanotubes bear positive charges under acidic conditions retaining the nanotube structure but pH-responsive switching of D-π-A naphthalimide alignments along the nanotube between a left-handed chiral and random arrangement was observed. The peptide nanotube is a stable scaffold for attaining pH-responsive alignment switching of side-chain chromophores.

Fig. 1. (A) Chemical structures of CP3Npi. (B) Schematic illustration of peptide nanotube composed of CP3Npi. (C) Chemical structures of reference compounds.

Fig. 2. (A) The DLS profile of CP3Npi suspension prepared from CP3NPi in HFIP/water (1/39, v/v). (B) The ATR-FTIR spectrum of CP3Npi suspension prepared from CP3NPi in HFIP/water (1/39, v/v). (C) AFM image and height profile on silicon wafer of peptide nanotube prepared from CP3NPi in HFIP/water (1/39, v/v).

Fig. 3. (A) Normalized UV-vis absorption spectra of CP3Npi and compound Ref1 in HFIP and HFIP/water (1/39, v/v). (B) CD spectra of CP3Npi in HFIP and HFIP/water (1/39 v/v).

Fig. 4. (A and B) Normalized UV-vis absorption and CD spectra of CP3Npi in HFIP/HCl aq. with various concentration of HCl (0.5, 1, 5, 10, 50, 100, 500, 1000 mM). (C) pH-Responsive UV, CD and DLS cycle profiles. (D) Schematic illustration of pH responsive behavior of CP3Npi peptide nanotube.