Chemoresponsive surface-tethered polypeptide brushes based on switchable secondary conformations

Abstract

Surface-tethered chemoresponsive polypeptides prepared by surface-initiated vapor deposition polymerization (SI-VDP) were used to investigate the conversion efficiency between α-helical and β-sheet conformations of poly(γ-methyl-L-glutamate) (PMLG) homopolypeptide and PMLG-b-poly(β-benzyl-L-glutamate) (PBLG) copolypeptide brushes. The end-grafted PMLG brush predominantly adopted a β-sheet conformation after exposure to formic acid vapor, and it was converted back to a predominantly α-helical conformation after treatment with a mixture of dichloroacetic acid (DCA) and chloroform (CHCl₃). This conformational inter-conversion process was reversible and repeatable for a number of cycles. Factors that affected the conversion efficiency between α-helix and β-sheet were studied, including PMLG chain mobility, concentration of DCA, and temperature of formic acid vapor. For end-grafted PMLG brushes with one end chain restriction, 60% conversion efficiency from α to β was achieved with 40 °C formic acid vapor treatment for 2 days, while for the PMLG in end-grafted PMLG-b-PBLG brushes, 40% conversion was achieved after 9 days, which was slower than end-grafted PMLG brushes due to the additional chain restriction from the PBLG block. In comparison, in spin-coated PMLG films in which the PMLG chains had no end chain restrictions, 80% conversion was achieved within 2 days in the same condition. The effects of solvent on the morphologies of surface-grafted PMLG brushes were also observed. The changes of the properties of these films, including surface wettability, thickness, and surface morphologies, were characterized by contact angle measurement, ellipsometry, and atomic force microscopy, respectively.