Direct Evidence of Controlled Structure Reorganization in a Nanoorganized Polypeptide Multilayer Thin Film

Abstract

Aspects of poly(l-lysine) (PLL) and poly(l-glutamic acid) (PLGA) multilayer thin film assembly, physical properties of the films, and changes in film structure on pH shift have been studied by far-UV circular dichroism spectroscopy (CD), ultraviolet spectroscopy (UVS), ellipsometry, quartz crystal microbalance (QCM), and atomic force microscopy (AFM). We show that CD can be used to assess the secondary structure content of a polypeptide multilayer film deposited on quartz substrates by electrostatic layer-by-layer assembly (LbL). Measurements have revealed that, under conditions where PLL and PLGA displayed a random coillike conformation in solution, the polypeptides formed predominantly β-sheet structures in multilayer films fabricated by LbL. A substantial conformational change occurred on exposure of such films to a strongly acidic (pH ≤ 2.5) or strongly basic (pH ≥ 12.0) aqueous medium, from β-sheet to predominantly α-helical structure. pH shift thus may be a useful postpreparation approach for stimuli-responsive modification of the surface roughness, porosity, and permeability of preassembled polypeptide films or structures made from such films, e.g., microcapsules.