Steric forces between brush layers of poly( l-glutamic acid) and their dependence on secondary structures as determined by FT-IR spectroscopy

Abstract

Interactions between apposed poly( l-glutamic acid) monolayers in water were investigated by direct surface forces measurement. The average degree of polymerization was 21. The monolayers of anchored poly( l-glutamic acid) were prepared on mica surfaces by the Langmuir-Blodgett deposition of monolayers of an amphiphile containing the polypeptide as the hydrophilic head group. Various secondary structures of poly( l-glutamic acid) were formed in this monolayer at the air-water interface, through intra- and intermolecular hydrogen bonding, depending on the surface pressure and the pH value of the aqueous subphase, and transferred onto mica surfaces. Formation of the secondary structures was characterized by Fourier transform-infrared (FT-IR) spectroscopy prior to the forces measurement. Surface force profiles consisted of a long-range electrostatic and a short-range steric repulsion. Secondary structures of poly( l-glutamic acid), identified by FT-IR with respect to their composition and orientation, were found to determine the short-range steric repulsion. The obtained elastic compressibility moduli of the polypeptide monolayers were 5.6 ± 2.2 MPa for the β 2-structure rich layer, and 0.2 ± 0.1 MPa for the layer of ionized extended chains.