Effect of side chain structure on aggregation state and mechanical properties of synthetic polypeptide monolayers at the air-water interface

Aggregation structure of poly(γ-methyl-L-glutamate)(PMLG) and poly(γ-n-hexyl-L-glutamate)(PnHLG) monolayers at the air-water interface was investigated on the basis of the transmission electron microscopic observation. The bright field image of PMLG and PnHLG monolayers exhibited a homogeneous structure at surface pressure of 18 mN·m^-1 and 15 mN·m^-1, respectively. The bright field image of PMLG monolayers showed linear humps at high surface pressure, suggesting the collapse of monolayers and the orientation of α-helix axis perpendicular to the compression direction. Surface pressure anisotropy was investigated by means of orthogonal Wilhelmy plate method. The surface pressure anisotropy for PMLG was greater than that for PnHLG. Dynamic viscoelastic properties of polypeptides monolayers at the air-water interface were examined on the basis of transient response of surface pressure. The magnitude of dynamic storage modulus E' of the PnHLG monolayer was lower than that of PMLG monolayer. This is ascribed to the active side chain motion of PnHLG at room temperature. The mechanical properties and aggregation structure of polypeptide monolayers were revealed to depend on their thermal molecular motion of side chain.