Second-harmonic generation optical activity of a polypeptide α-helix at the air∕water interface

Abstract

Quantitative measurements of second-harmonic generation optical activity (SHG-OA) have been performed for α -helical polypeptides poly-( γ -benzyl- L -glutamate) and poly-( γ -ethyl- L -glutamate) adsorbed at the air∕water interface, with the fundamental frequency ℏ ω = 2.96 eV ( λ = 417 nm ) . The chiral component of the nonlinear susceptibility χ X Y Z ( 2 ) is small for both polymers, being comparable in magnitude with the susceptibility χ X X Z ( 2 ) of the clean air∕water interface. The microscopic origin of the nonlinear response has been investigated by using semiempirical ZINDO∕S calculations in conjunction with standard time-dependent perturbation theory to evaluate the molecular hyperpolarizability tensor of a model α -helix composed of glycine residues. Calculated nonlinear susceptibilities (per monomer unit) are in good agreement with experimental measurements for both the chiral and achiral response. The computational results indicate that charge transfer transitions of the α -helix have a large influence on the achiral components of the hyperpolarizability tensor, and produce characteristic features in the response under suitable experimental conditions. The dominant origin of SHG-OA for the model α -helix is a structural effect due to the tilt of the plane of each amide group of the helix relative to the helical axis. SHG-OA is associated with the orientational distribution of isolated, achiral chromophores, and is present in the absence of electronic coupling between the amide subunits of the polypeptide α -helix.