Second-harmonic generation optical activity of a polypeptide alpha-helix at the air/water interface

Quantitative measurements of second-harmonic generation optical activity (SHG-OA) have been performed for alpha-helical polypeptides poly-(gamma-benzyl-L-glutamate) and poly-(gamma-ethyl-L-glutamate) adsorbed at the airwater interface, with the fundamental frequency variant Planck's over 2piomega = 2.96 eV (lambda = 417 nm). The chiral component of the nonlinear susceptibility chi(XYZ) ((2)) is small for both polymers, being comparable in magnitude with the susceptibility chi(XXZ) ((2)) of the clean airwater interface. The microscopic origin of the nonlinear response has been investigated by using semiempirical ZINDOS calculations in conjunction with standard time-dependent perturbation theory to evaluate the molecular hyperpolarizability tensor of a model alpha-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 alpha-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 alpha-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 alpha-helix.