Structure of Phenylalanine Adsorbed on Polystyrene from Nonlinear Vibrational Spectroscopy Measurements and Electronic Structure Calculations

Abstract

We have used nonlinear vibrational spectroscopy to determine the structure of l -phenylalanine adsorbed at the solution−polystyrene interface. Quantitative analysis of nonlinear susceptibility tensor elements was enabled by collecting spectra under various calibrated polarization schemes. We have proposed a technique for fitting the spectra that is well-suited to the challenging line shape resulting from coherent interferences between vibrational modes. Molecular-level vibrational hyperpolarizabilities have then been determined by electronic structure calculations with use of an effective solvent model. These hyperpolarizability values are used to predict spectral amplitudes for comparison with experimental results in order to determine the orientation distribution of the phenylalanine CH 2 plane. We have found that the orientation of phenylalanine is relatively constrained at the polymer surface. Our data and analysis suggest amino acid aromatic rings may be inserted into polystyrene surface aromatic rings to achieve this quasi-immobilization.