Investigation of vibrational changes due to adsorption of glycine on gold

The affinity of glycine to gold nanoparticles in aqueous environments is of importance in a wide range of applications including protein engineering and wet chemical synthesis. In an aqueous environment, glycine exists predominantly as a zwitterion, however, the adsorption of zwitterionic glycine on gold and its impact on the vibrational spectra has not been well studied. This work uses density functional theory to investigate the adsorption of neutral and zwitterionic glycine molecules on two-, four-, six- and 20-atom gold clusters. The neutral glycine molecule preferentially binds to gold via the amine group rather than via the carboxyl group, with interaction energies ranging from 21 down to ≈11 kcal mol ⁻¹ for two to 20 atom gold clusters, respectively. For the zwitterion in an implicit solvent medium, binding can only occur via the carboxyl group. The interaction energy ranges from 23 to 13 kcal mol ⁻¹ for two to 20-atom gold clusters, respectively. Bader charges analysis and charge difference density maps show that electronic charge is transferred to the gold cluster. Vibrational frequencies and IR intensities were calculated for neutral and zwitterionic glycine and for all binding configurations on the gold clusters. The IR spectra of both neutral and zwitterion glycine molecules show the sharpest peak for C–O stretching modes. This C–O stretching mode redshifts by around 100 cm ⁻¹ neutral glycine binds via carboxyl group. In the case of the zwitterion, the sharpest peak remains at the same position after adsorption. The size of the gold cluster has a negligible impact on the vibrational IR spectra for both neutral and zwitterion glycine molecules.