Fourier transform infrared reflection absorption spectroscopy (FT-IRAS) of some biologically important molecules adsorbed on metal surfaces

Abstract

The technique of Fourier transform infrared reflection absorption spectroscopy (FT-IRAS) has been used to characterize thin films of proteins and amino acids formed on metal surfaces. The films were spontaneously adsorbed from solution onto each metal. All infrared spectra were taken on dried films. The infrared spectra were obtained by reflecting the radiation from the film covered surface at a high angle of incidence (∼75°) and with the radiation polarized parallel to the plane of incidence.

The IRAS spectra of fibrinogen adsorbed on gold, copper, aluminium and titanium all showed a large blue shift of the Amide I frequency ca. 20 cm⁻¹, relative to the same band in the aqueous and solid phases. Similar shifts were also observed for lysozyme and pepsin adsorbed on these metal surfaces. An even larger shift ca. 32 cm⁻¹ was observed for β-lactoglobulin adsorbed on gold and titanium. These distinct shifts indicate that conformational change of the proteins occurs upon adsorption on metal surfaces, however, other alternative explanations for these blue shifts are also discussed.

In addition, model experiments of the smallest amino acid glycine on gold indicate that the molecule, in its zwitterionic form, adsorbs with a preferred orientation on the surface such that the NH₃ ⁺ group is close to the gold surface.