Investigation of molecular structures and adsorption mechanisms of phosphonodipeptides by surface-enhanced Raman, Raman, and infrared spectroscopies

Fourier-transform Raman (FT-RS), infrared (FT-IR), and surface-enhanced Raman scattering (SERS) spectra of l-Phe-l-Ala-PO3H2 (MD1), l-Phe-l-Val-PO3H2 (MD2), l-Phe-β-Ala-CH(OH)-PO3H2 (MD3), and l-Phe-l-Ala-CH(OH)-PO3H2 (MD4) phosphonodipeptides adsorbed on colloidal silver surface have been measured. The respective vibrational band assignments have been proposed. The analysis of the SERS spectra shows that these peptides interact with the silver surface mainly through the aromatic ring of the Phe residue. In the case of MD1 and MD2 the aromatic ring seems to be almost parallel to the metal surface, while for MD3 and MD4 it links to the metal in perpendicular orientation. The analysis of the relative band intensities suggests a contribution from the CE (chemical-enhancement) mechanism to the MD2 SERS spectra. Additionally, characteristic vibrations of the phosphonate, amino, and methane groups to the SERS spectra indicate that these groups are involved in interaction with the silver surface. We suggest that the O-P-O fragment of MD3 and MD4 and the PO fragment of MD1 and MD2 interact with the silver surface.