Density functional theoretical (DFT) and surface-enhanced Raman spectroscopic study of guanine and its alkylated derivatives Part 2: Surface-enhanced Raman scattering on silver surfaces

The surface-enhanced Raman scattering (SERS) spectra of guanine and its alkylated derivatives 7-methylguanine (7-MeG) and 9-ethylguanine (9-EtG) adsorbed to a silver colloid and a silver electrode are reported. The influence of the pH and the surface potential are studied. The interaction between the three molecules and the silver surfaces is interpreted on the basis of density functional theoretical (DFT) assignments of the SERS bands, the relative enhancement factors of the vibrational modes, the surface selection rules, the pH dependence of the SERS spectra, and a comparison of the SERS spectra of the different molecules. The wavenumber value and SERS intensity of a normal mode dominated by carbonyl stretching, in particular, is shown to provide valuable information concerning the orientation of the adsorbate on silver surfaces. The pKa values of all studied compounds are lowered substantially in the presence of the SERS substrates, so that acidic conditions have to be employed to obtain SERS spectra of neutral molecules. In their neutral forms, guanine and 7-MeG adsorb to both surface substrates with the H2N–C2–N3–C4–N9 side directed towards the surface, whereas 9-EtG is adsorbed via N7 and the carbonyl group. At alkaline pH, the colloidal surface stabilizes the N9-deprotonated tautomer of guanine, while only the N1-deprotonated tautomer is present on the electrode surface and in aqueous solution.