Surface-enhanced Raman spectroscopy of ellipticine, 2-N-methylellipticinium and their complexes with DNA

Surface-enhanced Raman spectroscopy (SERS) in silver colloidal solutions of ellipticine, 2-N-methylellipticinium and their complexes with DNA were investigated. On varying the pH of silver sols, large spectral changes are observed in the SER spectra of ellipticine which are related to the reversible transformation between the neutral and protonated forms. From intensity measurements the protonation pK of ellipticine absorbed on silver collo ds was estimated (ca. 7.5). This value is very close to that previously obtained in a pure aqueous medium from UV-visible absorption and fluorescence data. This indicates that the interstion of ellipticine with the silver surface does not perturb the protonation equilibrium at N-2. The intensity changes observed in the SER spectrum of the ellipticine-DNA complex at pH 8.5 are interpreted as arising from a shift of the protonation equilibrium in the presence of DNA (pKa ≈ 8.6) and not from perturbations induced by intercalation of the drug between base pairs. The results of this study suggest that adsorption of ellipticine (and the ellipticinium derivative) on silver colloids takes place through the indole moiety with the chromophore plane perpendicular to the silver surface.