SURFACE-ENHANCED RAMAN-SPECTROSCOPY OF ELLIPTICINES ADSORBED ONTO SILVER COLLOIDS

Surface-enhanced Raman spectra (SERS) of ellipticines (antitumor drugs) and their complexes with DNA, adsorbed on silver sols at various pH, were investigated. A rough assignment of the Raman modes, based on the model chromophores indole and isoquinoline, and deduced from the comparison between SER spectra onto colloids with near-IR FT Raman spectra of the powder, is proposed. The SER spectra of ellipiticine are very sensitive to the pH of the aqueous colloidal solution, particularly the skeletal modes related to the indole moiety. This reflects that the indolic moiety of the ellipticine molecule is close to the silver surface and expresses the important delocalization of the pi-electron system. Ellipticines either neutral or protonated bind DNA by a pure intercalation process and the SER spectra of the ellipticine-DNA complexes have been observed to be very similar to those of the unbound molecules except for a rather important decrease of the overall intensity. This decrease could be attributed either to a screening effect due to the polyelectrolytic character of DNA or to a distance effect from the silver surface upon intercalation. All the results of this study seem to indicate that the ellipticine chromophore plane is perpendicular to the Ag surface with its long side significantly tilted with respect to this surface.