Surface-Enhanced Raman Evidence of Protonation, Reorientation, and Ag+ Complexation of Deoxyadenosine and Deoxyadenosine-5′-Monophosphate (dAMP) on Ag and Au Surfaces

Surface-enhanced Raman (SERS) spectra of deoxyadenosine and 5′-dAMP on Ag and Au surfaces showed the protonation of both compounds in the N1 position, their orientation geometry on metal surfaces, and the formation of Ag + complexes at alkaline pH on hydroxylamine-reduced Ag colloids. Interestingly, substitution at the N9 position caused dramatic changes in the relative band intensities within the spectra of both deoxyadenosine and 5′-dAMP compared to that of simple adenine, although they continued to be dominated by adenine vibrations. Concentration-dependent spectra of 5′-dAMP were observed, which matched that of adenine at high concentrations and that of deoxyadenosine at lower concentration (<10 –4 M). This indicates that 5′-dAMP undergoes reorientation from flat to a more perpendicular orientation as the concentration increases. The result of these effects is that spectra of three different forms of deoxyadenosine and five of 5′-dAMP can be observed, depending on the exact experimental conditions used. The most highly scattering form is the Ag + complex, so selection of conditions that give this form dramatically increases the detection sensitivity. Finally, these data are important for correctly interpreting SERS spectra of adenine-containing DNA and RNA sequences.