Adenine− and Adenosine Monophosphate (AMP)−Gold Binding Interactions Studied by Surface-Enhanced Raman and Infrared Spectroscopies

Understanding the interactions of biomolecules with noble metal surfaces is critical to our development of functional biomedical nanodevices and accurate biosensors. Here we use surface enhanced Raman spectroscopy (SERS) and surface enhanced infrared absorption spectroscopy (SEIRA) on Au nanoshell substrates to study the interactions of adenine and two adenine derivatives, thiolated polyadenine single-stranded DNA (polyA) and adenosine monophosphate (AMP), with Au surfaces. pH-dependent conformational changes of these molecular species adsorbed on Au nanoshell surfaces were observed using SERS, and confirmed with SEIRA. The combined SERS-SEIRA spectra show significant pH dependence, consistent with adenine protonation and reduced Au−adenine binding at low pH. The spectra are also consistent with adenine binding “end-on” to the Au surface via a ring nitrogen, with the bond to the external NH₂ group aligned near the surface normal. For AMP, spectral evidence indicates binding through either a ring nitrogen and/or the external NH₂ group. Density functional calculations on adenine and comparisons with the literature allow us to assign the observed spectral features and to gain insight to the local binding geometry of the adsorbates.