SERS investigation of NAD+ adsorbed on a silver electrode

The surface-enhanced Raman scattering (SERS) spectra of nicotinamide adenine dinucleotide (NAD(+)) at different pH values and electrode potentials were investigated at a silver electrode. For comparison purposes, the spectra of adenine, N-methylnicotinamide, and nicotinamide mononucleotide were also recorded. Good quality spectra are reported, and NAD+ adsorbs spontaneously onto the electrode at all of the pH values (9.3-3.6) studied. Normal mode calculations of the in-plane vibrations of adenine were performed to assist in band assignments and determination of adsorbate orientation. These calculations utilized Urey-Bradley force field calculations and the PM3 semiempirical molecular orbital method. From these calculations, it is concluded that at physiological pH values NAD(+) adsorbs on the silver electrode in a conformer in which the adenine ring binds to the surface through the external amino group on carbon 6 and through nitrogen 7. The ribose and phosphate parts of the molecule are also involved in adsorption. In the surface conformer NAD(+) is in an extended configuration with the adenine ring oriented normal (side on) with respect to the surface and the nicotinamide ring oriented flat with respect to the, surface much like in an enzyme bound case.