Determination of the orientation of 6-mercaptopurine adsorbed on a silver electrode by surface-enhanced Raman spectroscopy and normal mode calculations

6-Mercaptopurine, 6MP, is a sulfur analogue of adenine which is commonly used as an antitumor agent. The coordination chemistry of 6MP with metals is of interest because the drug contains a number of electronically versatile binding sites. In this study the interaction between 6MP and a silver electrode surface was investigated using surface-enhanced Raman spectroscopy (SERS). High-quality solution Raman spectra of 6MP along with surface-enhanced Raman scattering spectra of 6MP adsorbed on a roughened silver electrode surface are presented. Urey-Bradley force field and semiempirical calculations with the PM3 method were used to generate vibrational frequencies for 6MP based on a specific set of possible molecular interaction sites. An error analysis of the calculated frequencies and the experimentally determined SERS frequencies allowed for the identification of the experimental data which most closely matched the calculated SERS data. It was concluded that 6MP attaches head-on through the N1 atom when the molecule is adsorbed onto a silver electrode surface.