A pH dependent surface-enhanced Raman scattering study of hypoxanthine

Surface-enhanced Raman scattering (SERS) of hypoxanthine (HX) molecule on silver colloid confirms that the molecule in the surface adsorbed state exists as [keto–N9 H9] form at acidic and normal pH but as [enol–O6 H1, N9 H9] form at alkaline pH. It is substantiated by the appearance of an intense carbonyl stretching mode at 1690 cm−1, a medium-intense in-plane CO bending mode at 655 cm−1 and disappearance of the C6–O6 H1 stretching mode in SER spectra in acidic and normal pH. Strongly intense Raman band at 1166 cm−1 assigned as C6–O6 H1 stretching mode confirms [enol–O6 H1, N9 H9] form in alkaline pH. The presence of Ag–N and Ag–O stretching modes in SER spectra further infer two modes of orientation of the HX molecule on the silver surface. At acidic pH (pH 2) the HX molecule is adsorbed on the metal surface through the N3 atom with the molecular plane lying almost perpendicular on the silver substrate, while at alkaline pH the molecule is oriented nearly parallel to the silver surface via nonbonding electrons of N1, N3, N7 and O6.