A pH dependent surface-enhanced Raman scattering study of hypoxanthine

Surface-enhanced Raman scattering (SERS) of hypoxanthine (HX) molecule on s ilver colloid confirms that the molecule in the surface adsorbed state exis ts 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 ca rbonyl stretching mode at 1690 cm(-1), a medium-intense in-plane C=O bendin g mode at 655 cm(-1) and disappearance of the C6-O6 H1 stretching mode in S ER 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 spect ra further infer two modes of orientation of the HX molecule on the silver surface. At acidic pH (pH 2) the EM molecule is adsorbed on the metal surfa ce through the N3 atom with the molecular plane lying almost perpendicular on the silver substrate, while at alkaline pH the molecule is oriented near ly parallel to the silver surface via nonbonding electrons of N1, N3, N7 an d O6. Copyright (C) 2000 John Wiley & Sons, Ltd.