Ionic and tautomeric conformers of adenine at different pH investigated by Raman spectroscopy combined with DFT calculations

Raman spectroscopy and density functional theory studies have been done to determine the ionic and tautomeric conformers of adenine (Ade) in aqueous solution at pH 1.5 (acidic), 7.0 (neutral) and 12.5 (alkaline). The experimental Raman spectra of the aqueous solution of Ade have been recorded for the spectral range of 500-1800cm(-1). As expected, the recorded Raman spectra of the aqueous solution of Ade at different pH have noticeable differences. The change in the spectral features of Ade at different pH is a result of the change in structure (neutral, cation and anion) of Ade in the aqueous solution. The shifting in the wavenumber position of the Raman bands, along with the change in relative intensity and appearance of new bands with the change of pH, has been explained. In order to determine the structure of Ade at various pH, theoretical Raman spectra, corresponding to the most probable structure of Ade at that particular pH, are calculated by density functional theory-based explicit model. The Raman bands observed at each pH of the aqueous solution of Ade are assigned with the help of a potential energy distribution calculation. The most plausible ionic and neutral conformers of Ade in aqueous solution at pH 1.5, 7.0 and 12.5 are determined by comparing the experimental and theoretical spectra. From this, it is concluded that at neutral pH (7.0), Ade-N9 and Ade-N7 tautomers of Ade predominantly exist along with some signature of imine form of Ade in the aqueous solution. However, single-protonated and double-protonated forms of Ade can exist simultaneously in the aqueous solution at pH 1.5. As per expectation, the anionic form of Ade has been found at alkaline pH (12.5). Copyright (c) 2016 John Wiley & Sons, Ltd.