Guanine adsorption on gold electrodes studied by in situ surface-enhanced infrared reflection absorption spectroscopy

Abstract

The adsorption of guanine on single-crystal and thin-film gold electrodes has been explored by combination of cyclic voltammetry and ATR-SEIRA ‘in situ’ spectroscopy at two pD values (8 and 11.6) employing D2O as a solvent. The experimental conditions are selected to study the adsorption / desorption of the two forms of guanine involved in the second acid-base equilibria and its tautomeric forms.The ATR-SEIRA spectra of adsorbed guanine were compared to the absorption FT-IR spectra in solution and interpreted in light of DFT calculations of the different acid-base and tautomeric forms with different solvation states of the CO group. Furthermore, the preponderance of each tautomeric form and the possible changes in orientation with the electric field have been determined by analyzing the two-dimensional correlation spectrum (2D COS) as well as the influence of the potential on the integrated intensities of the spectral signals in the 1800–1400 cm−1 region. It has been found that at pD 8 the adsorbed keto-amino tautomer of neutral guanine predominates in solution and in adsorbed state, and adsorbed guanine slightly rotate its molecular plane with the potential. At pD 11.6 the keto-amino tautomer of anionic guanine is the unique form detectable in the solution, while in adsorbed state the keto-imino tautomeric form is also present with its preponderance increasing with the electrode potential.