In situ surface enhanced infrared absorption spectroscopy study of the adsorption of cytosine on gold electrodes

Abstract

The structural facilities provided by the ‘in situ’ Surface-Enhanced Infrared Absorption Spectroscopy in the Attenuated Total Reflection mode (ATR-SEIRAS) are exploited for the characterization of cytosine adsorption on gold thin-film electrodes from acid, neutral and basic solutions, using H2O and D2O as solvents. The experimental conditions are selected in order to consider the two forms involved in the first acid-base equilibrium and in the tautomeric keto-enol and amino-imino equilibria of the molecule. The results are compared with the corresponding absorption FT-IR spectra of cytosine in solution. DFT calculations are used to optimize the molecular geometries and to estimate the vibrational properties of the different acid-base and tautomeric forms, either in solution or in the adsorbed state on gold surfaces.

The comparison between the spectra in solution and in the adsorbed state indicates the adsorption of the deprotonated form even at pH bellow the first pKa of cytosine. Therefore, only the tautomeric forms of the deprotonated cytosine are considered: the keto-amino (C1 and C4), the enol-amino (C2) and the keto-imino (C3) tautomers of cytosine and of deuterated cytosine molecules.

Interpretations of the absorption FT-IR spectra of the molecules in solution based on DFT calculations suggest the preponderance of the C1 tautomer in solution but with contributions of the C3 tautomer, in neutral and basic media. However, the interpretations of the ATR-SEIRAS experiments show the preponderance of tautomer C3 in the adsorbed state with contributions of tautomer C1 depending not only on the pH of the solution, but also on the potential applied to the electrode.