Exploring guanine-cytosine co-adsorption on gold electrodes in basic and neutral media: In situ insights from surface-enhanced infrared spectroscopy

In situ Surface-Enhanced Infrared Absorption Spectroscopy in the Attenuated Total Reflection mode (ATR-SEIRAS) has been used to investigate the co-adsorption of guanine (G) and cytosine (C) on gold thin nanostructured gold electrodes at pD 8.0 and 11.6, employing D 2 O as a solvent. At pD 8.0, at which both bases are in neutral form, the comparison between the ATR-SEIRA bands when guanine (G) and cytosine (C) are coadsorbed and the corresponding bands of each base adsorbed individually has revealed the existence of T-shaped pi interactions between the two nucleobases, with guanine almost normal to the electrode surface and cytosine parallel. The Watson-Crick or Hoogsteen hydrogen bond interactions, involving the carbonyls groups of the two nucleobase molecules, do not seem to be relevant. In addition, the co-adsorption of guanine maintains the same proportion of adsorbed cytosine tautomers and its evolution with potential than observed in the absence of guanine. At pD 11.6, at which guanine is deprotonated, the experimental results do not show T-shaped pi interaction between the complementary bases, as was proposed at pH 8, and only the competitive adsorption of both bases that originate a decrease in the ATR-SEIRAS band of each adsorbed base when the complementary one is also adsorbed.