In situ surface-enhanced infrared spectroscopy study of adenine-thymine co-adsorption on gold electrodes as a function of the pH

In situ attenuated total reflection surface enhanced infrared absorption spectroscopy (ATR-SEIRAS) has been used to study the co-adsorption of adenine and thymine on gold nanostructured electrodes from D2O and H2O solutions at acid, neutral and basic pH values. The comparison between the ATR-SEIRA band when adenine (A) and thymine (T) are co-adsorbed and the corresponding bands of both bases adsorbed individually has revealed the existence of interactions between the complementary bases at the gold-electrolyte interface, at acid, neutral and basic pH values. The most significant bands of adsorbed adenine undergo an enhancement in the presence of thymine. This can be explained if adenine adopts an orientation with the molecular plane more normal to the electrode surface and, at least at neutral pH, the surface concentrations of adenine increases in the presence of thymine. In the presence of adenine, the characteristic absorption bands of adsorbed thymine nearly disappear at potentials of the chemisorption region of thymine, suggesting the orientation of thymine molecular plane from normal to nearly parallel relative to the electrode surface, after the addition of adenine. The comparison of the spectral regions at 2900-3800 cm(-1) of adsorbed adenine in H2O solutions in the absence and in the presence of thymine only reveals the enhancement mentioned above. The bands corresponding to the NH2 stretching modes of adenine do not undergo any significant frequency shift, which permit us to discard the participation of adsorbed adenine in hydrogen bonding interactions (A-T) of the Watson-Crick or of the Hoogsteen types. Cooperative interactions including pi interactions A-T of the "edge to face" type and coplanar hydrogen bonding T-T are proposed to be responsible of the long range organization of the adlayer formed by the coadsorbed bases.