In situ Fourier transform infrared reflection absortion spectroscopy (FT-IRRAS) has been used in the external
(SNIFTIRS method) and the internal(ATR-SEIRAS) reflection configurations to determine the pH influence,
in the neutral and basic range, on the adsorption of adenine on Au(111) and gold nanofilm electrodes
from D2O and H2O solutions.
In D2O solutions, the main adsorbate band around 1640 cm−1, due to a ring stretching mode, shows
different characteristics in the spectra collected at pH values at which the neutral and the basic adenine
forms predominate in solution. The analysis of these differences, in comparison with the respective
spectra of adenine in solution, permits us to conclude that both forms of adenine can adsorb chemically.
The high sensitivity ofthe ATR-SEIRAS method has been used to analyze the contribution to the spectra of
each form of adsorbed adenine as a function of the pH of the solution. The pKa2 obtained for the adsorbed
species from this analysis is almost coincident with the pKa2 reported for adenine in solution, indicating
thatthe coordinationto the electrode andthe secondacid-base equilibriuminvolvesdifferent atoms ofthe
adenine molecule. This result confirms the previously proposed adsorption model for adenine, implying
the bonding of adenine to the electrode by the amine nitrogen (N10) and either the ring nitrogens N1 or
N7, while the second acid-base equilibrium of adenine involves the ring nitrogen N9.
Comparison of the 3400-3600 cm−1 region of the ATR-SEIRAS spectra of adenine obtained in H2O solutions
at different pH values, which corresponds to the characteristic–OH stretching mode ofthe interfacial
water molecules, permits us to discard the co-adsorption of water molecules in neutral and basic media,
contrary to the case of adenine adsorption from acid media.
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