Short communication Evidences of adenine–thymine Interactions at gold electrodes interfaces as provided by in-situ infrared spectroscopy Manuela Rueda a, ⁎, Francisco Prieto a , Julia Álvarez-Malmagro a , Antonio Rodes b a Department of Physical Chemistry, University of Seville, C/ Profesor García González n o 2, 41012 Seville, Spain b Department of Physical Chemistry and Institute of Electrochemistry, University of Alicante, Ap. 99, E-03080, Alicante, Spain abstract article info Article history: Received 25 June 2013 Received in revised form 13 July 2013 Accepted 15 July 2013 Available online 25 July 2013 Keywords: Adenine Thymine Adsorption Gold electrodes ATR-SEIRAS The co-adsorption of complementary DNA bases adenine and thymine on gold thin-film electrodes from 0.1 M HClO 4 solutions in H 2 O and D 2 O is studied by surface-enhanced infrared absorption spectroscopy in the attenu- ated total reflection mode (ATR-SEIRAS). The comparison of the spectra in the range 1750–1550 cm -1 for co-adsorbed adenine and thymine at controlled potentials to those of the individual adsorbed bases shows the enhancement of the signals associated to the vibration modes of adenine and the inhibition of those of thymine. The results can be explained by invoking the rearrangement of both molecules on the electrode surface in order to facilitate the Watson–Crick (W–C) and/or Hoogsteen (HG) interactions between the bases. The co-adsorption seems to be a cooperative process in which a low surface concentration of each base can induce the rearrange- ment of the complementary base molecules on the surface. © 2013 Elsevier B.V. All rights reserved. 1. Introduction Adenine and thymine are complementary nucleic acid bases that provide replication and transcription of the genetic codes in living cells by double H-bonding either in the Watson–Crick (W–C) [1] or the Hoogsteen (HG) [2] configurations (Fig. 1a). The interactions between DNA bases play decisive roles also in enzymatic and protein organization processes. The study of the interactions of these bases at the organized electrode interfaces can be interesting for understanding their activities in biolog- ical media and for biotechnology and nanotechnology applications. In-situ infrared spectroscopy can provide evidences about the dis- position of the molecules on the surface, the coordination sites with the metal and the intermolecular interactions. Particularly, surface- enhanced infrared absorption spectroscopy in the Kretschmann attenu- ated total reflection (ATR-SEIRAS) mode allows high sensitivity studies of adsorbed molecules with little interference from the solution [3]. We have studied adenine adsorption on gold electrodes in previous papers where an adsorption model was proposed consisting of slightly tilted adenine molecules coordinated to the metal by the N atoms of the amino group and by the N 7 atom [4–7]. On the other hand, for thy- mine adsorption on gold electrodes several adsorption states have been described depending on the applied potential [8–10]: a chemisorbed phase, at high potentials, in which the molecules coordinate to the metal with both oxygen atoms and a deprotonated N 3 atom in a verti- cally organisation, stabilized by π-stacking interactions and a condensed but weakly adsorbed adlayer stabilised by H-bonds, at lower potentials. Interestingly, the same atoms of adenine and thymine involved in the chemical interactions with the metal would also be participating in the W–C and HG interactions: N 10 H and N 1 (W–C interaction) or N 7 (HG interaction) of adenine and the oxygen on C 4 and N 3 H of thymine. In this paper the advantages of ATR-SEIRAS are explored in order to decide about the interactions of co-adsorbed adenine and thymine molecules on gold thin-film electrodes in acid media as compared to the separated adsorption of the two bases. Experiments were carried out both in water and deuterium oxide solutions and the spectra for deuterated and non-deuterated adsorbed species analysed. 2. Experimental Working solutions were 0.1 M HClO 4 (Merck Suprapur) in Purelab ultra water or in D 2 O (Sigma Aldrich 99.96%). Adenine and thymine (Sigma–Aldrich) were used as received. Adenine and thymine solutions with concentrations ranging from 1 × 10 -5 to 1 × 10 -3 M were pre- pared by spiking 1 × 10 -2 M stock solutions made in the same sup- porting electrolyte. Solutions were deareated by bubbling argon. The voltammetry experiments were performed with Au(111) single crystal electrodes prepared and cleaned as indicated in [6]. In the ATR- SEIRAS experiments, gold thin-film electrodes (ca 25 nm thick) were used, prepared by argon sputtering on one of the sides of a silicon prism bevelled at 60° (Pastec, Japan). Deposition rate was 0.01 nm s -1 . A gold foil and a reversible hydrogen electrode were used as counter and reference electrodes, respectively. All potentials are referred to the saturated calomel electrode. Electrochemistry Communications 35 (2013) 53–56 ⁎ Corresponding author. Tel.: +34 954556733; fax: +34 9 5 4233765. E-mail address: marueda@us.es (M. Rueda). 1388-2481/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.elecom.2013.07.026 Contents lists available at ScienceDirect Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom