Molecular recognition between guanine and cytosine-functionalized nucleolipid hybrid bilayers supported on gold (111) electrodes
Introduction
Molecular recognition reactions taking place at an electrode surface play an important role in the development of drug delivery or biosensor systems where the potential is commonly controlled [1]. The voltage controlled adsorption and co-adsorption of nucleobases at gold electrode surfaces have been extensively investigated [2], [3], [4], [5], [6], [7]. However, when the nucleoside bases are directly adsorbed onto the electrode, it is difficult to separate base-base interactions from the nucleobase-gold interactions. In a recent publication, a monolayer of 1,2-dipalmitoyl-sn-glycero-3-cytidine was deposited onto a gold (111) surface [8]. In this architecture, the two long acyl chains of the nucleolipid separate the cytosine moiety from the gold electrode surface eliminating the base-gold interactions and direct the cytidine head group towards the solution making it accessible for guanine base pairing [8]. Several studies have also demonstrated that molecular recognition can be conveniently investigated by spreading nucleolipid monolayers at the air-water interface [9], [10], [11], [12]. Recently, photon polarization modulation infrared reflection spectroscopy (PM-IRRAS) was employed to investigate the potential controlled orientation of DNA duplexes tethered to the gold electrode surface [13]. Accordingly, we have applied the combination of electrochemical and PM-IRRAS techniques to investigate the interaction of guanine with a 1,2-dipalmitoyl-sn-glycero-3-cytidine monolayer deposited on a gold (111) electrode surface. These studies revealed that the binding of guanine to the nucleolipid monolayer is strongly potential dependent [14]. However, the complex formation was strongly dependent on the charge density at the metal. In addition, guanine has a strong affinity to the gold electrode surface, while the physical adsorption interactions between the nucleolipid acyl chains and the gold surface are weak. Fig. 1 shows that the PM-IRRAS spectrum of the 1,2-dipalmitoyl-sn-glycero-3-cytidine monolayer with co-adsorb guanine in the 1800–1600 cm−1 region is significantly different than the transmission spectrum of a 1,2-dipalmitoyl-sn-glycero-3-cytidine vesicle solution incubated with guanine. This spectral region contains information about the C=O vibrational groups, which are involved in the cytosine-guanine complex formation. The spectral differences suggest that guanine may deeply penetrate into the 1,2-dipalmitoyl-sn-glycero-3-cytidine monolayer when the monolayer is physisorbed to the surface of an electrode.
To prove that the guanine molecules did not displace the nucleolipids within film, we have assembled a hybrid bilayer membrane (hBLM), using the components shown in Fig. 2, where the inner leaflet consisted of a 1-hexadecanethiol self-assembled monolayer (SAM) and the outer leaflet was formed by depositing a monolayer of 1,2-dipalmitoyl-sn-glycero-3-cytidine using the Langmuir Schaefer (LS) technique. The dipole moment of guanine is 6.5 D and hence the molecule is quite polar. The long aliphatic chain of 1-hexadecanethiol created a thick nonpolar region where the lipids are densely packed in a gel state. This hBLM is highly robust and eliminates the direct interaction between gold and guanine due to the strong affinity of the gold-sulfur bond. The objective of this paper was to characterize the interaction between cytidine and guanine using the hBLM system. This architecture ensures better lipid packing and allows the interaction between the two complementary bases to be studied at a broader range of potentials. Electrochemical measurements were used to monitor the stability of the hBLM and the PMI-IRRAS spectra to provide information about the orientation of the acyl chain, the cytidine nucleolipid as well as the interaction between cytidine and guanine. The information gained in this study is relevant for the development of future molecule-based sensors.
Section snippets
Reagents, solutions, electrodes and materials
1,2-dipalmitoyl-sn-glycero-3-cytidine diphosphate (16:0 CDP DG), was purchased from Avanti Polar Lipid and dissolved in chloroform to give a 1 mg mL−1 stock solution. The 1-hexadecanethiol obtained from Merck was dissolved in methanol to obtain a 1 mg mL−1 stock solution. Sodium fluoride powder (BioXtra, 99%) obtained from Sigma-Aldrich was cleaned in a UV ozone chamber (UVO cleaner, Jelight) for 15 min to oxidize and remove any organic impurities prior to electrolyte preparation. For
Electrochemical measurements
Differential capacitance (DC) curves were used to determine the behavior and stability of the hybrid 1-hexadecanethiol/1,2-dipalmitoyl-sn-glycero-3-cytidine bilayers, in the absence and presence of guanine, as a function of the applied potential. Fig. 3 compares the DC curves of the pure gold (111) electrode (1), the gold (111) electrode covered with a SAM of 1-hexadecanethiol (2) and the 1-hexadecanethiol/1,2-dipalmitoyl-sn-glycero-3-cytidine bilayer in presence (3) and absence (4) of guanine
Conclusions
We have compared the guanine-cytosine molecular recognition reaction of a hybrid bilayer, where the inner leaflet consisted of a SAM of the 1-hexadecane thiol and the outer leaflet of a cytosine-containing nucleolipid, with a physisorbed monolayer of the same nucleolipid molecule deposited directly onto the gold (111) electrode surface as described in [14]. In both films, guanine was present in the complexed and non-complexed forms, however, the molecular complex between guanine and the
Declaration of Competing Interest
Author declares that there is no conflicts of interest.
Acknowledgements
The FP and MR acknowledge research grants from Spanish Ministry of Economy and Competitiveness (CTQ2014-57515-C2-1-R) and Andalusian government (PAI-FQM202). JL acknowledges support of the Discovery grant from Natural Sciences and Engineering Council of Canada RG-03958. JAM acknowledges a FPU grant and a Visiting Academic grant from the Spanish Ministry of Science and Technology.
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